Atomic Mass of Hydrogen

Hydrogen is the lightest element and, at 1.0080 g/mol, was the natural zero point for the first atomic-weight tables: Dalton set it at 1 in 1803 and Prout speculated in 1815 that every other element was a whole-number multiple of it. Prout was almost right and wrong for an interesting reason — the whole numbers are there, but they belong to individual isotopes, and the fractional averages come from mixing them.

The stated value is an abridged standard atomic weight, because natural hydrogen is a variable mixture of ¹H (1.0078 u) and deuterium (2.0141 u, about 156 ppm) whose ratio depends on the source. IUPAC therefore publishes an interval, 1.00784 to 1.00811, rather than a single figure. That variability is a tool rather than a nuisance: deuterium ratios in ice cores and groundwater record the temperature at which the water last evaporated, which is how palaeoclimate records are read.