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What Is the Mass Number of an Atom?

The mass number of a nuclide is the sum of the number of protons and the number of neutrons in the atomic nucleus (the number of protons is the atomic number). The mass number is noted as a superscript for the symbol, for example, 206 Pb. The notation Pb-206 is also used.

What is the mass number of an atom?

The atomic mass of a nuclide (nuclide mass), minus the mass number, is called a mass surplus. It is between -0.1 and 0.22. If the nucleus mass is rounded to an integer, the mass number is obtained.

The (relative) atomic mass of an element is a weighted average over the mass numbers of the isotopes. The weighting is based on the relative presence of isotopes in nature.

Variation of the Mass Number in Radioactive Decay

The different types of radioactive decay are characterized by their changes in the mass number and atomic number, according to the law of radioactive displacement of Fajanas and Soddy.

Beta decay is possible because different isobars have mass differences of the order of a few masses of electrons. If possible, a nuclide will undergo a beta decay in an adjacent isobar with less mass. In the absence of other decay modes, a cascade of beta decays ends in the isobar with the lowest atomic mass.

Another type of radioactive decay without change in the mass number is the emission of a gamma ray from a nuclear isomer or a metastable excited state of an atomic nucleus. Since all protons and neutrons remain in the nucleus without changes in this process, the mass number does not change either.

Mass Number and Mass of Nucleons

Since the unbound masses of neutrons and protons are approximately equal to 1 uma, as a first approximation, the value of the mass number can be considered equal in numerical terms to the value of the sum of the masses in uma of the neutrons and protons. Although these values ​​are almost identical numerically, it is not correct to express the mass number in terms of mass in a neutron + proton, since the mass number is a dimensionless number, while the mass of neutrons and protons is not a number no dimensions (in fact, it has the dimensions of a mass).

In addition, the mass number is always identified by a natural number (that is, a positive integer), while the mass of the nucleons in uma, which we can approximate to the atomic mass (the mass of the electrons being insignificant in comparison with the mass of the nucleons), is represented by a real number. For example, the atomic mass of 24 Mg is equal to approximately 23,985 amu, while its mass number is equal to 24.

Uma Units, Atomic Mass Unit

The unit of unified atomic mass, also called dalton (Da), is a technical unit of measurement for the atomic mass now obsolete: it is replaced by submultiples of the kilogram or multiples of electron volts.

Relative Atomic Mass of an Element

Nor should the mass number be confused with the standard atomic weight (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 atomic mass unit unified. The atomic weight is a real mass (relative fact, that is, a relation), while the mass number is a counted number (and, therefore, an integer).

This weighted average may be quite different from the almost integer values ​​for individual isotopic masses. In addition, the weighted average mass may be almost whole, but at the same time it does not correspond to the mass of any natural isotope.


Published: February 27, 2020
Last review: February 27, 2020