Lewis diagrams only show how many bond pairs and lone pairs surround a given atom. However, all molecules are 3D and we need a method to illustrate the true shape of a molecule. That is where VSEPR Theory comes in (VSEPR Theory stands for “Valence Shell Electron Pair Repulsion Theory”).
Chemists need to know the shape of molecules (for instance, much drug design is based on the molecule fitting into the site in the body where healing is required).
Thus, we need to convey a sense of 3D in 2D. Like artists, we will use a form of perspective to indicate which parts “stick out” of the page and which parts “recede into” the page: wedge (out of page), broken line (into page), straight line (on page).
We can predict shapes by assuming that each pair of electrons (both bonding pairs and lone pairs) repel all other pairs of electrons.
Notice that there are a few exceptions to the Octet Rule in the examples. Be, B and Sn are happy to bond covalently and with less than a full octet. P and S will normally bound following the Octet Rule but when required, they will employ an extended valence to allow 10 or 12 electrons (respectively) around them.
Below you will find Lewis dot and structural diagrams for each of the examples in the chart.
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