Isotopes

Atoms belong to the same element if their nuclei contain the same number of protons. However, atoms of that element can have different numbers of neutrons. These different forms of the same element are called isotopes.

The nucleon number is the total number of protons and neutrons in a nucleus. Isotopes of an element have the same proton number but different neutron numbers, so they also have different nucleon numbers.

For example, carbon-12 and carbon-14 are isotopes of carbon. Both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8. Their nucleon numbers are therefore 12 and 14, respectively.