Why does oxygen have an atomic mass of 15.9994?
Because 1- nucleons lose weight when they get together (that is how hydrogen fusing into helium release energy; so mass is lost) 2- oxygen is a mix of 3 isotopes (mind you, the other two are actually heavier, so the atomic mass of O16 is actually less than what you put) Even an element with a single stable isotope, like gold, has a mass that is not a “round” number: 196.966569.
