5th century BC: Greek philosopher Democritus suggests the Universe consists of empty space and of invisible and indivisible particles called atoms.
1802: John Dalton, a Quaker-educated English physicist and chemist, lays groundwork of modern theory of the elements and the atom.
1897: Electron discovered by Britain's Joseph Thomson, who later proposes a "plum pudding" model of the atom. He suggests the atom is a slightly positive sphere with raisin-like electrons inside that have a negative charge.
1899-1919: New Zealand physicist Ernest Rutherford identifies atomic nucleus, the proton and alpha and beta particles.
1920s: Advances in quantum theory, about the behaviour of matter at the atomic level.
1932: Neutron, similar to the proton but with no electrical charge, is discovered by James Chadwick of Britain. The first antiparticle, the positron (the mirror particle to the electron), is discovered by American Carl Anderson.
1934: Italy's Enrico Fermi postulates the existence of the neutrino (Italian for "little neutral one"), a neutral-charge partner to the electron. Theory is confirmed in 1959.
1950s: Invention of particle accelerator leads to surge in discoveries of sub-atomic particles.
1964:
- British physicist Peter Higgs postulates existence of a particle, later known as the Higgs Boson, that provides mass to otherwise massless particles.
- Murray Gell-Mann and George Zweig of the United States propose that protons and neutrons are comprised of quarks.
1974: Development of the "Standard Model," a theory that everything in the Universe comprises 12 building blocks divided into two families, leptons and quarks, and these are governed by four fundamental forces.
1977-2000: Flurry of discoveries that strengthens Standard Model hypothesis, including the existence of bottom and top quarks, tau lepton, gluon, tau neutrino and the W and Z bosons which help carry the "weak" force.
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