Why is the atomic mass unit (amu), rather than the gram, used to express atomic mass?

1 Answer
Oct 5, 2017

Because atoms are ridiculously small.


And the "amu" is numerically equivalent to the "g/mol". For instance, if I were to be so lucky as to isolate "1 atom" of "N", it would have a mass of

14.007 cancel"amu" xx (1.6605 xx 10^(-24) "g")/(cancel"1 amu")

= ul(2.326 xx 10^(-23) "g")

which is immeasurably small. We don't care for masses that small because we physically can't see or measure it. Instead, we care for masses we can touch, like "1.000 g" or "12.50 g".

And that involves:

1.000 cancel"g N" xx cancel"1 mol N"/(14.007 cancel"g N") xx (6.022 xx 10^23)/(cancel"1 mol")

= ul(4.299 xx 10^22 "N atoms")

12.50 cancel"g N" xx cancel"1 mol N"/(14.007 cancel"g N") xx (6.022 xx 10^23)/(cancel"1 mol")

= ul(5.374 xx 10^23 "N atoms")

You can clearly tell that this number of atoms is impossible to count. And so Avogadro's number, 6.022 xx 10^23 "mol"^(-1), was invented to describe this many particles...

4.299 xx 10^22 "N atoms" xx ("1 mol")/(6.022 xx 10^23)

= ul"0.0714 mols N"

5.374 xx 10^23 "N atoms" xx ("1 mol")/(6.022 xx 10^23)

= ul"0.8924 mols N"

And as you can see, these numbers look much nicer and more physically useful.