Notation (Also seen in GCSE Physics 2)

We can represent an atom of element X in the following way:

Z is the proton number. This is the number of protons in the nucleus. In an uncharged atom the number of electrons orbiting the nucleus is equal to the number of protons.

In Chemistry it is called the atomic number

A is the nucleon number. This is the total number of nucleons in the nucleus (protons + neutrons) which can be written as A = Z + N.

In Chemistry it is called the atomic mass number

N is the neutron number. This is the number of neutrons in the nucleus.