The Molecular Formula for Water

It shows 1 oxygen atom and 2 hydrogen atoms

This is the three-dimensional molecular structure of water.
This is the three-dimensional molecular structure of water. Laguna Design / Getty Images

The molecular formula for water is H2O. One molecule of water consists of one oxygen atom covalently bonded to two hydrogen atoms.

There are three isotopes of hydrogen. The usual chemical formula for water assumes the hydrogen atoms consist of the isotope protium (one proton, no neutrons). Heavy water is also possible, in which one or more of the atoms of hydrogen consist of deuterium (symbol D) or tritium (symbol T). Other forms of the chemical formula for water include: D2O, DHO, T2O, and THO. It's theoretically possible to form TDO, although such a molecule would be extremely rare.

Although most people assume water is H2O, only completely pure water lacks other elements and ions. Drinking water usually contains chlorine, silicates, magnesium, calcium, aluminum, sodium, and trace amounts of other ions and molecules.

Also, water dissolves itself, forming its ions, H+ and OH-. A sample of water contains the intact water molecule along with hydrogen cations and hydroxide anions.