Most of us have heard, in one place or another, that the chemical formula for water is H2O. In other words, a molecule of water is made of two hydrogen atoms and one oxygen atom. But for a long time in the nineteenth century, chemists thought the formula was HO: one atom of hydrogen and one atom of oxygen in each water molecule.

That wrong formula arose from the work of the nineteenth-century English chemist John Dalton, who published A New System of Chemical Philosophy in 1842. 

Dalton's reasoning went like this. He was convinced that matter was made of atoms of different kinds: hydrogen was made of hydrogen atoms, oxygen was made of oxygen atoms, and so on. Dalton was also convinced that atoms could hook up in different combinations to make molecules, and that's why there are so many different chemical substances in the world. 

In Dalton's time, chemists knew of only one substance that could be made from hydrogen and oxygen: water. Dalton figured, therefore, that a molecule of water was made of one atom of hydrogen and one atom of oxygen, because that was the simplest possible combination. The formula must be HO.

Dalton's reasoning was sound, but he was missing some important facts. He didn't know -- no one knew in 1842 -- that water is not the only chemical that can form from hydrogen and oxygen. Dalton's mistake was in assuming that hydrogen and oxygen would combine in only one simple way.

John Dalton is one of the great figures in the history of science. His work in the early 1800's provided important evidence about the nature of atoms and how they combine. But because certain laboratory discoveries about hydrogen and oxygen had not yet been made in his time, Dalton came up with the wrong formula for water.

This moment of science comes from Indiana University, with production support from the Office of the Provost. 

I'm Yaël Ksander.