All matter is composed of elements, and all elements are composed of atoms.
The atom contains a nucleus, consisting of protons and neutrons, with electrons revolving around the nucleus. In the neutral atom, the number of electrons equals the number of protons. Isotopes Each individual arrangement of protons and neutrons is called a nuclide. Nuclides that do not have the same number of protons and neutrons are called isotopes. All isotopes of a particular element have almost identical chemical properties.