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In chemistry, a diatomic element is one that, when not chemically bonded with any other elements, will form a molecule having two atoms of the element, such as H2 or O2. Diatomic molecules rarely exist in their atomic form. They include most of the molecular elements or polyatomic molecules.

Earth's atmosphere is comprised almost completely (99%) of diatomic oxygen and diatomic nitrogen.

The seven diatomic elements are hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine. These elements, with the exception of hydrogen, when located on the periodic table, form a numeral "7".