The number of protons is what defines the element. Oxygen, for example, has eight protons. However, the number of neutrons in an atom can vary, leading to different forms of elements, called isotopes.

Image of the emission of 4 neutrons (blue spheres) from the exotic nucleus oxygen-28, which consists of 8 protons (red spheres) and 20 neutrons.

Elements - the basic building blocks of chemistry, like hydrogen, oxygen, and uranium - are made up of single atoms. Atoms in turn, are made of a nucleus containing protons and neutrons ...