Diatomic Molecule - Occurrence

Occurrence

Hundreds of diatomic molecules have been characterized in the terrestrial environment, laboratory, and interstellar medium. About 99% of the Earth's atmosphere is composed of diatomic molecules, specifically oxygen and nitrogen at 21% and 78%, respectively. 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). Other elements form homonuclear diatomics when evaporated, but these diatomic species repolymerize at lower temperatures. For example, heating ("cracking") elemental phosphorus gives diphosphorus, P₂.