The number of protons in the nucleus of an atom is known as the atomic number, which determines the chemical element to which the atom belongs. All atoms of a given element are not necessarily identical. For example, elements may have isotopes, which are atoms of the same element with different numbers of neutrons. The number of possible isotopes is not connected to the number of protons in an atom. The number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number. The nuclear force is a force that acts between the protons and neutrons of atoms.