The seven diatomic elements are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. According to Princeton, diatomic molecules are molecules composed of only two atoms of either the same or different chemical elements.
Five of the diatomic elements occur naturally as a gas. These include hydrogen, nitrogen, oxygen, fluorine and chlorine. Bromine is liquid and iodine is solid in the natural state.
Diatomic elements never exist as single atoms. They always occur in pairs or combine in a compound with another element. If a diatomic element occurs in a compound, it is considered to be heteronuclear. A diatomic molecule that contains two of the same element is homonuclear.
How diatomic molecules bond depends on whether the molecule is homonuclear or heteronuclear. Homonuclear molecules have covalent bonds in which they share electrons. These electrons are shared equally so there is no difference in electronegativity. Heteronuclear diatomic molecules have polar covalent bonds in which there is a difference in electronegativity.
Diatomic molecules are very common. Approximately 99 percent of Earth's atmosphere is made of the two diatomic molecules nitrogen and oxygen. In 1805, Gay-Lussac and von Humboldt determined that water is formed from the diatomic elements hydrogen and oxygen. Carbon monoxide, nitric oxide and hydrogen chloride are other prominent examples of diatomic molecules.