The 92 naturally occurring chemical elements (90, in fact, because promethium and technetium are no longer found in their natural state on Earth) are composed of a nucleus of subatomic nucleons orbited by negatively charged electrons. Nucleons are positively charged protons and neutral neutrons. As an atom contains equal numbers of protons and electrons with equal but opposite charges, it carries no net electrical charge. The mass of a proton is 1836 times that of an electron. The chemical properties of elements are largely, although not entirely, determined by the way their outermost shells of electrons interact with other elements. Ions are formed when atoms capture surplus electrons to give negatively charged anions or when they shed electrons to give positively charged cations. An atom may form several types of ions. Iron, for example, forms both ferric (Fe3+) ions and ferrous (Fe2+) ions, while it also occurs in the Fe0 elemental form.

A nuclide is an atomic nucleus characterized by the number Z of its protons and the number N of its neutrons regardless of its cloud of electrons. The mass number A is the sum of the nucleons N + Z. Different interactions act in the nucleus and explain its binding: the short-range (nuclear) strong force, the electromagnetic force, and the mysterious weak force. Two nuclides with the same number Z of protons but different numbers N of neutrons will be accompanied by the same suite of electrons and so have very similar chemical properties; they will be isotopes of the same element.