How does VSEPR work?
The key idea in
So if there are 2 electron pairs around a central atom, a linear molecule will result; if there are 3 electron pairs around a central atom, the electron pairs will arrange themselves in a trigonal plane; and if there are 4 electron pairs around a central atom, the electron pairs will assume a tetrahedral shape.
The important proviso, however, is that we do not describe molecular on the basis of electron pairs, but on the basis of bonds. Around carbon in methane there are 4 electron pairs in
On the other hand, around the oxygen atom in water there are also 4 electron pairs,
Now the shapes of the electron pairs mirror that of the Platonic solids: 2 electron pairs, linear; 3 electron pairs, trigonal planar; 4 electron pairs, tetrahedral; 5 electron pairs, trigonal bipyramidal; 6 electron pairs, octahedral. Whether the molecules assume these shapes depends on the number of bonding and lone pairs, but it is fairly straightforward to predict molecular geometry on the basis of
There should be many examples of well-answered questions on this topic on these boards. Good luck finding them.