Mass Number
Definition and meaning of Mass Number in chemistry.
Mass number is the total number of protons and neutrons (nucleons) in an atom's nucleus. Represented by the symbol A, it is used to identify and distinguish between different isotopes of the same element.
In more detail
Every nucleus contains protons and neutrons packed tightly together. The mass number equals the atomic number (Z, the number of protons) plus the number of neutrons (N), expressed as the formula A = Z + N. Because isotopes of the same element have different numbers of neutrons while maintaining the same number of protons, they have different mass numbers. While the mass number is always a whole integer, the actual atomic mass (measured in atomic mass units) is not exactly equal to it. The small difference arises from the mass defect caused by nuclear binding energy: depending on how tightly bound a nuclide's nucleons are relative to carbon-12 (the reference standard for the atomic mass unit), the atomic mass can be slightly greater than or slightly less than the mass number.
Key facts
| Symbol | A |
|---|---|
| Formula | A = Z + N (protons plus neutrons) |
| Field | General Chemistry |
| Notation example | C-14 or 14C (superscript before element symbol) |
Carbon has 6 protons (atomic number 6). Carbon-12 contains 6 neutrons, giving a mass number of 12 (6 + 6 = 12). Carbon-14 contains 8 neutrons, giving a mass number of 14 (6 + 8 = 14). Both are isotopes of carbon with identical chemical properties but different mass numbers and nuclear stability.
Frequently asked questions
Is mass number the same as atomic mass?
No. Mass number is a whole number count of nucleons. Atomic mass is measured in atomic mass units (amu) and is very close to, but not exactly equal to, the mass number. The small difference comes from the mass defect due to nuclear binding energy, and depending on the nuclide it can make the atomic mass slightly greater than or slightly less than the mass number.
Why do isotopes of the same element have different mass numbers?
Isotopes have the same number of protons but different numbers of neutrons. Since mass number counts both protons and neutrons, isotopes of an element have different mass numbers even though they have identical atomic numbers.