The dalton (symbol Da) is the immediate predecessor of the atomic mass unit. It is named after John Dalton in recognition of his achievement in introducing the atomic hypothesis into chemistry. Whereas the unified atomic mass unit is defined as 1/12 of the mass of a single atom of 12C, the dalton was defined as 1/16 of the mass of an atom of oxygen (which turned out to be a mixture of 16O, 17O and 18O in a 3150:1:5 ratio with mean atomic mass number of 16,0035).