AS-Level Amounts of Substance
The mass of an atom is determined by the number of protons and neutrons it has.
Different elements have different masses.
An element’s ‘relative atomic mass’ is the mass (in grams) of one mole’s worth of its atoms.
One mole of carbon-12 (12C) has a mass of 12g.
Each atom of carbon-12 has 6 protons and 6 neutrons.
The mass of each must be 1g per mole.
Therefore, atomic masses of elements are described as being ‘relative to 1/12 the mass of an atom of carbon-12’.
Relative Atomic Mass
Every element has a different mass for one atom of itself. The mass of an atom is determined by the number of protons and neutrons that it has. As every element has a different number of protons, each element has a different mass.
NOTE – for simplification purposes here I am ignoring isotopic masses, these will be addressed later in the notes.
In the periodic table, each element is given a mass number. The mass number refers to how many neutrons and protons one atom of that element has. Let’s take sodium as an example:
Sodium has a mass number of 23 and an atomic number of 11. So, there are 12 neutrons and 11 protons in one atom of sodium.
This is the mass for one atom of sodium. Virtually useless for everyday chemistry – who deals with one atom? These masses need to be in more useable values. To do this, we use a relative atomic mass.
A relative atomic mass just means for one mole of an element, the mass of that one mole will be the mass number of the element in grams. Remember – a mole is just a simple way of saying how many particles of something you have.
So, for sodium a mass number of 23 means that 1 mole of sodium will have a mass of 23 grams. This is why the units for relative atomic mass are gmol-1 (grams per 1 mole).
Relative to 1/12 of the mass of an atom of carbon12
The ‘relative’ part of relative atomic mass just refers to the assumption that one proton has a mass unit of one and a neutron has a mass unit of one.
This means that one mole’s worth of protons has a mass of one gram, and one mole’s worth of neutrons also has a mass of one gram.
This assumption is based upon the fact that one mole of carbon-twelve (carbon-twelve is just the most common form of carbon) has a mass of twelve grams.
As carbon has an atomic number of six, it must have six protons and six neutrons. Protons and neutrons have the same mass. So, if twelve particles make up one atom of carbon-twelve, then twelve moles’ worth of particles make up one mole of carbon-twelve atoms.
So, to get the relative mass of one particle (proton or neutron):
12g / 12 moles = 1 g per mole.
This is why the definition of relative atomic mass is: ‘the mass of one mole’s worth of atoms of an element, relative to 1/12th the mass carbon12’.
All this means is that we assume a proton or a neutron has a mass of 1 gram per mole, because twelve of these particles make up one mole of carbon-twelve (see above). So, we can simply multiply the number of protons and neutrons in one atom of an element by 1 gram per mole (or 1 / 12th the mass of one mole’s worth of carbon atoms) to get its relative atomic mass.