Confused about amu

[Harvest72]

Hi,

First off, amu is just a convienent way of expressing the weight of atoms, subatomic particles, right? It is my understanding that the amu is defined as 1/12 the mass of a Carbon-12 atom (12 amu = 1 Carbon-12 atom). Is it not also true that both protons and neutrons have masses of over 1 amu? If this is so, Carbon-12, containing 6 neutrons and 6 protons, as well as 6 electrons, should have a mass of slightly over 12 amu, right?

As in: 6*1.0002 + 6*1.0003 + electron masses (I know those numbers are not correct, just an example)

Another thing: the Law of Multiple Proportions doesn't make any sense to me. Well it sort of does, I just don't grasp the importance of it. If someone could help me out with that (please use multiple examples) that would be great!

Thanks,
Cam