Hydronium ion (H₃O⁺)
- Charge
- +1
- Atoms
- H × 3, O
- Atoms in total
- 4
- Mass of the ion
- 19.023 g/mol
Hydronium is a water molecule that has picked up an extra hydrogen ion: one oxygen atom bonded to three hydrogen atoms, with a 1+ charge. The added proton brings a positive charge but no electrons, so the group ends up one electron short. Every acid dissolved in water produces it, and the pH scale is a measure of how much of it a solution contains.
- Chemists often write H⁺(aq) as shorthand, but a bare proton in water always attaches to water molecules, so H₃O⁺ is the more realistic picture.
- Pure water makes tiny, equal amounts of hydronium and hydroxide, OH⁻, as molecules pass protons to each other: 2H₂O ⇌ H₃O⁺ + OH⁻.
- Some acids form solid hydrates that are really hydronium salts, such as hydronium perchlorate, H₃OClO₄, the crystal form of perchloric acid monohydrate.