Atomic Mass of Sodium

Sodium is mononuclidic: every stable sodium atom on earth is ²³Na, with no isotopic mixture to average and therefore no natural variation to blur the value. That is why the table gives 22.98976928 g/mol — eleven significant figures, against carbon's five — and why the uncertainty is smaller than any chemist could ever exploit. Twenty-one other elements share this property, among them fluorine, aluminium, phosphorus and gold.

The symbol Na comes from natrium, the Latin for the natron that Egyptian embalmers used, and Humphry Davy isolated the metal in 1807 by electrolysing molten caustic soda — one of the first elements ever obtained by electricity. In water chemistry sodium is the counter-ion in nearly every reagent on the shelf, and the ion that an ion-exchange softener trades for the calcium and magnesium it removes, at roughly 8 mg of sodium added per mg-equivalent of hardness taken out.