Seventeen particles, 61 variations: the Standard Model's list of basic ingredients
Atoms were named for being uncuttable, and they turned out not to be. Today's Standard Model lists seventeen particles that, as far as anyone can tell, have no smaller parts: twelve fermions and five bosons. Counting colours, flavours and antimatter twins, that becomes 61, and even this tidy list leaves out gravity.
Atoms themselves were controversial until 1905, when Albert Einstein's paper on Brownian motion settled that molecules were real rather than mathematical conveniences. Their inner parts followed: first the electron late in the 19th century, then the proton in 1919, with the photon identified during the 1920s and finally, in 1932, the neutron. Protons and neutrons later proved to be composites, built from quarks. Whether something counts as elementary is not measured directly; it depends on the theory in use.
Matter particles are fermions, arranged in three generations of four. Six are leptons, such as the electron and three neutrinos, which carry neither electric nor colour charge. The other six are quarks, which carry fractional electric charge and are never seen alone. The force between them, carried by gluons, strengthens as quarks are pulled apart, unlike electromagnetism, which weakens with distance. So quarks appear only in threes, as in protons, or as quark-antiquark pairs. Evidence for them came from firing electrons at protons: above a certain energy some bounced off at sharp angles, revealing smaller charged pieces inside.
Bosons carry forces. Photons handle electromagnetism, eight kinds of gluon the strong force, and the W and Z bosons the weak force, which lets a neutron turn into a proton. The Higgs boson gives particles their intrinsic mass; CERN announced its observation on 4 July 2012, with Peter Higgs in the audience and a statistical confidence of 5 sigma.
The model is extremely successful yet provisional. It omits gravity, contains unexplained parameters, and may not fit with general relativity. Supersymmetry, which would give every particle a heavier shadow partner, and the graviton both remain undiscovered as of 2026.
Source: Elementary particle
