Overview: Most people recognize that atoms are the fundamental building blocks of all matter around us. An atom itself is composed of protons, neutrons, and electrons. The simplest atom is the hydrogen atom because it consists of only one proton and one electron. If a neutron is added to the nucleus, the atom is still hydrogen, just a more massive version. Atoms of the same element (i.e. those with the same number of protons) but different numbers of neutrons are called isotopes.