Scientists used to think that all atoms of a particular element were identical. We have discovered that some elements have two or three different types of atom.

They will all have the same number of protons but they could have a different number of neutrons.

e.g. some atoms of carbon have 6 neutrons, while other atoms of carbon have 7 neutrons, and others have 8 neutrons.

We say these atoms are isotopes of each other.

Isotopes are atoms that have the same atomic number but a different mass number