# How do we weigh one C atom? [closed]

As far as I get it, we did weigh one carbon (12,6-isotope) atom. Then we said that electrons have nearly zero weight and protons weight nearly the same as neutrons. Hence we took the weight of that one atom and divided it by twelve and this way we got the mass of a proton/neutron:

$$w_\mathrm {n,p} = 1.6726 \cdot 10^{-24}\ \mathrm g$$

Because that isn't easy to calculate with, we introduce a number which multiplied with the weight of a proton/neutron gives us one gram:

$$N_\mathrm A = w_\mathrm {n,p}^{-1} = 1\ \mathrm{mol}$$

So far so good. Now hydrogen weights 1 g/mol and carbon weights 12 g/mol. Two questions to that:

1. How did we actually measure the weight of one carbon atom in the first place?
2. Why is the molar mass not precisely 1 and 12?
• 1.) did you check wikipedia? Its explained thoroughly there. 2.) nuclear binding energy, $E=mc^2$ – Karl Jan 9 at 0:40
• Your equation for the Avogadro constant is not correct. – Faded Giant Jan 9 at 7:14