VSEPR & Shapes of Molecules

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One for the chemistry students (and teachers!) out there today, with a look at how we can work out the shapes of some simple molecules using Valence Shell Electron Pair Repulsion (VSEPR) theory. These shapes are decided by the arrangement of electrons around the central atom in the molecule.

VSEPR works on the assumption that the shape adopted is that which minimises repulsions between the electron pairs in the molecule. It doesn’t take into account factors such as the size of bonded atoms or groups, and as such doesn’t always predict the shape of certain compounds correctly, in particular those of transition metals. However, it generally gives good predictions for compounds of main group elements.

In order to work out the shape adopted, we simply need to find the number of electron pairs surrounding the central atom in a molecule. This can be determined in a few short steps, using the method provided in the graphic. Note that this method assumes that only single bonds are present in the molecule; these are formed using a pair of shared electrons (1 from each atom), whereas double bonds involve two pairs of shared electrons.

A complication of VSEPR is that all electrons do not repel equally. The presence of electrons that are not involved in bonding around the central atom, so-called ‘lone pairs’, are closer to the central atom, and as such repel electron pairs around them more strongly. Consequently, if they are present in a molecule, they can have an effect on the shape, taking up a space that would usually be occupied by a bonding pair, and making the angles between the other bonds in the molecule marginally smaller (by approximately 2.5˚ per lone pair present around the central atom). This leads to the range of different shapes for the same number of electron pairs, as indicated in the graphic.

If you want to learn more about working out the shapes of molecules using VSEPR, check out the provided links in the ‘further reading’ section below.



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References & Further Reading