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I hope this blog post finds you well, wherever in the world you are. It is likely that you are either on holiday or working remotely at the moment (due to the ongoing global COVID-19 pandemic) so this month I thought I would give you a heads up or some revision on VSEPR. This stands for Valence Shell Electron Pair Repulsion (Theory) but you may be more familiar with it as the shape of molecules and / or bond angles. It comes under the SL part of unit 4 (bonding).

First things first – in order to use this theory, you need to be comfortable with Lewis diagrams (sometimes called ‘dot and cross’ diagrams). These are visual representations we use to show covalent bonding. I am making the assumption that you are happy with this idea – if not, I would suggest looking at this video:

 

The VSEPR theory revolves around the idea that electron pairs (both bonded and unbonded) influence the shape of a molecule. In order to deduce the shape of a molecule, you need to determine the number of bonded and unbonded pairs of elctrons in a molecule and then apply some rules (that unfortunately, you need to learn).

Image sourced: By Benjah-bmm27 – Own work, Public Domain,

https://commons.wikimedia.org/w/index.php?curid=2035641

 

Sometimes, the term ‘pairs of electrons’ is not used and is instead replaced by the term ‘electron domains’. This is actually a better term as we think of a single pair of electrons (a single bond), two pairs of electrons (double bond) or three pairs of electrons (a triple bond) all being an electron domain (or equivalent).

So, how do you use the VSEPR theory?

  1. Draw a Lewis structure of the molecule. Ideally and if possible you will draw a Lewis diagram where each atom has 8 outer shell electrons. However, this may not be possible and you may find one of the atom has 4, 6, 10 or even 12 outer shell (valence) electrons. If this is the case, just go with it!
  2. Find the central atom in the molecule (or atom closest to the centre if this is not possible).
  3. 3, Use this table to decide on the name, shape and bond angle (this is the bit you need to learn):
Number of electron domains Number of non bonded electron pairs Name of shape Bond angle(s) Example
2 0 Linear 180o BeCl2
3 0 Trigonal Planar 120o BF3
2 1 V shaped 115o SO2
4 0 Tetrahedral 109.5o CH4
3 1 Trigonal pyramidal 107o NH3
2 2 V shaped 104.5o H2O
5 0 Trigonal bipyramidal 90o & 120o PCl5
4 1 Distorted tetrahedral 88o SCl4
6 0 Octahedral 90o SF6
4 2 Square planar 90o XeF4