Molecules are the smallest particles into which a substance can be divided while maintaining the physical properties of the substance. Each type of molecule corresponds to a specific chemical compound. The CAS Substance Databases contain information on about 23 million compounds. Molecules are composites of one or more atoms.
Atoms are the smallest neutral particles into which matter can be divided by chemical reactions. An atom consists of a small, heavy nucleus surrounded by a relatively large, light cloud of electrons. Each type of atom corresponds to a specific chemical element, of which 110 have been named. Refer to the periodic table for an overview.
Atomic nuclei consist of protons and neutrons. Each type of nucleus is called a nuclide. Nuclear reactions can change one nuclide into another. The Table of Nuclides at KAERI contains information on more than 3000 nuclides.
Hadrons are built from quarks and/or antiquarks, tightly bound by the strong nuclear force. Hadrons are further classified by their quark content.
Baryons contain three quarks each, and are classified by their appearance in atomic nuclei.
Hyperons such as the Δ, Λ, Ξ and Ω particles are generally short-lived and heavier than nucleons. They do not normally appear in atomic nuclei.
Mesons are built from a quark and an antiquark, and include the pions, the kaons and many other types of mesons. The strong force between the protons and neutrons in an atomic nucleus is mediated by mesons.
Exotic baryons have been discovered only recently.
Tetraquark particles consist of two quarks and two antiquarks.
Pentaquark particles consist of four quarks and one antiquark.
Fermions have half-integral spin; for all known elementary particles this is 1/2. Each fermion has its own distinct antiparticle. Fermions are the basic building blocks of all matter. They are classified according to whether they feel the strong nuclear force or not.
Quarks interact strongly. They exist in six flavors: up, down, strange, charm, top and bottom.
Leptons do not interact strongly. They also exist in six flavors: electron, muon, tauon, electron-neutrino, muon-neutrino and tauon-neutrino.
The gravitino (spin 3/2) is the superpartner of the graviton in supergravity theories. As yet the gravitino is entirely hypothetical.
Bosons have integral spin (0, 1, or 2). These particles are their own anti-particle. Bosons mediate the fundamental forces of nature, and are classified accordingly.
