What is an isotope?

Isotopes are different forms of the same element by mass. While an atom is classified as a particular element based on the number of protons in the nucleus, the specific isotope of that element is determined by the number of neutrons in the nucleus.

For example:

Chlorine is reported as having a relative atomic mass of 35.5. This is because chlorine actually has 2 isotopes, Cl³⁵ and Cl³⁷. The figure of 35.5 is an average of these two, weighted by natural abundance.

Both of these isotopes have 17 protons, which defines them as chlorine atoms, but the Cl³⁵ has 22 neutrons, whereas the Cl³⁷ has 24.