Cracking the SAT Chemistry Subject Test

Focus on the central carbon atom. It has four sites at which it is surrounded by
electron pairs. How can these four sites be situated as far from each other as
possible around the central carbon atom? You might be tempted to say that they
should be 90° apart from each other, as the structural formula shows. But that’s
thinking in two dimensions, not three. The four sites can actually be 109° apart if
they arrange themselves in a tetrahedron (a symmetrical, four-sided figure):