Been many years since I took general and org chem and am currently reading general chem textbook as refresher. Came across defintion of amu (atomic mass unit). Text says that the mass of the carbon-12 atom is defined to be exactly 12 amu. However, a couple of pages over, I read that the mass of the proton is about 1.007 amu, and the mass of the neutron is about 1.009 amu. Add up the masses of 6 protons and 6 neutrons for the carbon-12 atom, and you come up with more than 12 amu. I can't resolve this apparent discrepancy. I've consulted other texts, etc. Does some type of relativistic effect bear on this situation? In other words, does, for example, an isolated proton have a mass of 1.007 amu, but then when part of an atom have a slightly different mass because of some relativistic effect? HELP!