Oxygen isotopes and atomic weights

It was discovered that oxygen atoms had three isotopes: oxygen-16, oxygen-17, and oxygen-18. This finding led to the selection of carbon-12 as the standard for atomic weights due to the variability in oxygen's isotopic composition.
 William Giauque United States 1929 Oxygen isotopes and atomic weights

It was discovered that oxygen atoms had three isotopes: oxygen-16, oxygen-17, and oxygen-18. This finding led to the selection of carbon-12 as the standard for atomic weights due to the variability in oxygen's isotopic composition.