Every atom of an element has the same number of protons. This is the atomic number, also called the proton number. The total number of protons and neutrons in the nucleus is the mass number, also called the nucleon number. Protons and neutrons are both nucleons.
A nucleus is written as a symbol with the mass number at the top left and the atomic number at the bottom left. For example, carbon-13 is written 136C. Its mass number is 13 and its atomic number is 6.
To find the number of neutrons, subtract the atomic number from the mass number. For carbon-13 this is 13 − 6 = 7 neutrons.
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Carbon-12 and carbon-13 are isotopes of carbon: both have 6 protons, but carbon-12 has 6 neutrons and carbon-13 has 7.
