Using valency to stretch Year 7

In his recent article Peter Nelson¹ argues that teachers should use of the concept valency to underpin the electronic theories of bonding that are taught in GCSE level chemistry courses. I would argue that we should use these concepts even earlier. 

In my experience as a secondary school chemistry teacher, the electronic theories of covalent and ionic bonding are perhaps the first time pupils meet the concept of bonding in depth. At Key Stage 3 pupils would have been taught bonding in terms of rigid sticks between atoms that can be broken and reformed in chemical reactions. Furthermore, teachers provide little experimental proof to illustrate how the electronic theories of bonding originated, resulting in a 'you will just have to trust me on this' approach. The intellectual leap (of faith) we are asking students to make on this topic as they move from Key Stage 3 to Key Stage 4 is enormous. 

Meeting the challenge

I believe that this gulf of understanding needs to be bridged at a much earlier age. Primary school children understand that bonds exist between atoms and can be made or broken, so I think that we should stretch our pupils with the theory of valency in Key Stage 3. I have found that not only is this possible with Year 7 pupils but that it improves their confidence in and enjoyment of chemistry.

The topic of valency follows on naturally from lessons on elements, mixtures and compounds. I first challenge pupils to predict the products of burning magnesium wire in air to form magnesium oxide. The pupils easily predict the result but can only guess the ratio of magnesium to oxygen. I then repeat the experiment, carefully weighing the magnesium and the magnesium oxide formed before taking the students through the calculations required to determine the formula of MgO.

At this stage my pupils cannot explain why the formula should be MgO and not MgO₂ or Mg₂O. I tell my pupils that we will use the result to test an even simpler way of determining the formula of a compound. 

To illustrate this point, I ask my pupils to test my memory by asking me to give the formula of any compound that they can think of. After meeting their challenge, I explain that although I can only do this because I have been studying chemistry for years, they will be able to do the same by the end of the lesson without remembering one formula. I explain that all they need to replicate the feat is a Periodic Table and a list of numbers. It is at this point I introduce the cross over method for determining the formulae of simple compounds.

The cross over method

First I ask pupils to label main group elements with a new number next to the group number, its valency. I then challenge them to deduce the formulae for a list of compounds, including magnesium oxide, using the following method: 

1. Compound: magnesium oxide 
2. Elements:        Mg   O 
3. Valencies:        2      2 
                                  ↔
4. Cross over:      2      2 
5. Ratio:                1      1 
6. Formula:          Mg   O 

The pupils are astounded with the results. As I go through the answers they realise the power of the method discovered. For some, the demystifying of compound names and formulae is profound and boosts their confidence in the subject. With their new knowledge of what a chemical formula is, how to deduce it and why it is fixed, they soon prefer to use formulae instead of laboriously writing out compounds in their long, word form. Indeed, I use their new chemistry skills and confidence to introduce formulae equations and the balancing of them without introducing a word equation. The pupils also make fewer errors in balancing equations by trying to alter formulae rather than multiplying them. 

Although valency, determining formulae and balancing equations are not on many schemes of work for Year 7 chemistry pupils, their combined effect when applied together is powerful. I believe that studying these subjects in Year 7 gives my pupils a much better preparation and underpinning for the theories of electronic bonding that follow in later years than is currently prescribed for them at Key Stage 3. 

Stuart Walker is a chemistry teacher at McAuley Catholic High School, Doncaster