 Let's make a water molecule. This is an oxygen and this is hydrogen. You can see their relative size. Oxygen has six valence electrons and each of the hydrogen has one. So that gives us a total of eight electrons. And we want to satisfy everybody's octet rule. So that means we need to have eight things around oxygen and two shared electrons with the hydrogen. So first if we have two unshared pairs and two attached pairs, we know that we've got four things attached around the oxygen. So we are going to have a tetrahedron. So first classify and then let's attach our hydrogens. Remember that's about 109. And then we'll attach our unshared pairs. So we want these to be about 109 degrees and this angle to be about 109 degrees. Now this looks a little confusing because what I want you to think about with the unshared pairs of electrons is there's a pair of electrons out here occupying a lot of time in this area. So that means there's going to be a negative charge over here. You've got another pair of electrons over here and so they're occupying some space and they actually push these even a little closer together and cause that angle to be about 105 instead of 109. When you describe the geometry of this molecule, you describe it as the molecule looks without the unshared pairs. You have to put the unshared pairs in to give you your shape but when you describe the molecule this is described as bent not tetrahedron. You describe it as the molecule looks without the unshared pairs. You have to put the unshared pairs in to give you your shape but when you describe the molecule this is described as bent not tetrahedron.