The third part of the Bonding Model is called Hybridization. It explains why we get the molecular geometry we do for molecules.
For 1st year chemistry students, we keep the explanation very simple. If you want to know the more in-depth explanation, you can take AP Chemistry.
In an atom, the electrons are located in orbitals called Atomic orbitals. What we have found is that when atoms get closed together that their valence electrons interact (i.e. shared electrons) that the bonding (and its physical properties) do need work correctly if we assume that the valence electrons are in atomic orbitals. So we have made up a new type of orbital when atoms bond (share electrons) called hybridized orbitals (also called bonding orbitals)</b>.
Click on [Hybridization Flash Animation] and listen to what hybridized orbitals are.
We only talk about hybridized orbitals on the central atoms. The simple way of determining hybridization is as follows (also in notes on VsperTable including example):