Chem 101 unit 2.1.2 Relative masses, what do they mean?


The reading for this unit includes the section on “relative masses of atoms” ( Link below) which talks about the realization in Dalton’s time (1800s) that relative masses could be determined by comparing “combining weights”. I am struggling to reach a clear understanding of this section and so I have written the questions below hoping someone can clear my confusion.

Was the “relative mass” they mention simply a way of expressing how much heavier or lighter an atom of an element is than the atoms of other elements. i.e. H has 1 and O is 7 ergo O atoms are heavier than H atoms.

If not, what do they mean by relative mass?

Any help is greatly appreciated.


Yes, relative atomic mass gives an easy way to express the comparative mass of an atom. Rather than saying that an atom has a mass of some infinitesimal fraction of a kg we can say that an atom of carbon has (on average) 12 times the mass of a hydrogen atom. Since we most often want to know the proportions of elements in a compound the use of relative masses makes sense.

For example, if we wanted to produce water by combining hydrogen and oxygen, knowing the formula of water is H2O and that the oxygen has a relative mass of 16, we can calculate that we would need 8kg of oxygen for each kg of hydrogen.

Technically, relative masses are referenced to 1/12 of the average mass of carbon-12, rather than hydrogen. In practice this doesn’t really make much difference.


Thank you, that was helpful.