By Drew Lichtenstein, Updated Aug 30, 2022
An element is a substance composed entirely of one type of atom. The periodic table catalogs every known atom, but the atom itself is not the smallest known particle. Instead, each atom consists of three fundamental parts: electrons, protons, and neutrons. Protons and neutrons are further made of even smaller constituents called quarks.
Electrons are elementary particles; no known substructure exists. They carry a negative charge and determine an atom’s electrical properties. A neutral atom contains equal numbers of electrons and protons, while adding or removing electrons creates ions—positively or negatively charged versions of the same element. Electrons occupy orbitals around the nucleus and are the particles that bond with other atoms to form molecules.
Protons define the identity of an element. The number of protons in an atom’s nucleus—its atomic number—determines the element’s position on the periodic table (hydrogen has one, carbon six, etc.). Protons are massive compared to electrons and, together with neutrons, give an atom its mass. They are found exclusively in the nucleus.
Neutrons have a mass similar to protons but carry no electric charge. While altering the number of electrons does not change the element itself, varying the neutron count produces different isotopes—variants of the same element with distinct mass numbers. Some isotopes are unstable and decay, emitting radiation and releasing energy.
Protons and neutrons are not fundamental; they are composed of quarks—six types discovered in 1961: up, down, charm, strange, bottom, and top. A baryon is formed when three quarks bind together, which is why protons and neutrons are classified as baryons. Quark–antiquark pairs form mesons, but these particles are extremely short‑lived, lasting only fractions of a millisecond.
