Under standard conditions, hydrogen is a gas of diatomic molecules with the formula H2, called dihydrogen, or sometimes hydrogen gas, molecular hydrogen, or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly combustible.
One molecule of hydrogen dissociates into two atoms (H 2 → 2H) when an energy equal to or greater than the dissociation energy (i.e., the amount of energy required to break the bond that holds together the atoms in the molecule) is supplied.