Homonuclear molecules, or homonuclear species, are molecules composed of only one element. Homonuclear molecules may consist of various numbers of atoms, depending on the element's properties. Some elements form molecules of more than one size. The most familiar homonuclear molecules are diatomic, meaning they consist of two atoms, though not all diatomic molecules are homonuclear. Homonuclear diatomic molecules include hydrogen (H2), oxygen (O2), nitrogen (N2) and all of the halogens. Ozone (O3) is a common triatomic homonuclear molecule. Homonuclear tetratomic molecules include arsenic (As4) and phosphorus (P4).
Allotropes are different chemical forms of the same element (not containing any other element). In that sense, allotropes are all homonuclear. Many elements have multiple allotropic forms. In addition to the most common form of gaseous oxygen, O2, and ozone, there are other allotropes of oxygen. Sulfur forms several allotropes containing different numbers of sulfur atoms, including diatomic, triatomic, hexatomic and octatomic (S2, S3, S6, S8) forms, though the first three are rare. The element carbon is known to have a number of homonuclear molecules, including diamond and graphite.