A diatomic molecule is a chemical species made up of exactly two atoms bonded together. This structural grouping classifies molecules based purely on their atomic count. The atoms forming the molecule can be from the same element, such as two oxygen atoms, or from two different elements, like hydrogen and chlorine.
The Meaning of Diatomic
The term “diatomic” is derived from Greek origins, where the prefix “di-” means two and “atomic” refers to atoms. This classification limits the atomic count to exactly two.
This two-atom structure contrasts with monatomic substances, which consist of single, unbonded atoms (such as the noble gases neon or argon). Diatomic molecules are also structurally different from polyatomic molecules, which contain three or more atoms. Examples of polyatomic molecules include ozone (\(\text{O}_3\)) or water (\(\text{H}_2\text{O}\)).
Diatomic molecules can be homonuclear, meaning both atoms are the same element, or heteronuclear, meaning the two atoms are different elements, like in carbon monoxide (CO).
Elements That Form Diatomic Molecules
A select group of elements naturally exist as diatomic molecules under standard temperature and pressure conditions because this paired state is more stable than the single-atom form. These elements are all nonmetals and are commonly referred to as the seven diatomic elements. The list includes Hydrogen (\(\text{H}_2\)), Nitrogen (\(\text{N}_2\)), Oxygen (\(\text{O}_2\)), Fluorine (\(\text{F}_2\)), Chlorine (\(\text{Cl}_2\)), Bromine (\(\text{Br}_2\)), and Iodine (\(\text{I}_2\)).
The atmosphere contains large quantities of two of these diatomic elements, with nitrogen gas (\(\text{N}_2\)) making up approximately 78% and oxygen gas (\(\text{O}_2\)) making up about 21%. The halogens—Fluorine, Chlorine, Bromine, and Iodine—also exist as diatomic molecules, though their physical states vary; for example, bromine is a liquid, while iodine is a solid.
These diatomic elements are rarely found as isolated single atoms because they are highly reactive and unstable in that form.
The Chemistry Behind Diatomic Bonds
The formation of these molecules is driven by chemical stability. Atoms attempt to achieve a full outer electron shell, matching the electron configuration of the stable noble gases. For most elements, this means having eight valence electrons, a concept known as the octet rule.
To satisfy this requirement, the two atoms in a diatomic molecule share their valence electrons through covalent bonding. In a covalent bond, electrons are shared between the two atomic nuclei. This sharing allows each atom to effectively count the shared electrons toward its own octet, creating a more stable molecular state.
For example, a single oxygen atom needs two electrons to complete its octet. When two oxygen atoms bond to form \(\text{O}_2\), they share two pairs of electrons, forming a double bond. Hydrogen is an exception to the octet rule, following the duplet rule, needing only two electrons to fill its outer shell, which it achieves by sharing one pair of electrons with another hydrogen atom.