Higher and foundation tier

A shortened version of the periodic table is shown below. For each element there are two number in the box e.g. for magnesium the numbers 24 and 12 are alongside its symbol, Mg. The number 12 is the atomic number, the proton number. The other number is the relative atomic mass, Ar. It is the average mass of magnesium atoms taking into account the presence of any isotopes of magnesium. The mass of 1 mole of magnesium is simply the Ar expressed in grams, so 1 mole of magnesium is 12g, and of course 1 mole of any substance contains 6 x 1023 particles.

periodic table

Examples using other elements from the periodic table.

The mass of 1 mole of an element is just the Ar expressed in grams

Relative formula mass, Mr

carbon dioxide molecule Carbon dioxide is a small molecule made up of three atoms, 1 atom of carbon and 2 atoms of oxygen.
To calculate the relative formula mass, Mr of this molecule is a simple task, simply add up the relative atomic masses, Ar, for each of the atoms present. The calculated mass of the molecule is called the relative formula mass, it is given the symbol Mr.

Example 1- Calculate the relative formula mass of carbon dioxide.

Mr of CO2 = (Ar of carbon) + (2xAr of oxygen)
= 12 + 32
Mr of CO2 = 44
a molecule of glucose

Example 2 - Calculate the relative formula mass, Mr of glucose (C6H12O6)

Mr of glucose (C6H12O6) = (6xAr of carbon) + (12xAr of hydrogen) + (6xAr of oxygen)
= (6x12) + (12 x1) + (6x16) =180
Mr of C6H12O6 = 180

If we express the Ar or Mr in grams then this will give us the mass of 1 mole of the substance, that is the mass of 6x1023 particles of that substance.
e.g.1- Carbon dioxide has a relative formula mass, Mr, of 44. This means that 44g of carbon dioxide will contain 6x1023 molecules of carbon dioxide. Or 3 moles of atoms. 2 moles of oxygen atoms and 1 mole of carbon atoms.
e.g.2- Glucose has a relative formula mass, Mr, of 180. This means that 180g of glucose will contain 6x1023 molecules of glucose. Or 24 moles of atoms, 6 moles of carbon atoms and 12 moles of hydrogen atoms and 6 moles of oxygen atoms.

Atom confusion

oxygen molecule To avoid confusion and mix-ups great care needs to be taken to ensure you are very clear about the type of particle you are discussing e.g. oxygen gas is a diatomic gas, this means that exists as a molecule, formula O2.
If we had 10 molecules of oxygen gas we would have 20 atoms of oxygen, since each molecule contains 2 atoms of oxygen. If we had 6x1023 molecules, that is 1 mole of molecules then we would have 2x6x1023 atoms of oxygen present, that is 2 moles of oxygen atoms.
The relative atomic mass of an oxygen atom is 16, Ar =16. So the mass of 1 mole of oxygen molecules is 32g. The mass of 1 mole of oxygen atoms is 16g. To avoid confusion you must be clear about the type of particle, atom, molecule or ion you are discussing.

Key points

Check your understanding - complete the following practice questions