Ionic Bonding Part 2 (Formulae) Edexcel
Ionic Bonding Part 2 (Formulae) Edexcel
Ionic Bonding Part 2 (Formulae) Edexcel
Its similar to what we did in part 1 with the dots and crosses but without having to draw them out every
time.
The dots and crosses are useful to aid understanding initially but once you can do them, you dont need
to worry about it, unless of course they ask you to do it in an exam question.
To be good at working out formulae, you need to use the periodic table properly and follow a few rules.
The Rules
If the element is in:
Group 1, you will form a 1+ ion Group 7, you will form a 1- ion
Group 2, you will form a 2+ ion Group 6, you will form a 2- ion
Group 3, you will form a 3+ ion Group 5, you will form a 3- ion
This accounts for most of the elements that you will see in ionic bonding. You can combine any
combination of these ions.
Sodium Oxide
We can see that there is a mismatch in the charge of the 2 ions, 1+ versus 2-. We need to balance these
charges, so we need 2 x Na to give us 2+ to balance the 2- from the oxygen.
Formula Na2O
Sodium Nitrite
Sodium 1+
Nitrogen 3- ion
Again, there is a mismatch in charge, so we need 3 x sodium ions to balance the 3- charge.
Formula Na3N
If you can do these examples then you will be able to work out most ionic formulas. Try combining any
ions from groups 1-3 (metals) with any from groups 5-7 (non-metals).
Other examples
The transition metals are a group of elements that can also change their state.
They will tell you in the question, normally in the form of roman numerals e.g. Fe(II) means Fe2+ or
Mn(VII)O4- means Mn7+.
Or going backwards
If they give you the formula of an ionic compound, you can work out the oxidation state of the metal ion.
For example: Fe2O3
We are saying we need two Fe ions and three O ions to balance the overall charge.
As we have an O2- ion, and there are 3 of them, this gives a 6+ charge. To balance the 6+ charge, we must
have the Fe3+ ion, as there are 2 of them, to give a 6+ charge overall.
Group Ions
There are ions that consist of more than one element, which are known as group ions. You can treat
these group ions as a single ion.
A few examples:
Iron(II) Hydroxide
The ions: Fe2+ and OH- Fe(OH)2
If we wrote FeOH2 without the brackets it implies that we have 2 x H but only 1 x O.
Calcium Carbonate
The ions: Ca2+ and CO32- CaCO3
We dont need the brackets this time as the 2+ and 2- cancel out.
Once you know the charge on the positive and negative ions you can simply cross the charges over to
get the formula.
Simply cross the 2 from the oxygen over to the iron side and the 3 from the iron over to the oxygen
side. This instantly gives us the formula of the ionic compound.
When you are crossing over the numbers, you can simply forget about the charges. It all just magically
balances.