Talk:Avogadro constant

The following statement is incorrect:

Such an atom consists of 6 protons, 6 neutrons and 6 electrons, and N_A is therefore equal to 12 grams divided by the sum of the masses of a proton, neutron and electron.

While an atom of Carbon-12 does include the various subatomic particles mentioned above, it also has some mass due to the nuclear binding energy. Therefore, the sum of the masses of 6 free protons, neutrons and electrons will be different from that of an atom of Carbon-12. -- Matt Stoker

Ah yes, that makes sense. I originally added the above sentence, and I was a little worried that the numbers didn't come out quite right... :-)

But now you've got me thinking. The example on mole unit adds molecular masses like this: atomic mass of carbon is 12, atomic mass of hydrogen is 1, so molecular mass of C₂H₆ is 2*12+6 = 30. This is incorrect, isn't it? It neglects the mass in the binding energy. AxelBoldt

Strictly speaking, you are correct that the chemical binding energy contributes mass. However, this mass is very small compared to the mass due to the nuclear binding energy and so can usually be neglected. For example, the chemical binding energy in a hydrogen molecule contributes a mass of ~2.4E-9 g/mol. By contrast the nuclear binding energy in Deuterium contributes a mass of ~0.0029 g/mol (over six orders of magnitude greater than the chemical binding energy). For the purpose of defining Avogadros Number to arbitrary precision, both nuclear binding energy and chemical binding energy must be accounted for and I would assume the definition refers to 0.012 kg of free Carbon-12 atoms (ie. no chemical bonding), since graphite or diamond would have a slightly different mass. However, for most other applications the chemical binding energy contribution to mass can be neglected, so the example under mole unit should be fine. --Matt Stoker