What is the Difference Between Atomic and Mass Numbers?

Mass number

The atomic mass number is represented by the symbol (letter) ‘A’. This is not to be confused with the relative atomic mass Ar.

The mass number gives the integral number of nucleons, protons and neutrons found in the nucleus of an atom.

The relative mass is a value that is not necessarily integral that compares a mass to the mass of a carbon isotope, assigned a value of exactly 12.0000 units.


Atomic number

This is represented by the symbol (letter) ‘Z’. It shows us the number of protons in an atom (and the number of electrons in a neutral atom.)

Example: How many protons and electrons does an atom of iron contain?

The atomic number of iron is 26 therefore it contains 26 protons

The number of electrons = number of protons, therefore there are 26 electrons