Mass number: definition in chemistry
Mass number: The total number of protons and neutrons in an atom's nucleus, given the symbol A.
The mass number is always a whole number, and it identifies a particular isotope, as in carbon-14 or ²³⁵U. Subtracting the atomic number from the mass number gives the number of neutrons. It is close to, but not the same as, the atom's actual mass in atomic mass units, which is a measured value with decimals.
Examples
- Carbon-12 — 6 protons + 6 neutrons = mass number 12
- Sodium-23 has 11 protons and 12 neutrons
- Uranium-235 has 235 − 92 = 143 neutrons
- Written at upper left of the symbol, as in ³⁷Cl
On the periodic table
Most tables do not show mass numbers, since each element can have several isotopes; the atomic mass printed instead is an average of them.