By Doug Bennett (Updated August 30, 2022)
While atoms are the smallest units of matter with consistent properties, the true foundation of the physical world lies in particles even tinier than atoms—subatomic particles. These fundamental constituents—including protons, neutrons, electrons, quarks, and radioactive alpha and beta particles—compose and sometimes dismantle the matter around us.
Discovered by Ernest Rutherford in 1919, protons reside in atomic nuclei and carry a positive electric charge. Each proton’s mass is roughly one atomic mass unit (amu), and together with neutrons, they account for most of an atom’s mass. The number of protons defines an element’s atomic number.
James Chadwick identified neutrons in 1932. Like protons, neutrons are about one amu in mass but are electrically neutral. Variations in neutron count give rise to isotopes of a single element.
Sir J.J. Thomson first observed electrons in 1897. Electrons orbit the nucleus in an electron cloud, possessing a mass 1,840 times smaller than that of a proton and a negative charge. Electrons drive chemical bonding through loss, gain, or sharing with other atoms.
Alpha particles consist of two protons and two neutrons—the nucleus of a helium atom. They are emitted during alpha decay of heavy, unstable nuclei. Though low‑energy and short‑range, alpha particles can be highly damaging to biological tissue upon contact.
Beta particles are high‑energy electrons (β⁻) or positrons (β⁺) produced by beta decay. They travel at speeds close to light, penetrating up to 100 times deeper into materials than alpha particles.
Quarks are the smallest known subatomic entities and constitute the true elementary particles of matter. Six flavors—up, down, charm, strange, top, bottom—exist, each with one of three “color” charges (red, blue, green). Protons are made of two up and one down quark; neutrons comprise two down and one up quark.
For more detailed scientific background, see the Wikipedia entry on particles and the NIST definition of the atomic mass unit.
