Occurrence

Where Dihydrogen Is Found

Hydrogen is the most abundant element in the universe, making up roughly 70% of its total mass, and it dominates the composition of the Sun and giant planets such as Jupiter and Saturn.

On Earth the picture is quite different. Because hydrogen is so light, free $H_2$ gas makes up only about 0.15% by mass of the atmosphere — most of what once existed has long since escaped into space. Combined with other elements, however, hydrogen is far more abundant: it accounts for about 15.4% of the mass of the Earth's crust and oceans together.

Most terrestrial hydrogen exists in combined form rather than as free gas:

• In water ($H_2O$), by far the largest reservoir …