Mass Number
Definition and meaning of Mass Number in chemistry.
Mass number is the total count of protons and neutrons in an atom's nucleus. These particles together are called nucleons. Chemists use the symbol A for mass number, and it helps identify specific isotopes of an element.
In more detail
A nucleus packs protons and neutrons tightly together at its center. The mass number equals the atomic number (Z), which counts protons, plus the number of neutrons (N). This gives the formula A = Z + N. Isotopes of the same element always share the same atomic number, since that defines the element, but they can have different numbers of neutrons.
This means isotopes of one element have different mass numbers. Mass number is always a whole number, since it counts particles. However, an atom's actual measured mass, given in atomic mass units (amu), is not exactly equal to its mass number.
This small difference is called the mass defect. It comes from the nuclear binding energy holding protons and neutrons together, measured relative to carbon-12, the reference standard for the atomic mass unit. Depending on the nuclide, the true atomic mass can be slightly higher or slightly lower than the whole-number mass number.
Mass number should not be confused with atomic number, which only counts protons and defines which element an atom is. Changing an atom's mass number by adding or removing neutrons produces a different isotope, but it never changes what element the atom is.
Key facts
| Symbol | A |
|---|---|
| Formula | A = Z + N (protons plus neutrons) |
| Field | General Chemistry |
| Notation example | carbon-14, written as C-14 or 14C |
| Always a whole number | Yes, unlike atomic mass |
Carbon always has 6 protons, since that defines it as carbon. Carbon-12 has 6 neutrons, giving a mass number of 12 (6 + 6). Carbon-14 has 8 neutrons, giving a mass number of 14 (6 + 8). Both isotopes behave the same chemically, but carbon-14 is radioactive while carbon-12 is stable.
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 close to, but not exactly equal to, the mass number due to the mass defect from nuclear binding energy.
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 despite sharing the same atomic number.
Does mass number appear on the periodic table?
No. The periodic table lists atomic mass, a weighted average of all naturally occurring isotopes of an element. Mass number applies to one specific isotope and is always a whole number.