Introducing the Project:

I’ve been trying to find most coherent ways to understand and teach chemical bonding for ages 16-19. For several years now, I’ve been slowly refining some methods of talking about this often-challenging topic with the 14-16 age group, but I’m just stepping up to teaching A-level Chemistry, which for me was a long time ago and before I was really paying attention!

When I was a student I noticed that the language and diagrams used for each type of chemical bond (eg covalent, ionic, metallic) are very different in style, which makes them hard to compare. I think it’s because they are are meant to emphasise particular characteristics rather than as a way to visualise and compare the concepts consistently. Whilst there are multiple factors involved the formation and behaviour of different types of chemical bonds, at the smallest scale all of them involve only 3 players: electromagnetic forces, positively charged nuclei and a bunch of identical electrons occupying various regions called orbitals.

So I figured should be possible to sketch covalent, metallic and ionic bonding (and in-between types) with one consistent model or visual analogy, to make comparisons easier. What follows are my explorations of the topic, most recent first for your convenience but I’m leaving the older stuff below so you can read from the bottom up if you prefer to follow along as I learn.

Most Recent – Draft 3:

Here we are, with a lot more labelling and some initial hypotheses about the ‘polar metallic bonding’ situation. I’ve not had much time for study since the previous post so this is more about cleaning up the diagram.

I did shift from the ‘more area of the orbital = more time the electrons spend in that place’ to a model where I used varying colour intensity to represent that instead. This freed up area to be used to represent difference in the size and shape of the orbital:

Draft 2

So I started creating a neater, electronic version of my original sketches, and included semi-metallic bonding (between metallic and covalent bonding types) this time. Here’s a screen shot of the progress so far, still missing some labelling:

But I came across further questions:

1. Firstly, what should the shape of the bonding orbital be? For example should it be ‘thin’ or ‘broad’ in the region between the atoms in a covalent bond? As it turns out many possible shapes are possible and will vary by exact material (see references), but I initially used my physics intuition about electrons being more or less attracted to the atoms and therefore more or less tightly bound in the region around them.

2. Secondly, as soon as I started to place my examples on the diagram of the ‘bonding triangle’ it became vary obvious that there was a huge gap! As you can see, I don’t currently have a good idea of what goes on in the transition up the left side of the triangle – from metallic bonding to ionic bonding. After some searching for electron density maps of compounds like potassium hydride, or other probably-in-between situations, on a hunch I tried searching for ‘polar metallic bonds’ and started to find things. Still more to work on however, but it seems like maaaaaybe it’s similar to what you get in a polarised ionic situation on the right hand edge of the triangle. Or perhaps sort of a ‘covalent bond that turns into a delocalised orbital at the metal end’ situation? Quite a bit to figure out. Learning is interesting! What I find really interesting is that I hadn’t noticed how big the gap in my understanding was until I started to map out the diagram above…when explaining, a picture really is equal a thousand words!

3. What am I trying to show here? How detailed do I want to be? How detailed is it helpful to be? Not very detailed I think…I have decided for now that I don’t really want to pitch beyond A-level (since that is what I teach) but if possible want to know the subject beyond that to avoid any simplifications that might give a wrong impression to A-level students. In particular, I already have one modification I’d like to make…I initially used the ‘bigger area = electron spends more time there’ way to represent things, but I’m now concerned that this might give the wrong intuition about the orbital shapes sometimes. If necessary I’ll change it to a ‘colour-coded model’ but I’m not decided for now as the size-based model is very intuitive and may not give wrong impressions on the ones I’m showing. I don’t want to make this diagram inaccessible by adding too much detail. For this is far from easy – it turns out that in many ways the visualisation of chemical bonds is still an active research topic!

Interesting references (which I have not finished reading myself yet):

1. Interesting paper talking about many techniques and showing some interesting diagrams developed from a new one. I don’t understand enough of the nuances to take the resulting diagrams and modify my drawings yet, but it sure does look cool!: https://pubs.acs.org/doi/pdf/10.1021/ct500490b
2. Royal Society of Chemistry explanation article: https://edu.rsc.org/feature/chemical-bonding/2020113.article
3. A whole journal focused on polar metallic bonding…pretty heavy stuff! https://www.mdpi.com/journal/crystals/special_issues/Polar_Metallic_Bonding

The Very First Post:

My student yesterday was struggling with the difference between a polar covalent bond and a polarized anion. He thought the following sketch was helpful:

I thought it might be worth developing that diagram further, as I haven’t seen many other diagrams that prioritise consistency of presentation to allow comparison of concepts. If there are some out there, maybe you can point me to them! I’m mostly doing this for my own understanding.

I have a bit more research and checking to do since I’m a physicist not a chemist by trade (possibly the chemists will tear me to shreds? Be gentle…haha), but if this turns out to be useful maybe I’ll make it into a handy visual guide for my current and future students. I’ve already noticed one subtle mistake I need to fix* and some things I could add** but I thought I’d post it up as a work in progress for those who may find it interesting.

*I probably should have placed the resultant charges alongside rather than ‘inside’ the black circles…(The black circles generally represent the inner electron shells). Also, I am aware that no real elements exist which have only one outer electron and still have the range of electro-negativities needed participate in all these types of bonding. I’m keeping it to a single bond in each case for simplicity.

**Ideas for development into an infographic:

• a key
• a diagram for semiconductors
• potentially each little diagram could show more than one pair of atoms/ions nearby. This would make the development of simple or giant structure more intuitive without strictly teaching it
• Mentioning electronegativity/charge density of the involved elements is probably a good idea since that is the main determining factor in these broad patterns.
• It may be best to do the electronegativity bit by simply arranging the little diagrams in appropriate positions on a (mostly – plain) sketch graph of the ‘bonding triangle’. This again reinforces the “sliding scale“ between all these types of bonding.
• Possibly each diagram could have a couple of examples of bonds following that pattern. So the diagram for polar covalent bonding might have “Eg H-O, B-F”
• A little box to the side mentioning that other factors are sometimes significant e.g. atomic radius differences, lone electron pairs getting involved… to make sure that everybody knows this diagram is giving the broad strokes of the pattern and not the fine details.

Let’s see where this goes. Comments?