The atoms of a chemical element can exist in different types that have very similar behavior, but weigh different amounts. These are called isotopes of the element. Atoms of the same element have the same number of protons, but different isotopes have different numbers of neutrons. Different isotopes of the same element have different masses because they have a different number of neutrons. Mass is the word for how much substance (or matter) something has. Things with different masses also have different weights. 