Relative Masses

Chemistry: Cambridge International A Level, topic 1.1.1

Relative Atomic Mass

Relative atomic mass (A_r) is the average mass of an element compared to one-twelfth the mass of a single carbon-12 atom.

Almost every element has multiple stable isotopes.
This means that some atoms of an element will weigh more than others.
The average mass is the average of the masses of the elements, weighted according to their abundance.
E.g. ³⁵Cl is 75% abundant and ³⁷Cl is 25% abundant.
- A_r = (0.75 × 35) + (0.25 × 37) = 35.5

Relative molecular mass (M_r) is the average mass of a molecule compared to one-twelfth the mass of a single carbon-12 atom.

Recall that the A_r takes into account the differences in mass and abundance of isotopes.
This means that we can simply add the A_r values for all the atoms in a molecule to calculate the M_r!

The relative formula mass is similar to the relative molecular mass. It even uses the same symbol - M_r!
It is used when considering ionic compounds.
Ionic compounds form giant ionic lattices, so we must consider the formula mass. Ionic compounds don't make compounds.
The relative formula mass is calculated the same way as the M_r. Simply add the A_r values of the ions.
For example the relative formula mass of NaCl would be 23 (A_r of Na + 35.5 (A_r of Cl) = 58.5.