# Unified Atomic Mass unit

1. Jun 2, 2006

### al_201314

Hi guys

I understand that the unified atomic mass unit is defined as 1/12 of the mass of a carbon-12 isotope and has a value of 1.66 X 10^-27kg. Why is this value slightly different with when I use the calculation 0.012/6.02 X10^23?

Why in theory, is the mass of a protn different from a neutron?

Thanks!

2. Jun 2, 2006

### al_201314

oh dear I realised what I missed out.. but I'd appreciate any help regarding the difference in mass of a proton and a neutron.

I have one more question, given the avagadro's constant, I was told to find the mass of an atom of carbon-12. I took 0.012/6.02X10^23 which gives me 1.99 X10^-26kg but the answer I was given was ^-25. Was there a mistake on my part or the answer is wrong?

Thanks!

Last edited: Jun 2, 2006
3. Jun 2, 2006

### Hootenanny

Staff Emeritus
The simple "hand-waving" answer is that protons and neutrons contain different quarks which have different masses. A proton is composed of two up quarks and a down quark. Wheras neutrons consist of two down quarks and an up quark. The exact mass of these quarks are unknow but it is commonly accepted that a down quark is more massive than an up quark.

~H