Ratio of the average mass of a chemical element 's atoms to 112 the mass of an atom of the carbon-12 isotope .
The original standard of atomic weight, established in the 19th century, was hydrogen, with a value of 1. From с 1900 until 1961, the reference standard was oxygen, with a value of 16, and the unit of atomic mass was defined as 116 the mass of an oxygen atom. Oxygen, however, contains small amounts of two isotopes that are heavier than the most abundant one, and 16 is actually a weighted average of the masses of the three isotopes of oxygen. Therefore, the standard was changed to one based on carbon-12. The new scale required only minimal changes to the values that had been used for chemical atomic weights.