Final answer:
An element is a pure substance made of atoms with the same number of protons. Atoms of the same element can have different neutrons, known as isotopes, but share the same chemical properties.
Step-by-step explanation:
An element or pure substance consists only of atoms with the same number of protons. This defines the identity of the element. For example, all atoms of helium have two protons, and no other elements have atoms with two protons. Atoms of the same element that have different numbers of neutrons are known as isotopes, but they still retain the same chemistry due to having the same number of protons and electrons.