Atom

The atom is the smallest unit of an element that retains the chemical properties of that element. An atom has an electron cloud consisting of negatively charged electrons surrounding a dense nucleus. The nucleus contains positively charged protons and electrically neutral neutrons. When the number of protons in the nucleus equals the number of electrons, the atom is electrically neutral; otherwise it is an ion and has a net positive or negative charge. An atom is classified according to its number of protons and neutrons: the number of protons determines the chemical element and the number of neutrons determines the isotope of that element.

The name atom comes from the Greek ἄτομος/átomos, α-τεμνω, which means uncuttable, something that cannot be divided further. The concept of an atom as an indivisible component of matter was first proposed by early Indian and Greek philosophers. In the 17th and 18th centuries, chemists provided a physical basis for this idea by showing that certain substances could not be further broken down by chemical methods. During the late 19th and early 20th centuries, physicists discovered subatomic components and structure inside the atom, thereby demonstrating that the 'atom' was not indivisible. The principles of quantum mechanics were used to successfully model the atom. [cite web
first=Hans | last=Haubold | coauthors=Mathai, A. M. | year=1998 | url=http://www.columbia.edu/~ah297/unesa/universe/universe-chapter3.html | title=Microcosmos: From Leucippus to Yukawa | work=Structure of the Universe | publisher=Common Sense Science | accessdate=2008-01-17] [Harrison (2003:123–139).]

Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses that can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.9% of an atom's mass is concentrated in the nucleus, [Most isotopes have more nucleons than electrons. In the corner case of hydrogen-1, with a single electron and nucleon, the proton is , or 99.95% of the total atomic mass.] with protons and neutrons having roughly equal mass. Each element has at least one isotope with unstable nuclei that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus. [cite web | author=Staff | date=2007-08-01 | url=http://www2.slac.stanford.edu/vvc/theory/nuclearstability.html | title=Radioactive Decays | publisher=Stanford Linear Accelerator Center, Stanford University | accessdate=2007-01-02] Electrons occupy a set of stable energy levels, or orbitals, and can transition between these states by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

History

The concept that matter is composed of units and cannot be divided into arbitrarily tiny quantities has been around for millennia, but these ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. The nature of atoms in philosophy varied considerably over time and between cultures and schools, and often had spiritual elements. Nevertheless, the basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.Ponomarev (1993:14–15).]

The earliest references to the concept of atoms date back to ancient India in the 6th century BCE. [Gangopadhyaya (1981).] The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects (first in pairs, then trios of pairs). [Teresi (2003:213–214).] The references to atoms in the West emerged a century later from Leucippus whose student, Democritus, systemized his views. In approximately 450 BCE, Democritus coined the term "átomos" ( _el. ἄτομος), which means "uncuttable" or "the smallest indivisible particle of matter", i.e., something that cannot be divided. Although the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.

Further progress in the understanding of atoms did not occur until the science of chemistry began to develop. In 1661, natural philosopher Robert Boyle published "The Sceptical Chymist" in which he argued that matter was composed of various combinations of different "corpuscules" or atoms, rather than the classical elements of air, earth, fire and water. [Siegfried (2002:42–55).] In 1789 the term "element" was defined by the French nobleman and scientific researcher Antoine Lavoisier to mean basic substances that could not be further broken down by the methods of chemistry. [cite web
url=http://web.lemoyne.edu/~GIUNTA/EA/LAVPREFann.HTML
title=Lavoisier's Elements of Chemistry
work=Elements and Atoms
publisher=Le Moyne College, Department of Chemistry
accessdate=2007-12-18 ]

In 1803, English instructor and natural philosopher John Dalton used the concept of atoms to explain why elements always react in a ratio of small whole numbers—the
law of multiple proportions—and why certain gases dissolve better in water than others. He proposed that each element consists of atoms of a single, unique type, and that these atoms can join together to form chemical compounds. [Wurtz (1881:1–2).] [Dalton (1808).]

Additional validation of particle theory (and by extension atomic theory) occurred in 1827 when botanist Robert Brown used a microscope to look at dust grains floating in water and discovered that they moved about erratically—a phenomenon that became known as "Brownian motion". J. Desaulx suggested in 1877 that the phenomenon was caused by the thermal motion of water molecules, and in 1905 Albert Einstein produced the first mathematical analysis of the motion, thus confirming the hypothesis. [cite journal
last=Einstein | first=Albert
title=Über die von der molekularkinetischen Theorie der Wärme geforderte Bewegung von in ruhenden Flüssigkeiten suspendierten Teilchen
journal=Annalen der Physik | month=May | year=1905
volume=322 | issue=8 | pages=549–560 | language=German
url=http://www.physik.uni-augsburg.de/annalen/history/papers/1905_17_549-560.pdf
format=PDF | doi=10.1002/andp.19053220806
accessdate=2007-02-04 ] [Mazo (2002:1–7).] [cite web
last=Lee | first=Y. K. | coauthors=Hoon, Kelvin
year=1995
url=http://www.doc.ic.ac.uk/~nd/surprise_95/journal/vol4/ykl/report.html
title=Brownian Motion | publisher=Imperial College, London
accessdate=2007-12-18 ]

The physicist J. J. Thomson, through his work on cathode rays in 1897, discovered the electron and its subatomic nature, which destroyed the concept of atoms as being indivisible units.cite web
author=The Nobel Foundation | year=1906
url=http://nobelprize.org/nobel_prizes/physics/laureates/1906/thomson-bio.html
title=J.J. Thomson | publisher=Nobelprize.org
accessdate=2007-12-20 ] Thomson believed that the electrons were distributed throughout the atom, with their charge balanced by the presence of a uniform sea of positive charge (the plum pudding model).

However, in 1909, researchers under the direction of physicist Ernest Rutherford bombarded a sheet of gold foil with helium ions and discovered that a small percentage were deflected through much larger angles than was predicted using Thomson's proposal. Rutherford interpreted the gold foil experiment as suggesting that the positive charge of an atom and most of its mass was concentrated in a nucleus at the center of the atom (the Rutherford model), with the electrons orbiting it like planets around a sun. Positively charged helium ions passing close to this dense nucleus would then be deflected away at much sharper angles. [cite journal
last=Rutherford | first=E.
title=The Scattering of α and β Particles by Matter and the Structure of the Atom
journal=Philosophical Magazine
year=1911 | volume=21 | pages=669–88
url=http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Rutherford-1911/Rutherford-1911.html
accessdate=2008-01-18]

While experimenting with the products of radioactive decay, in 1913 radiochemist Frederick Soddy discovered that there appeared to be more than one type of atom at each position on the periodic table. [cite web
url=http://nobelprize.org/nobel_prizes/chemistry/laureates/1921/soddy-bio.html
title=Frederick Soddy, The Nobel Prize in Chemistry 1921
publisher=Nobel Foundation
accessdate=2008-01-18] The term isotope was coined by Margaret Todd as a suitable name for different atoms that belong to the same element. J.J. Thomson created a technique for separating atom types through his work on ionized gases, which subsequently led to the discovery of stable isotopes. [cite journal
last=Thomson | first=Joseph John
title=Rays of positive electricity
journal=Proceedings of the Royal Society
year=1913 | volume=A 89 | pages=1–20
url=http://web.lemoyne.edu/~giunta/canal.html
accessdate=2007-01-18 ]

of thehydrogen atom, showing an electron jumping between fixed orbits and emitting a photon of energy with a specific frequency.]

Meanwhile, in 1913, physicist Niels Bohr revised Rutherford's model by suggesting that the electrons were confined into clearly defined orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states. [cite web
last=Stern | first=David P. | date=May 16, 2005
url=http://www-spof.gsfc.nasa.gov/stargaze/Q5.htm
title=The Atomic Nucleus and Bohr's Early Model of the Atom
publisher=NASA Goddard Space Flight Center
accessdate=2007-12-20 ] An electron must absorb or emit specific amounts of energy to transition between these fixed orbits. When the light from a heated material is passed through a prism, it produced a multi-colored spectrum. The appearance of fixed lines in this spectrum was successfully explained by the orbital transitions. [cite web
last=Bohr | first=Niels | date=December 11, 1922
url=http://nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-lecture.html
title=Niels Bohr, The Nobel Prize in Physics 1922, Nobel Lecture
publisher=The Nobel Foundation
accessdate=2008-02-16 ]

Chemical bonds between atoms were now explained, by Gilbert Newton Lewis in 1916, as the interactions between their constituent electrons. [cite journal
last=Lewis | first=Gilbert N.
title=The Atom and the Molecule
journal=Journal of the American Chemical Society
date=April 1916 | volume=38 | issue=4
pages=762–786 | doi=10.1021/ja02261a002 ] As the chemical properties of the elements were known to largely repeat themselves according to the periodic law, [cite book
first=Eric R. | last=Scerri | year=2007
title=The Periodic Table | pages=205–226
publisher=Oxford University Press US
isbn=0195305736 ] in 1919 the American chemist Irving Langmuir suggested that this could be explained if the electrons in an atom were connected or clustered in some manner. Groups of electrons were thought to occupy a set of electron shells about the nucleus. [cite journal
last=Langmuir | first=Irving
title=The Arrangement of Electrons in Atoms and Molecules
journal=Journal of the American Chemical Society
year=1919 | volume=41 | issue=6 | pages=868–934
url=http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Langmuir-1919b.html
accessdate=2008-09-01
doi=10.1021/ja02227a002 ]

The Stern–Gerlach experiment of 1922 provided further evidence of the quantum nature of the atom. When a beam of silver atoms was passed through a specially-shaped magnetic field, the beam was split based on the direction of an atom's angular momentum, or spin. As this direction is random, the beam could be expected to spread into a line. Instead, the beam was split into two parts, depending on whether the atomic spin was oriented up or down. [cite journal
last=Scully | first=Marlan O.
coauthors=Lamb Jr., Willis E.; Barut, Asim
title=On the theory of the Stern-Gerlach apparatus
journal=Foundations of Physics
year=1987 | month=June | volume=17
issue=6 | pages=575–583
doi=10.1007/BF01882788 ]

In 1926, Erwin Schrödinger, using Louis de Broglie's 1924 proposal that particles behave to an extent like waves, developed a mathematical model of the atom that described the electrons as three-dimensional waveforms, rather than point particles. A consequence of using waveforms to describe electrons is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at the same time; this became known as the uncertainty principle, formulated by
Werner Heisenberg in 1926. In this concept, for each measurement of a position one could only obtain a range of probable values for momentum, and vice versa. Although this model was difficult to visualise, it was able to explain observations of atomic behavior that previous models could not, such as certain structural and spectral patterns of atoms larger than hydrogen. Thus, the planetary model of the atom was discarded in favor of one that described atomic orbital zones around the nucleus where a given electron is most likely to exist. [cite web
last=Brown | first=Kevin | year=2007
url=http://www.mathpages.com/home/kmath538/kmath538.htm
title=The Hydrogen Atom | publisher=MathPages
accessdate=2007-12-21] [cite web
last=Harrison | first=David M. | month=March | year=2000
url=http://www.upscale.utoronto.ca/GeneralInterest/Harrison/DevelQM/DevelQM.html
title=The Development of Quantum Mechanics
publisher=University of Toronto
accessdate=2007-12-21 ]

The development of the mass spectrometer allowed the exact mass of atoms to be measured. The device uses a magnet to bend the trajectory of a beam of ions, and the amount of deflection is determined by the ratio of an atom's mass to its charge. The chemist Francis William Aston used this instrument to demonstrate that isotopes had different masses. The mass of these isotopes varied by integer amounts, called the whole number rule. [cite journal
title=The constitution of atmospheric neon
journal=Philosophical Magazine | year=1920
first=Francis W. | last=Aston
volume=39 | issue=6 | pages=449–55 ] The explanation for these different atomic isotopes awaited the discovery of the neutron, a neutral-charged particle with a mass similar to the proton, by the physicist James Chadwick in 1932. Isotopes were then explained as elements with the same number of protons, but different numbers of neutrons within the nucleus. [cite web
last=Chadwick | first=James | date=December 12, 1935
url=http://nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-lecture.html
title=Nobel Lecture: The Neutron and Its Properties
publisher=Nobel Foundation
accessdate=2007-12-21 ]

In the 1950s, the development of improved particle accelerators and particle detectors allowed scientists to study the impacts of atoms moving at high energies. [cite web
last=Kullander | first=Sven | date=August 28, 2001
url=http://nobelprize.org/nobel_prizes/physics/articles/kullander/
title=Accelerators and Nobel Laureates
publisher=The Nobel Foundation
accessdate=2008-01-31 ] Neutrons and protons were found to be hadrons, or composites of smaller particles called quarks. Standard models of nuclear physics were developed that successfully explained the properties of the nucleus in terms of these sub-atomic particles and the forces that govern their interactions. [cite web
author=Staff | date=October 17, 1990
url=http://nobelprize.org/nobel_prizes/physics/laureates/1990/press.html
title=The Nobel Prize in Physics 1990
publisher=The Nobel Foundation
accessdate=2008-01-31 ]

Around 1985, Steven Chu and co-workers at Bell Labs developed a technique for lowering the temperatures of atoms using lasers. In the same year, a team led by William D. Phillips managed to contain atoms of sodium in a magnetic trap. The combination of these two techniques and a method based on the Doppler effect, developed by Claude Cohen-Tannoudji and his group, allows small numbers of atoms to be cooled to several microkelvin. This allows the atoms to be studied with great precision, and later led to the discovery of Bose-Einstein condensation. [cite web
author=Staff | date=October 15, 1997
url=http://nobelprize.org/nobel_prizes/physics/laureates/1997/
title=The Nobel Prize in Physics 1997
publisher=Nobel Foundation
accessdate=2008-02-10 ]

Historically, single atoms have been prohibitively small for scientific applications. Recently, devices have been constructed that use a single metal atom connected through organic ligands to construct a single electron transistor. [cite journal
author=Park, Jiwoong "et al" | journal = Nature
year = 2002 | volume = 417 | issue = 6890 | pages=722–25
title = Coulomb blockade and the Kondo effect in single-atom transistors
url=http://adsabs.harvard.edu/abs/2002Natur.417..722P
doi=10.1038/nature00791 | accessdate=2008-01-03 ] Experiments have been carried out by trapping and slowing single atoms using laser cooling in a cavity to gain a better physical understanding of matter. [cite journal
first=P. | last=Domokos | coauthors=Janszky, J.; Adam, P.
title=Single-atom interference method for generating Fock states
journal=Physical Review a | volume=50
pages=3340–44 | year=1994 | doi=10.1103/PhysRevA.50.3340
url=http://adsabs.harvard.edu/abs/1994PhRvA..50.3340D
accessdate=2008-01-03 ]

Components

ubatomic particles

Though the word "atom" originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the atom is composed of various subatomic particles. The constituent particles of an atom are the electron, the proton and the neutron, except that hydrogen-1 has no neutrons and a positive hydrogen ion has no electrons.

The electron is by far the least massive of these particles at 9.11Esp|−31 kg, with a negative electrical charge and a size that is too small to be measured using available techniques. [Demtröder (2002:39–42).] Protons have a positive charge and a mass 1,836 times that of the electron, at 1.6726Esp|−27 kg, although this can be reduced by changes to the atomic binding energy. Neutrons have no electrical charge and have a free mass of 1,839 times the mass of electrons, [Woan (2000:8).] or 1.6929Esp|−27 kg. Neutrons and protons have comparable dimensions—on the order of 2.5Esp|−15 m—although the 'surface' of these particles is not sharply defined. [MacGregor (1992:33–37).]

In the Standard Model of physics, both protons and neutrons are composed of elementary particles called quarks. The quark is a type of fermion, and is one of the two basic constituents of matter—the other being the lepton, of which the electron is an example. There are six types of quarks, each having a fractional electric charge of either +2/3 or −1/3. Protons are composed of two up quarks and one down quark, while a neutron consists of one up quark and two down quarks. This distinction accounts for the difference in mass and charge between the two particles. The quarks are held together by the strong nuclear force, which is mediated by gluons. The gluon is a member of the family of gauge bosons, which are elementary particles that mediate physical forces. [cite web
author=Particle Data Group | year=2002
url=http://www.particleadventure.org/
title=The Particle Adventure
publisher=Lawrence Berkeley Laboratory
accessdate=2007-01-03] [cite web
first=James | last=Schombert
date=April 18, 2006
url=http://abyss.uoregon.edu/~js/ast123/lectures/lec07.html
title=Elementary Particles
publisher=University of Oregon
accessdate=2007-01-03]

Nucleus

All the bound protons and neutrons in an atom make up a tiny atomic nucleus, and are collectively called nucleons. The radius of a nucleus is approximately equal to fm, where "A" is the total number of nucleons. [Jevremovic (2005:63).] This is much smaller than the radius of the atom, which is on the order of 10⁵ fm. The nucleons are bound together by a short-ranged attractive potential called the residual strong force. At distances smaller than 2.5 fm this force is much more powerful than the electrostatic force that causes positively charged protons to repel each other.Pfeffer (2000:330–336).]

Atoms of the same element have the same number of protons, called the atomic number. Within a single element, the number of neutrons may vary, determining the isotope of that element. The total number of protons and neutrons determine the nuclide. The number of neutrons relative to the protons determines the stability of the nucleus, with certain isotopes undergoing radioactive decay. [cite web
last=Wenner | first=Jennifer M. | date=October 10, 2007
url=http://serc.carleton.edu/quantskills/methods/quantlit/RadDecay.html
title=How Does Radioactive Decay Work?
publisher=Carleton College | accessdate=2008-01-09 ]

The neutron and the proton are different types of fermions. The Pauli exclusion principle is a quantum mechanical effect that prohibits "identical" fermions (such as multiple protons) from occupying the same quantum physical state at the same time. Thus every proton in the nucleus must occupy a different state, with its own energy level, and the same rule applies to all of the neutrons. (This prohibition does not apply to a proton and neutron occupying the same quantum state.)

A nucleus that has a different number of protons than neutrons can potentially drop to a lower energy state through a radioactive decay that causes the number of protons and neutrons to more closely match. As a result, atoms with roughly matching numbers of protons and neutrons are more stable against decay. However, with increasing atomic number, the mutual repulsion of the protons requires an increasing proportion of neutrons to maintain the stability of the nucleus, which modifies this trend. Thus, there are no stable nuclei with equal proton and neutron numbers above atomic number Z = 20 (calcium); and as Z increases toward the heaviest nuclei, the ratio of neutrons per proton required for stability increases to about 1.5.

The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. Nuclear fusion occurs when multiple atomic particles join to form a heavier nucleus, such as through the energetic collision of two nuclei. At the core of the Sun, protons require energies of 3–10 KeV to overcome their mutual repulsion—the coulomb barrier—andfuse together into a single nucleus. [cite web
last=Mihos | first=Chris | date=July 23, 2002
url=http://burro.cwru.edu/Academics/Astr221/StarPhys/coulomb.html
title=Overcoming the Coulomb Barrier
publisher=Case Western Reserve University
accessdate=2008-02-13 ] Nuclear fission is the opposite process, causing a nucleus to split into two smaller nuclei—usually through radioactive decay. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. In such processes that change the number of protons in a nucleus, the atom becomes an atom of a different chemical element. [cite web
author=Staff | date=March 30, 2007
url=http://www.lbl.gov/abc/Basic.html
title=ABC's of Nuclear Science
publisher=Lawrence Berkeley National Laboratory
accessdate=2007-01-03] [cite web
first=Arjun | last=Makhijani | coauthors=Saleska, Scott
date=March 2, 2001
url=http://www.ieer.org/reports/n-basics.html
title=Basics of Nuclear Physics and Fission
publisher=Institute for Energy and Environmental Research
accessdate=2007-01-03]

If the mass of the nucleus following a fusion reaction is less than the sum of the masses of the separate particles, then the difference between these two values is emitted as energy, as described by Albert Einstein's mass–energy equivalence formula, "E" = "mc"², where "m" is the mass loss and "c" is the speed of light. This deficit is the binding energy of the nucleus. [Shultis "et al" (2002:72–6).]

The fusion of two nuclei that have lower atomic numbers than iron and nickel is usually an exothermic process that releases more energy than is required to bring them together. [cite journal
last = Fewell | first = M. P.
title=The atomic nuclide with the highest mean binding energy
journal=American Journal of Physics
year=1995 | volume=63 | issue=7 | pages=653–58
url=http://adsabs.harvard.edu/abs/1995AmJPh..63..653F
accessdate = 2007-02-01
doi=10.1119/1.17828 ] It is this energy-releasing process that makes nuclear fusion in stars a self-sustaining reaction. For heavier nuclei, the total binding energy begins to decrease. That means fusion processes with nuclei that have higher atomic numbers is an endothermic process. These more massive nuclei can not undergo an energy-producing fusion reaction that can sustain the hydrostatic equilibrium of a star.cite web
last=Raymond | first=David | date=April 7, 2006
url=http://physics.nmt.edu/~raymond/classes/ph13xbook/node216.html
title=Nuclear Binding Energies
publisher=New Mexico Tech
accessdate=2007-01-03 ]

Electron cloud

The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force. This force binds the electrons inside an electrostatic potential well surrounding the smaller nucleus, which means that an external source of energy is needed in order for the electron to escape. The closer an electron is to the nucleus, the greater the attractive force. Hence electrons bound near the center of the potential well require more energy to escape than those at the exterior.

Electrons, like other particles, have properties of both a particle and a wave. The electron cloud is a region inside the potential well where each electron forms a type of three-dimensional standing wave—a wave form that does not move relative to the nucleus. This behavior is defined by an atomic orbital, a mathematical function that characterises the probability that an electron will appear to be at a particular location when its position is measured. [cite journal
last=Mulliken | first=Robert S.
title=Spectroscopy, Molecular Orbitals, and Chemical Bonding
journal=Science | year=1967
volume=157 | issue=3784 | pages=13–24
doi=10.1126/science.157.3784.13
pmid=5338306 ] Only a discrete (or d) set of these orbitals exist around the nucleus, as other possible wave patterns will rapidly decay into a more stable form.cite web
last=Brucat | first=Philip J. | year=2008
url=http://www.chem.ufl.edu/~itl/2045/lectures/lec_10.html
title=The Quantum Atom | publisher=University of Florida
accessdate=2007-01-04 ] Orbitals can have one or more ring or node structures, and they differ from each other in size, shape and orientation. [cite web
last=Manthey | first=David | year=2001
url=http://www.orbitals.com/orb/
title=Atomic Orbitals | publisher=Orbital Central
accessdate=2008-01-21 ]

Each atomic orbital corresponds to a particular energy level of the electron. The electron can change its state to a higher energy level by absorbing a photon with sufficient energy to boost it into the new quantum state. Likewise, through spontaneous emission, an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon. These characteristic energy values, defined by the differences in the energies of the quantum states, are responsible for atomic spectral lines.

The amount of energy needed to remove or add an electron (the electron binding energy) is far less than the binding energy of nucleons. For example, it requires only 13.6 eV to strip a ground-state electron from a hydrogen atom. [cite web
last=Herter | first=Terry | year=2006
url=http://instruct1.cit.cornell.edu/courses/astro101/lectures/lec08.htm
title=Lecture 8: The Hydrogen Atom
publisher=Cornell University | accessdate=2008-02-14 ] Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms that have either a deficit or a surplus of electrons are called ions. Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals. [Smirnov (2003:249–72).]

Properties

Nuclear properties

By definition, any two atoms with an identical number of "protons" in their nuclei belong to the same chemical element. Atoms with equal numbers of protons but a different number of "neutrons" are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons (hydrogen-1, by far the most common form, sometimes called protium), one neutron (deuterium), two neutrons (tritium) and more than two neutrons. [cite web
last=Matis | first=Howard S. | date=August 9, 2000
url=http://www.lbl.gov/abc/wallchart/chapters/02/3.html
title=The Isotopes of Hydrogen
work=Guide to the Nuclear Wall Chart
publisher=Lawrence Berkeley National Lab
accessdate=2007-12-21 ] The known elements form a set of atomic numbers from hydrogen with a single proton up to the 118-proton element ununoctium. [cite news
last=Weiss | first=Rick | date=October 17, 2006
title=Scientists Announce Creation of Atomic Element, the Heaviest Yet
publisher=Washington Post
url=http://www.washingtonpost.com/wp-dyn/content/article/2006/10/16/AR2006101601083.html
accessdate=2007-12-21 ] All known isotopes of elements with atomic numbers greater than 82 are radioactive.Sills (2003:131–134).] cite news
last=Dumé | first=Belle | date=April 23, 2003
title=Bismuth breaks half-life record for alpha decay
publisher=Physics World
url=http://physicsworld.com/cws/article/news/17319
accessdate=2007-12-21 ]

About 339 nuclides occur naturally on Earth, of which 269 (about 79%) are stable. [cite web
last=Lindsay | first=Don | date=July 30, 2000
url=http://www.don-lindsay-archive.org/creation/isotope_list.html
title=Radioactives Missing From The Earth
publisher=Don Lindsay Archive
accessdate=2007-05-23 ] Of the chemical elements, 80 have one or more stable isotopes. Elements 43, 61, and all elements numbered 83 or higher have no stable isotopes. As a rule, there is, for each atomic number (each element) only a handful of stable isotopes, the average being 3.4 stable isotopes per element which has any stable isotopes. Sixteen elements have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten (for the element tin).CRC Handbook (2002).]

Stability of isotopes is affected by the ratio of protons to neutrons, and also by presence of certain "magic numbers" of neutrons or protons which represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus. Of the 269 known stable nuclides, only four have both an odd number of protons "and" odd number of neutrons: ²H, ⁶Li, ¹⁰B and ¹⁴N. Also, only four naturally-occurring, radioactive odd-odd nuclides have a half-life over a billion years: ⁴⁰K, ⁵⁰V, ¹³⁸La and ^180mTa. Most odd-odd nuclei are highly unstable with respect to beta decay, because the decay products are even-even, and are therefore more strongly bound, due to nuclear pairing effects.

Mass

Because the large majority of an atom's mass comes from the protons and neutrons, the total number of these particles in an atom is called the mass number. The mass of an atom at rest is often expressed using the unified atomic mass unit (u), which is also called a Dalton (Da). This unit is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66Esp|−27 kg. Hydrogen-1, the lightest isotope of hydrogen and the atom with the lowest mass, has an atomic weight of 1.007825 u. [cite web
last=Chieh | first=Chung
date=January 22, 2001
url=http://www.science.uwaterloo.ca/~cchieh/cact/nuctek/nuclideunstable.html
title=Nuclide Stability
publisher=University of Waterloo
accessdate=2007-01-04 ] An atom has a mass approximately equal to the mass number times the atomic mass unit. [cite web
url=http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&ascii=html&isotype=some
title=Atomic Weights and Isotopic Compositions for All Elements
publisher=National Institute of Standards and Technology
accessdate=2007-01-04 ] The heaviest stable atom is lead-208, with a mass of 207.9766521 u. [cite journal
last=Audi | first=G. | coauthors=Wapstra, A. H.; Thibault C.
title=The Ame2003 atomic mass evaluation (II)
journal=Nuclear Physics
year=2003 | volume=A729 | pages=337–676
url=http://www.nndc.bnl.gov/amdc/web/masseval.html
accessdate=2008-02-07 ]

As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles. The mole is defined such that one mole of any element will always have the same number of atoms (about 6.022Esp|23). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element will have a mass of 0.001 kg, or 1 gram. Carbon, for example, has an atomic mass of 12 u, so a mole of carbon atoms weighs 0.012 kg.Mills "et al" (1993).]

ize

Atoms lack a well-defined outer boundary, so the dimensions are usually described in terms of the distances between two nuclei when the two atoms are joined in a chemical bond. The radius varies with the location of an atom on the atomic chart, the type of chemical bond, the number of neighboring atoms (coordination number) and a quantum mechanical property known as spin. [cite journal
last = Shannon | first = R. D.
title=Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides
journal=Acta Crystallographica, Section a
year=1976 | volume=32 | pages=751
url=http://journals.iucr.org/a/issues/1976/05/00/issconts.html
accessdate=2007-01-03
doi=10.1107/S0567739476001551 ] On the periodic table of the elements, atom size tends to increase when moving down columns, but decrease when moving across rows (left to right). [cite web
last=Dong | first=Judy | year=1998
url=http://hypertextbook.com/facts/MichaelPhillip.shtml
title=Diameter of an Atom
publisher=The Physics Factbook
accessdate=2007-11-19 ] Consequently, the smallest atom is helium with a radius of 32 pm, while one of the largest is caesium at 225 pm. [Zumdahl (2002).] These dimensions are thousands of times smaller than the wavelengths of light (400–700 nm) so they can not be viewed using an optical microscope. However, individual atoms can be observed using a scanning tunneling microscope.

Some examples will demonstrate the minuteness of the atom. A typical human hair is about 1 million carbon atoms in width. [cite web
author=Staff | year=2007
url=http://oregonstate.edu/terra/2007winter/features/nanotech.php
title=Small Miracles: Harnessing nanotechnology
publisher=Oregon State University
accessdate=2007-01-07 —describes the width of a human hair as 10⁵ nm and 10 carbon atoms as spanning 1 nm.] A single drop of water contains about 2 sextillion (2Esp|21) atoms of oxygen, and twice the number of hydrogen atoms. [Padilla "et al" (2002:32)—"There are 2,000,000,000,000,000,000,000 (that's 2 sextillion) atoms of oxygen in one drop of water—and twice as many atoms of hydrogen."] A single carat diamond with a mass of 2Esp|-7 kg contains about 10 sextillion atoms of carbon. [A carat is 200 milligrams. By definition, Carbon-12 has 0.012 kg per mole. The Avogadro constant defines 6Esp|23 atoms per mole.] If an apple were magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple. [Feynman (1995).]

Radioactive decay

Every element has one or more isotopes that have unstable nuclei that are subject to radioactive decay, causing the nucleus to emit particles or electromagnetic radiation. Radioactivity can occur when the radius of a nucleus is large compared with the radius of the strong force, which only acts over distances on the order of 1 fm.cite web
url=http://www.splung.com/content/sid/5/page/radioactivity
title=Radioactivity | publisher=Splung.com
accessdate=2007-12-19 ]

The most common forms of radioactive decay are: [L'Annunziata (2003:3–56).] [cite web
last=Firestone | first=Richard B. | date=May 22, 2000
url=http://isotopes.lbl.gov/education/decmode.html
title=Radioactive Decay Modes
publisher=Berkeley Laboratory
accessdate=2007-01-07 ]
* Alpha decay is caused when the nucleus emits an alpha particle, which is a helium nucleus consisting of two protons and two neutrons. The result of the emission is a new element with a lower atomic number.
* Beta decay is regulated by the weak force, and results from a transformation of a neutron into a proton, or a proton into a neutron. The first is accompanied by the emission of an electron and an antineutrino, while the second causes the emission of a positron and a neutrino. The electron or positron emissions are called beta particles. Beta decay either increases or decreases the atomic number of the nucleus by one.
* Gamma decay results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. This can occur following the emission of an alpha or a beta particle from radioactive decay.

Other more rare types of radioactive decay include ejection of neutrons or protons or clusters of nucleons from a nucleus, or more than one beta particle, or result (through internal conversion) in production of high-speed electrons which are not beta rays, and high-energy photons which are not gamma rays.

Each radioactive isotope has a characteristic decay time period—the half-life—that is determined by the amount of time needed for half of a sample to decay. This is an exponential decay process that steadily decreases the proportion of the remaining isotope by 50% every half life. Hence after two half-lives have passed only 25% of the isotope will be present, and so forth.

Magnetic moment

Elementary particles possess an intrinsic quantum mechanical property known as spin. This is analogous to the angular momentum of an object that is spinning around its center of mass, although strictly speaking these particles are believed to be point-like and cannot be said to be rotating. Spin is measured in units of the reduced Planck constant ($hbar$), with electrons, protons and neutrons all having spin ½ $hbar$, or "spin-½". In an atom, electrons in motion around the nucleus possess orbital angular momentum in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin. [cite web
last=Hornak | first=J. P. | year=2006
url=http://astro.rit.edu/htbooks/nmr/bnmr.htm
title=Chapter 3: Spin Physics | work=The Basics of NMR
publisher=Rochester Institute of Technology
accessdate=2007-01-07 ]

The magnetic field produced by an atom—its magnetic moment—is determined by these various forms of angular momentum, just as a rotating charged object classically produces a magnetic field. However, the most dominant contribution comes from spin. Due to the nature of electrons to obey the Pauli exclusion principle, in which no two electrons may be found in the same quantum state, bound electrons pair up with each other, with one member of each pair in a spin up state and the other in the opposite, spin down state. Thus these spins cancel each other out, reducing the total magnetic dipole moment to zero in some atoms with even number of electrons.cite web
last=Schroeder | first=Paul A.
date=February 25, 2000
url=http://www.gly.uga.edu/schroeder/geol3010/magnetics.html
title=Magnetic Properties
publisher=University of Georgia
accessdate=2007-01-07]

In ferromagnetic elements such as iron, an odd number of electrons leads to an unpaired electron and a net overall magnetic moment. The orbitals of neighboring atoms overlap and a lower energy state is achieved when the spins of unpaired electrons are aligned with each other, a process known as an exchange interaction. When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. Paramagnetic materials have atoms with magnetic moments that line up in random directions when no magnetic field is present, but the magnetic moments of the individual atoms line up in the presence of a field. [cite web
last=Goebel | first=Greg
date=September 1, 2007
url=http://www.vectorsite.net/tpqm_04.html
title= [4.3] Magnetic Properties of the Atom
work=Elementary Quantum Physics
publisher=In The Public Domain website
accessdate=2007-01-07]

The nucleus of an atom can also have a net spin. Normally these nuclei are aligned in random directions because of thermal equilibrium. However, for certain elements (such as xenon-129) it is possible to polarize a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called hyperpolarization. This has important applications in magnetic resonance imaging. [cite journal
last=Yarris | first=Lynn | title=Talking Pictures
journal=Berkeley Lab Research Review
date=Spring 1997
url=http://www.lbl.gov/Science-Articles/Research-Review/Magazine/1997/story1.html
accessdate=2008-01-09] [Liang and Haacke (1999:412–26).]

Energy levels

When an electron is bound to an atom, it has a potential energy that is inversely proportional to its distance from the nucleus. This is measured by the amount of energy needed to unbind the electron from the atom, and is usually given in units of electronvolts (eV). In the quantum mechanical model, a bound electron can only occupy a set of states centered on the nucleus, and each state corresponds to a specific energy level. The lowest energy state of a bound electron is called the ground state, while an electron at a higher energy level is in an excited state. [cite web
last=Zeghbroeck | first=Bart J. Van | year=1998
url=http://physics.ship.edu/~mrc/pfs/308/semicon_book/eband2.htm
title=Energy levels | publisher=Shippensburg University
accessdate=2007-12-23 ]

In order for an electron to transition between two different states, it must absorb or emit a photon at an energy matching the difference in the potential energy of those levels. The energy of an emitted photon is proportional to its frequency, so these specific energy levels appear as distinct bands in the electromagnetic spectrum. [Fowles (1989:227–233).] Each element has a characteristic spectrum that can depend on the nuclear charge, subshells filled by electrons, the electromagnetic interactions between the electrons and other factors. [cite web
last=Martin | first=W. C.
coauthors=Wiese, W. L. | month=May | year=2007
url=http://physics.nist.gov/Pubs/AtSpec/
title=Atomic Spectroscopy: A Compendium of Basic Ideas, Notation, Data, and Formulas
publisher=National Institute of Standards and Technology
accessdate=2007-01-08 ]

When a continuous spectrum of energy is passed through a gas or plasma, some of the photons are absorbed by atoms, causing electrons to change their energy level. Those excited electrons that remain bound to their atom will spontaneously emit this energy as a photon, traveling in a random direction, and so drop back to lower energy levels. Thus the atoms behave like a filter that forms a series of dark absorption bands in the energy output. (An observer viewing the atoms from a different direction, which does not include the continuous spectrum in the background, will instead see a series of emission lines from the photons emitted by the atoms.) Spectroscopic measurements of the strength and width of spectral lines allow the composition and physical properties of a substance to be determined.cite web
url=http://www.avogadro.co.uk/light/bohr/spectra.htm
title=Atomic Emission Spectra — Origin of Spectral Lines
publisher=Avogadro Web Site
accessdate=2006-08-10]

Close examination of the spectral lines reveals that some displaya fine structure splitting. This occurs because of
spin-orbit coupling, which is an interaction between thespin and motion of the outermost electron. [cite web
last=Fitzpatrick | first=Richard
date=February 16, 2007
url=http://farside.ph.utexas.edu/teaching/qm/lectures/node55.html
title=Fine structure
publisher=University of Texas at Austin
accessdate=2008-02-14 ] When an atom is in an external magnetic field, spectral lines become split into three or more components; a phenomenon called the Zeeman effect. This is caused by the interaction of the magnetic field with the magnetic moment of the atom and its electrons. Some atoms can have multiple electron configurations with the same energy level, which thus appear as a single spectral line. The interaction of the magnetic field with the atom shifts these electron configurations to slightly different energy levels, resulting in multiple spectral lines. [cite web
last=Weiss | first=Michael | year=2001
url=http://math.ucr.edu/home/baez/spin/node8.html
title=The Zeeman Effect
publisher=University of California-Riverside
accessdate=2008-02-06] The presence of an external electric field can cause a comparable splitting and shifting of spectral lines by modifying the electron energy levels, a phenomenon called the Stark effect. [Beyer (2003:232–236).]

If a bound electron is in an excited state, an interacting photon with the proper energy can cause stimulated emission of a photon with a matching energy level. For this to occur, the electron must drop to a lower energy state that has an energy difference matching the energy of the interacting photon. The emitted photon and the interacting photon will then move off in parallel and with matching phases. That is, the wave patterns of the two photons will be synchronized. This physical property is used to make lasers, which can emit a coherent beam of light energy in a narrow frequency band. [cite web
last=Watkins
first=Thayer
url=http://www.sjsu.edu/faculty/watkins/stimem.htm
title=Coherence in Stimulated Emission
publisher=San José State University
accessdate=2007-12-23]

Valence

The outermost electron shell of an atom in its uncombinedstate is known as the valence shell, and the electrons inthat shell are called valence electrons. The number ofvalence electrons determines the bondingbehavior with other atoms. Atoms tend to chemically react with each other in a manner that will fill (or empty) their outer valence shells. [cite web
last=Reusch | first=William | date=July 16, 2007
url=http://www.cem.msu.edu/~reusch/VirtualText/intro1.htm
title=Virtual Textbook of Organic Chemistry
publisher=Michigan State University
accessdate=2008-01-11 ]

The chemical elements are often displayed in a periodic table that is laid out to display recurring chemical properties, and elements with the same number of valence electrons form a group that is aligned in the same column of the table. (The horizontal rows correspond to the filling of a quantum shell of electrons.) The elements at the far right of the table have their outer shell completely filled with electrons, which results in chemically inert elements known as the noble gases. [cite web
author=Husted, Robert et al | date=December 11, 2003
url=http://periodic.lanl.gov/default.htm
title=Periodic Table of the Elements
publisher=Los Alamos National Laboratory
accessdate=2008-01-11 ] [cite web
first=Rudy | last=Baum | year=2003
url=http://pubs.acs.org/cen/80th/elements.html
title=It's Elemental: The Periodic Table
publisher=Chemical & Engineering News
accessdate=2008-01-11 ]

tates

Quantities of atoms are found in different states of matter that depend on the physical conditions, such as temperature and pressure. By varying the conditions, materials can transition between solids, liquids, gases and plasmas. [Goodstein (2002:436–438).] Within a state, a material can also exist in different phases. An example of this is solid carbon, which can exist as graphite or diamond. [cite journal
last=Brazhkin | first=Vadim V.
title=Metastable phases, phase transformations, and phase diagrams in physics and chemistry
journal=Physics-Uspekhi
year=2006 | volume=49 | pages=719–24
doi=10.1070/PU2006v049n07ABEH006013 ]

At temperatures close to absolute zero, atoms can form a Bose–Einstein condensate, at which point quantum mechanical effects, which are normally only observed at the atomic scale, become apparent on a macroscopic scale. [Myers (2003:85).] [cite news
author=Staff | date=October 9, 2001
title=Bose-Einstein Condensate: A New Form of Matter
publisher=National Institute of Standards and Technology
url=http://www.nist.gov/public_affairs/releases/BEC_background.htm
accessdate=2008-01-16 ] This super-cooled collection of atomsthen behaves as a single super atom, which may allow fundamental checks of quantum mechanical behavior. [cite web
last=Colton | first=Imogen | coauthors=Fyffe, Jeanette
date=February 3, 1999
url=http://www.ph.unimelb.edu.au/~ywong/poster/articles/bec.html
title=Super Atoms from Bose-Einstein Condensation
publisher=The University of Melbourne
accessdate=2008-02-06 ]

Identification

The scanning tunneling microscope is a device for viewing surfaces at the atomic level. It uses the quantum tunneling phenomenon, which allows particles to pass through a barrier that would normally be insurmountable. Electrons tunnel through the vacuum between two planar metal electrodes, on each of which is an adsorbed atom, providing a tunneling-current density that can be measured. Scanning one atom (taken as the tip) as it moves past the other (the sample) permits plotting of tip displacement versus lateral separation for a constant current. The calculation shows the extent to which scanning-tunneling-microscope images of an individual atom are visible. It confirms that for low bias, the microscope images the space-averaged dimensions of the electron orbitals across closely packed energy levels—the Fermi level local density of states. [cite web
last=Jacox | first=Marilyn | coauthors=Gadzuk, J. William
url=http://physics.nist.gov/GenInt/STM/stm.html
title=Scanning Tunneling Microscope
publisher=National Institute of Standards and Technology
month=November | year=1997 | accessdate=2008-01-11] [cite web
url=http://nobelprize.org/nobel_prizes/physics/laureates/1986/index.html
title=The Nobel Prize in Physics 1986
publisher=The Nobel Foundation
accessdate=2008-01-11 —in particular, see the Nobel lecture by G. Binnig and H. Rohrer.]

An atom can be ionized by removing one of its electrons. The electric charge causes the trajectory of an atom to bend when it passes through a magnetic field. The radius by which the trajectory of a moving ion is turned by the magnetic field is determined by the mass of the atom. The mass spectrometer uses this principle to measure the mass-to-charge ratio of ions. If a sample contains multiple isotopes, the mass spectrometer can determine the proportion of each isotope in the sample by measuring the intensity of the different beams of ions. Techniques to vaporize atoms include inductively coupled plasma atomic emission spectroscopy and inductively coupled plasma mass spectrometry, both of which use a plasma to vaporize samples for analysis. [cite journal
first=N. | last=Jakubowski
coauthors = Moens, L.; Vanhaecke, F
title = Sector field mass spectrometers in ICP-MS
journal = Spectrochimica Acta Part B: Atomic Spectroscopy
volume = 53 | issue = 13 | year = 1998
doi=10.1016/S0584-8547(98)00222-5 | pages = 1739–63]

A more area-selective method is electron energy loss spectroscopy, which measures the energy loss of an electron beam within a transmission electron microscope when it interacts with a portion of a sample. The atom-probe tomograph has sub-nanometer resolution in 3-D and can chemically identify individual atoms using time-of-flight mass spectrometry. [cite journal
last=Müller | first=Erwin W.
authorlink=Erwin Müller
coauthors=Panitz, John A., McLane, S. Brooks
year=1968
title=The Atom-Probe Field Ion Microscope
journal=Review of Scientific Instruments
volume=39 | issue=1 | pages=83–86
issn=0034-6748 | doi=10.1063/1.1683116 ]

Spectra of excited states can be used to analyze the atomic composition of distant stars. Specific light wavelengths contained in the observed light from stars can be separated out and related to the quantized transitions in free gas atoms. These colors can be replicated using a gas-discharge lamp containing the same element. [cite web
last=Lochner | first=Jim
coauthors=Gibb, Meredith; Newman, Phil
date=April 30, 2007
url=http://imagine.gsfc.nasa.gov/docs/science/how_l1/spectral_what.html
title=What Do Spectra Tell Us?
publisher=NASA/Goddard Space Flight Center
accessdate=2008-01-03 ] Helium was discovered in this way in the spectrum of the Sun 23 years before it was found on Earth. [cite web
last=Winter | first=Mark | year=2007
url=http://www.webelements.com/webelements/elements/text/He/hist.html
title=Helium | publisher=WebElements
accessdate=2008-01-03 ]

Origin and current state

Atoms form about 4% of the total mass density of the observable universe, with an average density of about 0.25 atoms/m³. [cite web
last=Hinshaw | first=Gary
date=February 10, 2006
url=http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
title=What is the Universe Made Of?
publisher=NASA/WMAP | accessdate=2008-01-07 ] Within a galaxy such as the Milky Way, atoms have a much higher concentration, with the density of matter in the interstellar medium (ISM) ranging from 10⁵ to 10⁹ atoms/m³. [Choppin "et al" (2001).] The Sun is believed to be inside the Local Bubble, a region of highly ionized gas, so the density in the solar neighborhood is only about 10³ atoms/m³. [cite journal
last=Davidsen | first=Arthur F.
title=Far-Ultraviolet Astronomy on the Astro-1 Space Shuttle Mission
journal=Science | year=1993 | volume=259
issue=5093 | pages=327–34
url=http://www.sciencemag.org/cgi/content/abstract/259/5093/327
accessdate=2008-01-07
doi=10.1126/science.259.5093.327
pmid=17832344 ] Stars form from dense clouds in the ISM, and the evolutionary processes of stars result in the steady enrichment of the ISM with elements more massive than hydrogen and helium. Up to 95% of the Milky Way's atoms are concentrated inside stars and the total mass of atoms forms about 10% of the mass of the galaxy. [Lequeux (2005:4).] (The remainder of the mass is an unknown dark matter. [cite web
first=Nigel | last=Smith | date=January 6, 2000
url=http://physicsworld.com/cws/article/print/809
title=The search for dark matter
publisher=Physics World | accessdate = 2008-02-14 ] )

Nucleosynthesis

Stable protons and electrons appeared one second after the Big Bang. During the following three minutes, Big Bang nucleosynthesis produced most of the helium, lithium, and deuterium in the universe, and perhaps some of the beryllium and boron. [cite journal
last=Croswell | first=Ken
title=Boron, bumps and the Big Bang: Was matter spread evenly when the Universe began? Perhaps not; the clues lie in the creation of the lighter elements such as boron and beryllium
journal=New Scientist | year=1991 | issue=1794 | pages=42
url=http://space.newscientist.com/article/mg13217944.700-boron-bumps-and-the-big-bang-was-matter-spread-evenly-whenthe-universe-began-perhaps-not-the-clues-lie-in-the-creation-of-thelighter-elements-such-as-boron-and-beryllium.html
accessdate=2008-01-14 ] [cite journal
last=Copi | first=Craig J.
coauthors=Schramm, David N.; Turner, Michael S
title=Big-Bang Nucleosynthesis and the Baryon Density of the Universe
journal=Science | year=1995 | volume=267 | pages=192–99
url=http://www.sciencemag.org/cgi/reprint/267/5195/192.pdf
doi = 10.1126/science.7809624
format=PDF | accessdate=2008-01-13 |pmid=7809624] [cite web
last=Hinshaw | first=Gary | date=December 15, 2005
url=http://map.gsfc.nasa.gov/m_uni/uni_101bbtest2.html
title=Tests of the Big Bang: The Light Elements
publisher=NASA/WMAP | accessdate=2008-01-13] The first atoms (complete with bound electrons) were theoretically created 380,000 years after the Big Bang—an epoch called , when the expanding universe cooled enough to allow electrons to become attached to nuclei. [cite web
last=Abbott | first=Brian | date=May 30, 2007
url=http://www.haydenplanetarium.org/universe/duguide/exgg_wmap.php
title=Microwave (WMAP) All-Sky Survey
publisher=Hayden Planetarium | accessdate=2008-01-13] Since then, atomic nuclei have been combined in stars through the process of nuclear fusion to produce elements up to iron. [cite journal
title=The synthesis of the elements from hydrogen
author = F. Hoyle
journal = Monthly Notices of the Royal Astronomical Society
volume = 106 | pages = 343–83 | year=1946
url=http://adsabs.harvard.edu/abs/1946MNRAS.106..343H
accessdate=2008-01-13 ]

Isotopes such as lithium-6 are generated in space through cosmic ray spallation. [cite journal
last=Knauth | first=D. C.
coauthors=Federman, S. R.; Lambert, David L.; Crane, P.
title=Newly synthesized lithium in the interstellar medium
journal=Nature | year=2000 | volume=405 | pages=656–58
doi=10.1038/35015028 ] This occurs when a high-energy proton strikes an atomic nucleus, causing large numbers of nucleons to be ejected. Elements heavier than iron were produced in supernovae through the r-process and in AGB stars through the s-process, both of which involve the capture of neutrons by atomic nuclei. [cite web
last=Mashnik | first=Stepan G.
title=On Solar System and Cosmic Rays Nucleosynthesis and Spallation Processes
url=http://arxiv.org/abs/astro-ph/0008382
month=August | year=2000 | publisher=Cornell University
accessdate=2008-01-14 ] Elements such as lead formed largely through the radioactive decay of heavier elements. [cite web
author=Kansas Geological Survey | date=May 4, 2005
url=http://www.kgs.ku.edu/Extension/geotopics/earth_age.html
title=Age of the Earth | publisher=University of Kansas
accessdate=2008-01-14 ]

Earth

Most of the atoms that make up the Earth and its inhabitants were present in their current form in the nebula that collapsed out of a molecular cloud to form the solar system. The rest are the result of radioactive decay, and their relative proportion can be used to determine the age of the Earth through radiometric dating.Manuel (2001:407–430,511–519).] [cite journal
last=Dalrymple | first=G. Brent
title=The age of the Earth in the twentieth century: a problem (mostly) solved
journal=Geological Society, London, Special Publications
year=2001 | volume=190 | pages=205–21
doi=10.1144/GSL.SP.2001.190.01.14
url=http://sp.lyellcollection.org/cgi/content/abstract/190/1/205
accessdate=2008-01-14 ] Most of the helium in the crust of the Earth (about 99% of the helium from gas wells, as shown by its lower abundance of helium-3) is a product of alpha decay. [cite web
last=Anderson | first=Don L.
authorlink=Don L. Anderson
coauthors=Foulger, G. R.; Meibom, Anders
date=September 2, 2006
url=http://www.mantleplumes.org/HeliumFundamentals.html
title=Helium: Fundamental models
publisher=MantlePlumes.org | accessdate=2007-01-14 ]

There are a few trace atoms on Earth that were not present at the beginning (i.e., not "primordial"), nor are results of radioactive decay. Carbon-14 is continuously generated by cosmic rays in the atmosphere. [cite news
last=Pennicott | first=Katie | date=May 10, 2001
title=Carbon clock could show the wrong time
publisher=PhysicsWeb
url=http://physicsworld.com/cws/article/news/2676
accessdate=2008-01-14 ] Some atoms on Earth have been artificially generated either deliberately or as by-products of nuclear reactors or explosions. [cite news
last=Yarris
first=Lynn
date=July 27, 2001
title=New Superheavy Elements 118 and 116 Discovered at Berkeley Lab
publisher=Berkeley Lab
url=http://enews.lbl.gov/Science-Articles/Archive/elements-116-118.html
accessdate=2008-01-14] [cite journal
author=Diamond, H. "et al"
title=Heavy Isotope Abundances in Mike Thermonuclear Device
journal=Physical Review
year=1960
volume=119
pages=2000–04
url=http://prola.aps.org/abstract/PR/v119/i6/p2000_1
accessdate=2008-01-14
doi=10.1103/PhysRev.119.2000
format=subscription required ] Of the transuranic elements—those with atomic numbers greater than 92—only plutonium and neptunium occur naturally on Earth. [cite web
author=Poston Sr., John W. | date=March 23, 1998
title=Do transuranic elements such as plutonium ever occur naturally?
publisher=Scientific American
url=http://www.sciam.com/chemistry/article/id/do-transuranic-elements-s/topicID/4/catID/3
accessdate=2008-01-15] [cite journal
last=Keller | first=C.
title=Natural occurrence of lanthanides, actinides, and superheavy elements
journal=Chemiker Zeitung
year=1973 | volume=97 | issue=10 | pages=522–30
url=http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=4353086
accessdate=2008-01-15 ] Transuranic elements have radioactive lifetimes shorter than the current age of the Earth [Marco (2001:17).] and thus identifiable quantities of these elements have long since decayed, with the exception of traces of plutonium-244 possibly deposited by cosmic dust. Natural deposits of plutonium and neptunium are produced by neutron capture in uranium ore. [cite web
url=http://www.oklo.curtin.edu.au/index.cfm
title=Oklo Fossil Reactors
publisher=Curtin University of Technology
accessdate=2008-01-15 ]

The Earth contains approximately 1.33Esp|50 atoms. [cite web
last=Weisenberger | first=Drew
url=http://education.jlab.org/qa/mathatom_05.html
title=How many atoms are there in the world?
publisher=Jefferson Lab
accessdate=2008-01-16 ] In the planet's atmosphere, small numbers of independent atoms of noble gases exist, such as argon and neon. The remaining 99% of the atmosphere is bound in the form of molecules, including carbon dioxide and diatomic oxygen and nitrogen. At the surface of the Earth, atoms combine to form various compounds, including water, salt, silicates and oxides. Atoms can also combine to create materials that do not consist of discrete molecules, including crystals and liquid or solid metals. [cite web
last=Pidwirny | first=Michael
url=http://www.physicalgeography.net/fundamentals/contents.html
title=Fundamentals of Physical Geography
publisher=University of British Columbia Okanagan
accessdate=2008-01-16] [cite journal
last=Anderson | first=Don L.
title=The inner inner core of Earth
journal=Proceedings of the National Academy of Sciences
year=2002 | volume=99 | issue=22 | pages=13966–68
url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=137819
accessdate=2008-01-16 | doi=10.1073/pnas.232565899
pmid=12391308 ] This atomic matter forms networked arrangements that lack the particular type of small-scale interrupted order associated with molecular matter. [Pauling (1960:5–10).]

Rare and theoretical forms

While isotopes with atomic numbers higher than lead (82) are known to be radioactive, an "island of stability" has been proposed for some elements with atomic numbers above 103. These superheavy elements may have a nucleus that is relatively stable against radioactive decay. [cite journal
title=Second postcard from the island of stability
author=Anonymous | journal=CERN Courier
date=October 2, 2001
url=http://cerncourier.com/cws/article/cern/28509
accessdate=2008-01-14 ] The most likely candidate for a stable superheavy atom, unbihexium, has 126 protons and 184 neutrons. [cite journal
last=Jacoby | first=Mitch
title=As-yet-unsynthesized superheavy atom should form a stable diatomic molecule with fluorine
journal=Chemical & Engineering News
year=2006 | volume=84 | issue=10 | pages=19
url=http://pubs.acs.org/cen/news/84/i10/8410notw9.html
accessdate=2008-01-14 ]

Each particle of matter has a corresponding antimatter particle with the opposite electrical charge. Thus, the positron is a positively charged antielectron and the antiproton is a negatively charged equivalent of a proton. For unknown reasons, antimatter particles are rare in the universe, hence, no antimatter atoms have been discovered in nature. [cite news
last=Koppes | first=Steve | date=March 1, 1999
title=Fermilab Physicists Find New Matter-Antimatter Asymmetry
publisher=University of Chicago
url=http://www-news.uchicago.edu/releases/99/990301.ktev.shtml
accessdate=2008-01-14] [cite news
last=Cromie | first=William J. | date=August 16, 2001
title=A lifetime of trillionths of a second: Scientists explore antimatter
publisher=Harvard University Gazette
url=http://www.hno.harvard.edu/gazette/2001/08.16/antimatter.html
accessdate=2008-01-14] However, antihydrogen, the antimatter counterpart of hydrogen, was first synthesized at the CERN laboratory in Geneva in 1996. [cite journal
last=Hijmans | first=Tom W.
title=Particle physics: Cold antihydrogen
journal=Nature | year=2002 | volume=419
pages=439–40 | doi=10.1038/419439a] [cite news
author=Staff | date=October 30, 2002
title=Researchers 'look inside' antimatter | publisher=BBC News
url=http://news.bbc.co.uk/2/hi/science/nature/2375717.stm
accessdate=2008-01-14 ]

Other exotic atoms have been created by replacing one of the protons, neutrons or electrons with other particles that have the same charge. For example, an electron can be replaced by a more massive muon, forming a muonic atom. These types of atoms can be used to test the fundamental predictions of physics. [cite journal
last=Barrett | first=Roger
coauthors=Jackson, Daphne; Mweene, Habatwa
title=The Strange World of the Exotic Atom
journal=New Scientist
year=1990 | issue=1728 | pages=77–115
url=http://media.newscientist.com/article/mg12717284.600-the-strange-world-of-the-exotic-atom-physicists-can-nowmake-atoms-and-molecules-containing-negative-particles-other-than-electronsand-use-them-not-just-to-test-theories-but-also-to-fight-cancer-.html
accessdate=2008-01-04 ] [cite journal
last=Indelicato | first=Paul
title=Exotic Atoms | journal=Physica Scripta
year=2004 | volume=T112 | pages=20–26
doi=10.1238/Physica.Topical.112a00020 ] [cite web
last=Ripin | first=Barrett H. | month=July | year=1998
url=http://www.aps.org/publications/apsnews/199807/experiment.cfm
title=Recent Experiments on Exotic Atoms
publisher=American Physical Society
accessdate=2008-02-15 ]

ee also

* Atomic mirror
* History of quantum mechanics
* Infinite divisibility
* Introduction to quantum mechanics
* List of basic chemistry topics
* List of particles
* Nuclear model
* Radioactive isotope
* Transuranium element

References

Notes

Book references

*cite book
last=L'Annunziata
first=Michael F.
year=2003 | title=Handbook of Radioactivity Analysis
publisher=Academic Press | isbn=0124366031
oclc=162129551
*cite book
last=Beyer | first=H. F. | coauthors=Shevelko, V. P.
year=2003
title=Introduction to the Physics of Highly Charged Ions
publisher=CRC Press | isbn=0750304812
oclc=47150433
*cite book
last=Choppin | first=Gregory R.
coauthors=Liljenzin, Jan-Olov; Rydberg, Jan
year=2001 | title=Radiochemistry and Nuclear Chemistry
publisher=Elsevier | isbn=0750674636
oclc=162592180
*cite book
first=J. | last=Dalton
authorlink=John Dalton | year=1808
title=A New System of Chemical Philosophy, Part 1
publisher=S. Russell
location=London and Manchester
*cite book
last=Demtröder | first=Wolfgang | year=2002
title=Atoms, Molecules and Photons: An Introduction to Atomic- Molecular- and Quantum Physics
publisher=Springer | edition=1st Edition
isbn=3540206310
oclc=181435713
*cite book
last=Feynman | first=Richard | year=1995 | authorlink=Richard Feynman
title=Six Easy Pieces | publisher=The Penguin Group
isbn=978-0-140-27666-4
oclc=40499574
*cite book
last=Fowles | first=Grant R. | year=1989
title=Introduction to Modern Optics
publisher=Courier Dover Publications
isbn=0486659577
oclc=18834711
*cite book
last=Gangopadhyaya | first=Mrinalkanti
title=Indian Atomism: History and Sources
publisher=Humanities Press | year=1981
location=Atlantic Highlands, New Jersey
isbn=0-391-02177-X
oclc=10916778
*cite book
last=Goodstein | first=David L. | year=2002
title=States of Matter | publisher=Courier Dover Publications
id=ISBN 048649506X
*cite book
last=Harrison | first=Edward Robert | year=2003
title=Masks of the Universe: Changing Ideas on the Nature of the Cosmos
publisher=Cambridge University Press
isbn=0521773512
oclc=50441595
*cite book
last=Jevremovic | first=Tatjana | year=2005
title=Nuclear Principles in Engineering
publisher=Springer | isbn=0387232842
oclc=228384008
*cite book
last=Lequeux | first=James | year=2005
title=The Interstellar Medium
publisher=Springer | isbn=3540213260
oclc=133157789
*cite book
last=Liang | first=Z.-P. | coauthors=Haacke, E. M.
editor=Webster, J. G. | year=1999
volume=vol. 2 | pages=pp. 412–26
title=Encyclopedia of Electrical and Electronics Engineering: Magnetic Resonance Imaging
publisher=John Wiley & Sons
url=http://ieeexplore.ieee.org/iel5/8734/27658/01233976.pdf?arnumber=1233976
format=PDF | accessdate=2008-01-09 | isbn=0471139467
*cite book
last=MacGregor | first=Malcolm H. | year=1992
title=The Enigmatic Electron
publisher=Oxford University Press | isbn=0195218337
oclc=223372888
*cite book
last=Manuel | first=Oliver | year=2001
title=Origin of Elements in the Solar System: Implications of Post-1957 Observations
publisher=Springer | isbn=0306465620
oclc=228374906
*cite book
last=Mazo | first=Robert M. | year=2002
title=Brownian Motion: Fluctuations, Dynamics, and Applications
publisher=Oxford University Press | isbn=0198515677
oclc=48753074
*cite book
last=Mills | first=Ian
coauthors=Cvitaš, Tomislav; Homann, Klaus; Kallay, Nikola; Kuchitsu, Kozo
title=Quantities, Units and Symbols in Physical Chemistry
publisher=International Union of Pure and Applied Chemistry, Commission on Physiochemical Symbols Terminology and Units, Blackwell Scientific Publications
location=Oxford | edition=2nd edition | year=1993
isbn=0-632-03583-8
oclc=27011505
*cite book
last=Myers | first=Richard | year=2003
title=The Basics of Chemistry | publisher=Greenwood Press
isbn=0313316643
oclc=50164580
*cite book
last=Padilla | first=Michael J.
coauthors=Miaoulis, Ioannis; Cyr, Martha | year = 2002
title = Prentice Hall Science Explorer: Chemical Building Blocks
publisher = Prentice-Hall, Inc.
location = Upper Saddle River, New Jersey USA
isbn = 0-13-054091-9
oclc=47925884
*cite book
last=Pauling | first=Linus | year=1960
title=The Nature of the Chemical Bond
publisher=Cornell University Press
isbn=0801403332
oclc=17518275
*cite book
last=Pfeffer | first=Jeremy I.
coauthor=Nir, Shlomo | year=2000
title=Modern Physics: An Introductory Text
publisher=Imperial College Press | isbn=1860942504
oclc=45900880
*cite book
last=Ponomarev | first=Leonid Ivanovich | year=1993
title=The Quantum Dice | publisher=CRC Press
isbn=0750302518
oclc=26853108
*cite book
last=Shultis | first=J. Kenneth | coauthors=Faw, Richard E.
title=Fundamentals of Nuclear Science and Engineering
year=2002 | publisher=CRC Press | isbn=0824708342
oclc=123346507
*cite book
last=Siegfried | first=Robert | year=2002
title=From Elements to Atoms: A History of Chemical Composition
publisher=DIANE | isbn=0871699249
oclc=186607849
*cite book
last=Sills | first=Alan D. | year=2003
title=Earth Science the Easy Way
publisher=Barron's Educational Series | isbn=0764121464
oclc=51543743
*cite book
last=Smirnov | first=Boris M. | year=2003
title=Physics of Atoms and Ions
publisher=Springer | id=ISBN 038795550X
*cite book
last=Teresi | first=Dick | publisher = Simon & Schuster
title = Lost Discoveries: The Ancient Roots of Modern Science
year=2003 | isbn=074324379X
url = http://books.google.com/books?id=pheL_ubbXD0C&dq
pages = 213–214
*cite book
last=Various | editor=Lide, David R. | year=2002
title=Handbook of Chemistry & Physics
edition=88th edition | publisher=CRC
url=http://www.hbcpnetbase.com/
accessdate=2008-05-23 | isbn=0849304865
oclc=179976746
*cite book
last=Woan | first=Graham | year=2000
title=The Cambridge Handbook of Physics
publisher=Cambridge University Press | isbn=0521575079
oclc=224032426
*cite book
first=Charles Adolphe | last=Wurtz | year=1881
title=The Atomic Theory
publisher=D. Appleton and company
location=New York
*cite book
last=Zaider | first=Marco | coauthors=Rossi, Harald H. | year=2001
title=Radiation Science for Physicians and Public Health Workers
publisher=Springer | isbn=0306464039
oclc=44110319
*cite book
last=Zumdahl | first=Steven S. | year=2002
title=Introductory Chemistry: A Foundation
edition=5th edition | publisher=Houghton Mifflin
url=http://college.hmco.com/chemistry/intro/zumdahl/intro_chemistry/5e/students/protected/periodictables/pt/pt/pt_ar5.html
accessdate=2008-02-05 | isbn=0-618-34342-3
oclc=173081482

External links

*cite web
last=Francis | first=Eden | year=2002 | url=http://dl.clackamas.cc.or.us/ch104-07/atomic_size.htm
title=Atomic Size | publisher=Clackamas Community College
accessdate=2007-01-09
*cite web
last=Freudenrich | first=Craig C. | url=http://dl.clackamas.cc.or.us/ch104-07/atomic_size.htm
title=How Atoms Work | publisher=How Stuff Works
accessdate=2007-01-09
*cite web
url=http://en.wikibooks.org/wiki/FHSST_Physics_Atom:The_Atom
work=Free High School Science Texts: Physics
title=Atom:The Atom | publisher=Wikibooks
accessdate=2007-01-09
*cite web
author=Anonymous | year=2007 | url=http://www.scienceaid.co.uk/chemistry/basics/theatom.html
title=The atom | publisher=Science aid+
accessdate=2007-01-09 —a guide to the atom for teens.
*cite web
author=Anonymous | date=2006-01-03 | url=http://www.bbc.co.uk/dna/h2g2/A6672963
title=Atoms and Atomic Structure
publisher=BBC | accessdate=2007-01-11
*cite web
author=Various | date=2006-01-03 | url=http://www.colorado.edu/physics/2000/index.pl?Type=TOC
title=Physics 2000, Table of Contents
publisher=University of Colorado | accessdate=2008-01-11
*cite web
author=Various | date=2006-02-03 | url=http://www.hydrogenlab.de/elektronium/HTML/einleitung_hauptseite_uk.html
title=What does an atom look like?
publisher=University of Karlsruhe | accessdate=2008-05-12