Hydrogen (H) is a chemical element with atomic number 1 (one single proton in its nucleus). Together with oxygen, hydrogen constitutes water (H₂O). Hydrogen is the most abundant element in the universe, with the vast majority of all atoms being hydrogen atoms. It is estimated that there's somewhere between one quinvigintillion (10⁷⁸) and one sesvigintillion (10⁸¹) hydrogen atoms in the whole universe.

The most common hydrogen isotope is ¹H - no neutrons, just a single proton and a single electron.