Atom between atoms. By this mechanism, atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals. By definition, any two atoms with an identical number of "protons" in their nuclei belong to the same chemical element. Atoms with equal numbers of protons but a different number of "neutrons" are different isotopes of the same element. For example, all hydrogen atoms admit exactly one proton, but isotopes exist with no neutrons (hydrogen-1, by far the most common form, also called protium), one neutron (deuterium), two neutrons (tritium) and more than two neutrons. The
Atomic number Atomic number The atomic number or proton number (symbol "Z") of a chemical element is the number of protons found in the nucleus of an atom. It is identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons. The sum of the atomic number "Z" and the number of neutrons, "N", gives the mass number "A" of an atom. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes)