The standard way to think about steric repulsion is very simple. An atom requires space for its electrons. If another atom intrudes on that space, both atoms are unhappy and the energy rises. Steric repulsion.
Unfortunately, concept doesn’t quite match reality here. It is so costly for atoms to “overlap” (unless they are bonded, of course) that they rarely do. For example the distance between the end C in butane, C-C-C-C, is practically the same in the relatively low energy gauche conformation (3.1 A) and for the very high energy “methyls eclipsed” conformation (2.9 A). (For comparison, the distance is about 3.8 A in the anti conformation.) We know that the “methyls eclipsed” conformation is destabilized by steric repulsion, but the distance between the methyl groups is still fairly large. What’s going on?
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