H2 is the symbol for a molecule containing two hydrogen atoms. Hydrogen is known as a diatomic element because it does not normally exist as a single atom.
A single hydrogen atom, also known as atomic hydrogen, has one unpaired electron. This makes the atom unstable. When two hydrogen atoms bond together, they share electrons. Forming a covalent bond stabilizes the atoms and creates a molecule of diatomic molecular hydrogen. Hydrogen forms ionic bonds with other elements, such as chlorine.
There are six other diatomic elements: bromine, chlorine, oxygen, nitrogen, fluorine and iodine. All of these diatomic elements are gases.