Occurrence

In its free (uncombined) state, hydrogen occurs as dihydrogen gas, H2. Hydrogen is the single most abundant element in the entire universe, making up roughly 70% of the universe's total mass, and it is also the principal element of the Sun and the rest of the solar system. On Earth itself hydrogen is comparatively much less abundant: measured by mass it is only the tenth most abundant element, though measured by number of atoms (rather than mass) it rises to the third most abundant element on Earth — a reminder that hydrogen atoms are individually ve …