Hundreds of diatomic molecules

Other elements that can form two atom molecules are phosphorus (P2) and sulfur (S2) although neither of these molecules are stable in atmospheric conditions.

The noble gases do not form diatomic molecules: this can be explained using molecular orbital theory (see molecular orbital diagram).

Compounds[edit]

All other diatomic molecules are chemical compounds of two elements, for example carbon monoxide (CO). Many elements and chemical compounds aside from these form diatomic molecules when evaporated.

Occurrence[edit]

Hundreds of diatomic molecules have been characterized[3] in the terrestrial environment, laboratory, and interstellar medium. About 99% of the Earth's atmosphere is composed of two diatomic molecules: oxygen (21%) and nitrogen (78%). The natural abundance of hydrogen (H2) in the Earth's atmosphere is only on the order of parts per million, but H2 is, in fact, the most abundant diatomic molecule in nature. The interstellar medium is, indeed, dominated by hydrogen atoms.

If a diatomic molecule consists of two atoms of the same element, such as H2 and O2, then it is said to be homonuclear, but otherwise it is heteronuclear. The bond in a homonuclear diatomic molecule is non-polar. In most diatomic molecules, the elements are nonidentical. Prominent examples include carbon monoxide, nitric oxide, and hydrogen chloride, but other important examples include gaseous MgO, SiO, and many other species not normally considered diatomic because they polymerize near room temperature. All halogens are diatomic.

Elements that consist of diatomic molecules, under typical laboratory conditions of 1 bar and 25 °C, include hydrogen (H2), nitrogen (N2), oxygen (O2), and the halogens (although it is not yet known whether astatine forms diatomic astatine molecules[4]).[5] Other elements form homonuclear diatomics when evaporated, but these diatomic species repolymerize at lower temperatures. For example, heating ("cracking") elemental phosphorus gives diphosphorus, P2.

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