# Mass number

Mass number

The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N=A−Z.[1]

The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or 12
C
, which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: 12
6
C
.[2] This is technically redundant, as each element is defined by its atomic number, so it is often omitted.

## Mass number changes in radioactive decay

Different types of radioactive decay are characterized by their changes in mass number as well as atomic number, according to the radioactive displacement law of Fajans and Soddy. For example, uranium-238 usually decays by alpha decay, where the nucleus loses two neutrons and two protons in the form of an alpha particle. Thus both the atomic number and the number of neutrons decrease by 2 (Z: 92→90, n: 146→144), which decreases the mass number by 4 (A = 238→234); the result is an atom of thorium-234 and an alpha particle (4
2
He2+
):[3]

 238 92U → 234 90Th + 4 2He2+

On the other hand, carbon-14 naturally decays by radioactive beta decay, whereby one neutron is transmuted into a proton with the emission of an electron and an anti-neutrino. Thus the atomic number increases by 1 (Z: 6→7) and the mass number remains the same (A = 14), while the number of neutrons decreases by 1 (n: 8→7).[4] The resulting atom is nitrogen-14, with seven protons and seven neutrons:

 14 6C → 14 7N + e− + ν e

Another type of radioactive decay without change in mass number is emission of a gamma ray from a nuclear isomer or metastable excited state of an atomic nucleus. Since all the protons and neutrons remain in the nucleus unchanged in this process, the mass number is also unchanged.

## Mass number and isotopic mass

The mass number gives an estimate of the isotopic mass measured in atomic mass units (u). For 12C the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of 12C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, 35Cl has a mass number of 35 and an isotopic mass of 34.96885.

There are two reasons for the difference between mass number and isotopic mass, known as the mass defect:

1. The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons.
2. The nuclear binding energy varies between nuclei. A nucleus with greater binding energy has a lower total energy, and therefore a lower mass according to Einstein's mass-energy equivalence relation E = mc2. For 35Cl the isotopic mass is less than 35 so this must be the dominant factor.

## Atomic mass of an element

The mass number should also not be confused with the relative atomic mass (also called atomic weight) of an element, which is the ratio of the average atomic mass of the different isotopes of that element (weighted by abundance) to the unified atomic mass unit.[5] This weighted average can be quite different from the near-integer values for individual isotopic masses.

For instance, there are two main isotopes of chlorine: chlorine-35 and chlorine-37. In any given sample of chlorine that has not been subject to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/mol).

## References

1. ^ "How many protons, electrons and neutrons are in an atom of krypton, carbon, oxygen, neon,platnum, gold, etc...?". Thomas Jefferson National Accelerator Facility. Retrieved 2008-08-27.
2. ^ "Elemental Notation and Isotopes". Science Help Online. Retrieved 2008-08-27.
3. ^ Suchocki, John. Conceptual Chemistry, 2007. Page 119.
4. ^ Curran, Greg (2004). Homework Helpers. Career Press. pp. 78–79. ISBN 1564147215.
5. ^ "IUPAC Definition of Relative Atomic Mass". International Union of Pure and Applied Chemistry. Retrieved 2008-08-27.

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### Look at other dictionaries:

• mass number — n an integer that approximates the mass of an isotope and that designates the number of nucleons in the nucleus <the symbol for carbon of mass number 14 is 14C or C14> compare ATOMIC MASS * * * (A) the number of nucleons (protons plus… …   Medical dictionary

• mass number — n. Physics Chem. the number of neutrons and protons in the nucleus of an atom: the approximate mass of a given nucleus is obtained by multiplying the mass number by the fundamental unit of mass, 1.6605 × 10 24 grams (1/ 12 the mass of C12 atom) …   English World dictionary

• mass number — ► NOUN Physics ▪ the total number of protons and neutrons in a nucleus …   English terms dictionary

• mass number — noun the sum of the number of neutrons and protons in an atomic nucleus • Syn: ↑nucleon number • Hypernyms: ↑mass unit * * * ˈmass number 7 [mass number] noun …   Useful english dictionary

• mass number — Physics. the integer nearest in value to the atomic weight of an atom and equal to the number of nucleons in the nucleus of the atom. Symbol: A [1920 25] * * * ▪ physics       in nuclear physics, the sum of the numbers of protons and neutrons… …   Universalium

• mass number — masės skaičius statusas T sritis Standartizacija ir metrologija apibrėžtis Atomo branduolio nukleonų skaičius. atitikmenys: angl. mass number; nuclear number; nucleon number vok. Massenzahl, f; Nukleonenzahl, f rus. массовое число, n; число… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

• mass number — masės skaičius statusas T sritis chemija apibrėžtis Atomo branduolio nukleonų skaičius. atitikmenys: angl. mass number; nuclear number; nucleon number rus. массовое число …   Chemijos terminų aiškinamasis žodynas

• mass number — masės skaičius statusas T sritis fizika atitikmenys: angl. mass number; nuclear number; nucleon number vok. Massenzahl, f; Massezahl, f; Nukleonenzahl, f rus. массовое число, n pranc. nombre de masse, m; nombre de nucléons, m …   Fizikos terminų žodynas

• mass number — mass′ num ber n. phs the number of nucleons in an atomic or isotopic nucleus Symbol: A II, 8) • Etymology: 1920–25 …   From formal English to slang

• mass number — noun Date: 1923 an integer that approximates the mass of an isotope and designates the number of nucleons in the nucleus …   New Collegiate Dictionary