Ho Ho Ho! When Water Was Diatomic

Chemical Education

Abstract

When he introduced the concept of atomic weights at the beginning of the 19^th Century CE, John Dalton assumed that water had the formula HO. This assumption resulted in a half century of confusion – partly because on a scale of H = 1, he defined the atomic weights O = 8 and C = 6, and partly because elements that could exhibit variable valency appeared to possess different atomic (or rather equivalent) weights. The correction of the formula of water, together with the recognition of the diatomic nature of the gases hydrogen and oxygen, were formalized following the Karlsruhe congress of 1861 and allowed the establishment of the "modern" and consistent atomic weights.