# Why does the atomic mass of an atom equal the weight of 1 mol of that atom in grams?

Sep 3, 2017

And you might just as well ask why there are 12 eggs in a dozen eggs......

#### Explanation:

A dozen is a convenient number for counting, because we can use our thumb to mark out a dozen with the tips and joints of the four fingers on each hand.

$\text{Dozens, Bakers' dozens, Botany Bay dozens}$ are thus all convenient units to measure a particular thing.

On the other hand, a mole contains $6.022140 \times {10}^{23}$ individual items. And thus a mole of ""^12C represents $6.022140 \times {10}^{23}$ individual ""^12C atoms.

Why should we use such an absurdly large number? Because $6.022140 \times {10}^{23}$ individual ""^12C atoms have a mass of $12 \cdot g$ precisely. The mole is this the link between the (sub!) micro world of atoms, and molecules, to the macro world of grams, and litres, what chemists can measure out in a laboratory.

And in every chemistry exam you should be given a copy of the Periodic Table, which tells you the molar mass of each known element, as well as its atomic number. It is up to you to learn how to use the Table effectively. Good luck.

I have said much the same thing here, and in the links.

Note that on the Table, the atomic mass of carbon is quoted as $12.011 \cdot g$....why does it diverge from $12$? This is because of the existence of the ""^13C isotope, which contains an extra nuclear neutron. This raises the atomic mass slightly.