Atomic Mass of Oxygen

Oxygen was the reference for chemical atomic weights for more than a century, set at exactly 16 because it forms compounds with nearly everything and was easy to weigh accurately. The trouble surfaced in 1929 when Giauque and Johnston found ¹⁷O and ¹⁸O in the atmosphere: physicists then defined their scale on ¹⁶O alone while chemists kept the natural mixture, leaving the two scales 275 ppm apart. Carbon-12 was adopted in 1961 to settle it.

The modern value, 15.999 g/mol, reflects 99.76 % ¹⁶O with small amounts of ¹⁷O and ¹⁸O. Their ratio varies with evaporation and condensation history, which is why δ¹⁸O in foraminifera shells and polar ice is the backbone of the palaeotemperature record stretching back 800000 years. Oxygen also happens to be the most abundant element in the Earth's crust by mass, at about 46 %.