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, A_{r}. 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 A_{r} expressed in grams, so
1 mole of magnesium is 12g, and of course 1 mole of any substance contains 6 x 10^{23} particles.

Examples using other elements from the periodic table.

- the element boron,
^{11}B, its A_{r}=11, so 11g of boron will contain 6x10^{23}or 1 mole of boron atoms. - the element calcium,
^{40}Ca, its A_{r}=40, so 40g of calcium will contain 6x10^{23}or 1 mole of calcium atoms. - the element aluminium,
^{27}Al, its A_{r}=27, so 27g of aluminium will contain 6x10^{23}or 1 mole of aluminium atoms.

The mass of 1 mole of an element is just the A_{r} expressed in grams

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, M_{r} of this molecule is a simple task, simply add up
the relative atomic masses, A_{r},
for each of the atoms present. The calculated mass of the molecule is called the relative
formula mass, it is given the symbol M_{r}.

example 1- Calculate the realtive formula mass of carbon dioxide.

M_{r} of CO_{2} = (A_{r} of carbon) + (2xA_{r} of oxygen)

= 12 + 32

M_{r} of CO_{2} = 44

example 2 - Calculate the relative formula mass, M_{r}, of glucose sugar,
C_{6}H_{12}O_{6}.

M_{r} of C_{6}H_{12}O_{6} = (6xA_{r} of carbon) + (12xA_{r} of hydrogen) + (6xA_{r} of oxygen)

= (6x12) + (12 x1) + (6x16) =180

M_{r} of C_{6}H_{12}O_{6} = 180

If we express the A_{r} or M_{r} in grams then this will give us the mass of 1 mole,
that is the mass of 6x10^{23} particles of that substance.

e.g.1- Carbon dioxide has a relative formula mass, M_{r}, of 44. This means that 44g of carbon
dioxide will contain 6x10^{23} 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, M_{r}, of 180. This means that 180g of glucose
will contain 6x10^{23} 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.

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 O_{2}.

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 6x10^{23} molecules, that is 1 mole of molecules then we would
have 2x6x10^{23} atoms of oxygen present, that is 2 moles of oxygen atoms.

The relative atomic mass of an oxygen atom is 16,
A_{r} =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.

- The relative atomic mass, A
_{r}is the average mass of an element atoms taking into account the presence of any isotopes. The relative atomic mass of any element is simply found in the periodic table. - The relative formula mass, M
_{r}, is simply found by adding up the relative atomic masses for each element in the compound. - The mass of 1 mole of a substance is simply the A
_{r}or M_{r}expressed in grams.