# D11.1 Predicting the Geometry of Bonds Around an Atom

The type of hybrid orbitals for each atom in a molecule correlates with the local 3D geometry of that atom. This is because the hybrid orbitals are arranged in specific orientations in space. Once you have drawn the best Lewis structure for a molecule, you can use it to assign hybridization to each atom, predict the geometric arrangement of bonds around each atom, and then predict the overall 3D structure for the molecule. This and the next few sections explain how this works.

First, for each atom in a Lewis structure, determine the number of valence atomic orbitals that are hybridized, nhyb, and use this value to predict hybridization:

• Count the number of σ bonds (nσ) the atom forms.
• A double (or triple) bond contains 1 σ bond and 1 (or 2) π bond(s).
• Are there any lone pairs on the atom?
• If yes: nhyb = nσ + 1
• If no: nhyb = nσ
• Use the value of nhyb to determine the number of valence atomic orbitals that are hybridized and hence the type of hybridization:
• For nhyb = 2, the atom is sp hybridized (two atomic orbitals are hybridized);
• for nhyb = 3, the atom is sp2 hybridized (three atomic orbitals are hybridized);
• for nhyb = 4, the atom is sp3 hybridized (four atomic orbitals are hybridized);
• An H atom in a molecule has nhyb = 1. It is not hybridized; its electron is in the 1s atomic orbital when forming a σ bond.

These rules derive from the idea that hybridized orbitals form stronger σ bonds. Therefore, the more σ bonds are formed to an atom, the more atomic orbitals are combined to form hybrid orbitals.

Exercise: Molecular geometry of BeCl2

Exercise: Molecular geometry of BF3 