Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron. It turns out that protons and neutrons are made of varieties of a still smaller particle called the quark. At this time it appears that the two basic constituents of matter are the lepton (of which the electron is one type) and quark; there are believed to be six types of each. Each type of lepton and quark also has a corresponding antiparticle: a particle that has the same mass but opposite electrical charge and magnetic moment. An isolated quark has never been found—quarks appear to almost always be found in pairs or triplets with other quarks and antiquarks (the resulting particles being classed as hadrons, more than 200 of which have been identified). Two theoretically predicted five-quark particles, called pentaquarks, have been produced in the laboratory. Four- and six-quark particles are also predicted but have not been found.
The most familiar lepton is the electron; the other five leptons are the muon, the tau particle, and the three types of neutrino associated with each: the electron neutrino, the muon neutrino, and the tau neutrino. The six quarks have been whimsically named up, down, charm, strange, top (or truth), and bottom (or beauty); the top quark, which has a mass greater than an entire atom of gold, is about 35 times heavier than the next biggest quark and may be the heaviest particle nature has ever created. The quarks found in ordinary matter are the up and down quarks, from which protons and neutrons are made. A proton, for instance, consists of two up quarks and a down quark, and a neutron consists of two down quarks and an up quark. The pentaquark consists of two up quarks, two down quarks, and the strange antiquark. (Quarks have fractional charges of one third or two thirds of the basic charge of the electron or proton.)
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.