Lesson 3 - Complex formulae

(c) For compounds where there are three elements...

When writing the formula of a compound containing three elements, we have a little more work to do. These 'ate' endings are a little more complicated and cannot be worked out from just the Periodic Table. They need to be learnt, a little bit like 'chemistry vocab'. They won’t be given in an examination, so learn them nice and early...

The method we use is the same though, we want to work out how many of the positive charged bits are needed to cancel out the negative charged bits to get the overall compound as neutral (0)

e.g. 1. What is the formula for sodium nitrate?

Sodium is Na⁺¹

Nitrate (use the table above to find!) is NO₃^-1.

This means I need one sodium for each nitrate.

So the formula is NaNO₃

e.g. 2. What is the formula for magnesium hydroxide?

Magnesium is Mg²⁺

Hydroxide is OH^-1

This means I need two hydroxides for each one magnesium. I have to be careful when writing the formula... because I need two lots of the whole of OH, so I need to put it in brackets when writing the formula.

So the formula is Mg(OH)₂

e.g. 3. What is the formula for lithium phosphate?

Lithium is Li⁺¹

Phosphate is PO₄^3-

This means I need three lithiums for each one phosphate.

So the formula is Li₃PO₄

e.g. 4. What is the formula for ammonium sulfate?

Ammonium is NH₄⁺¹

Sulfate is SO₄^2-

This means I need two ammoniums for each sulfate.

So the formula is (NH₄)₂SO₄

Extra point to note!

Sometimes the compound contains a metal where it is impossible for us to work out the 'charge' because it isn't in one of the simple groups on the Periodic Table. If this happens, we have to tell you the charge the metal will have so that you can use your rules to work out the formula.

e.g. Iron(III) chloride

The (III) uses Roman Numerals to tell us that the iron (Fe) must have a +3 charge.

So...

Fe = +3

Cl = -1

So the formula is FeCl₃