Mass Number
The total number of protons and neutrons in an atomic nucleus.
Formula / equation
A = Z + N
Unit
Dimensionless
In depth
Written as a superscript before the symbol, the mass number is always an integer because it simply counts nucleons. It is close to but never exactly equal to the isotopic mass in unified mass units, because nuclear binding energy converts a small fraction of the nucleon mass into energy.
Examples in the real world
Carbon-14 has A = 14 (6 protons, 8 neutrons); uranium-235 has A = 235 with 143 neutrons.