Determine the molecular geometry for the following anion. Here we have beryllium connected to three hydrogens and there's a negative charge. Now beryllium itself is in Group 2A, and we're going to say because of that it has two valence electrons. But here we have 3 hydrogens, so we would need 3 valence electrons. That's going to be OK because this -1 charge means we gained an electron from the outside, so we've added additional electron to beryllium.
Remember, hydrogens only have one valence electron, since they're in Group 1A, and they only make single bonds. Because this has a charge, we'd put in brackets and the charge on the outside. Now if we take a look here, we'd say that beryllium is connected to three electron groups, 3 surrounding hydrogens. It has no lone pairs on it.
So we have 3 electron groups, 0 lone pairs, and because of that our molecular geometry would be trigonal planar. This would be the molecular geometry of our particular anion.