Students have a hard time understanding the basic shapes of chemical molecules until they have played with the model kits for a while. I can talk about valence shell electron pair repulsion theory and generalizations for the shapes of molecules with various groups of elements. I can show them photos or animations of the molecular orbitals and shapes of molecule. It's not until they get their hands on the models and see the shapes for themselves that they really figure out what I'm talking about. I'm not complaining; that's just how learning goes.