Simply put, a diatomic molecule is one that consists of two atoms. Most diatomic molecules are of the same element though a few combine different elements. At room temperature, virtually all diatomic molecules are gases. Interestingly, some substances that have crystalline or other atomic arrangements at room temperature become diatomic at higher temperatures.
TL;DR (Too Long; Didn't Read)
A diatomic molecule has two atoms. The diatomic elements are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine.
The elements that form two-atom molecules at room temperature are hydrogen, nitrogen, oxygen and the halogens fluorine, chlorine, bromine and iodine. Chemists call these molecules “homonuclear” referring to the fact that both atoms have the same nuclear structure. Nitrogen stands out because its atoms share a strong triple bond, making it a very stable substance. The noble gases, such as helium and neon, rarely form molecules at all; they are monatomic.
Other elements have a metallic nature; at standard temperature and pressure most of them form crystalline solids, and the atoms share electrons freely. These elements do not form molecules with themselves or other metallic elements. While they do form molecules with nonmetals, such as cupric chloride or ferric oxide, many of these molecules have more than two atoms. The remaining metal-nonmetal compounds are ionic and also not diatomic under standard conditions.
A few compounds such as carbon monoxide, hydrogen chloride and nitric oxide have diatomic molecules. Like the diatomic elements, these compounds are gases at room temperature. Chemists call these compounds “heteronuclear” because their atomic nuclei come from different elements.
Diatomic Molecules and High Temperatures
At room temperature, the element lithium is a solid and does not form diatomic molecules. However, if you heat it up enough such that it becomes a gas, the gas phase is a diatomic molecule. Chemists use the prefix “di-“ to distinguish and characterize substances such as this, for example, they use the term, dilithium. No, this is not the science-fictional “Star Trek” antimatter fuel, this is an actual form of lithium. Other elements that also form diatomic molecular gases include sulfur as disulfur, tungsten as ditungsten, and carbon as dicarbon. Similarly, ionic compounds such as sodium chloride that aren’t diatomic at normal temperatures can become diatomic molecules when turned to a gas.
Diatomic Molecules and Low Temperatures
Oxygen, nitrogen and the other diatomic molecules that are gases at room temperature remain diatomic at temperatures low enough to turn them to liquids. Forces weaker than atomic bonds that attract neighboring molecules allow them to enter the liquid state when low temperatures slow the molecules down sufficiently.