Homonuclear molecules (or species) (homo 'equal') are composed of only one element. They may consist of several atoms, depending on the properties of the element, and some of these may have several allotropes.
Noble gases are exceptional cases of molecules of a single atom (monatomic), because they rarely form bonds (chemical combinations). As corroboration of the similarity of properties of the elements, they make up group 18 (8A or VIIIA) of the periodic table. However, most homonuclear molecules are diatomic: hydrogen (H2), oxygen (O2), nitrogen (N2) and all halogens; however, not all diatomic molecules are homonuclear.
There are also homonuclear molecules with more than two atoms. For example, oxygen, in addition to its diatomic molecule, also constitutes the truncated homonuclear ozone molecule (O3). Likewise, there are tetratomic molecules: arsenic (As4) and phosphorus (P4). Sulfur has three homonuclear molecular variations: diatomic (S2), hexatomic (S6) and octatomic (S8), although the first two are rare. The carbon element is characterized by constituting several homonuclear molecules, among which the buckminsterfullerene or buckyball (C60) stands out.
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