Yes, lone pairs do count when determining the hybridization of an atom.
Hybridization involves mixing atomic orbitals to form new hybrid orbitals that are suitable for bonding. To determine the hybridization, you need to consider the total number of sigma bonds and lone pairs around the central atom. This number corresponds to the number of hybrid orbitals required.
Here's a breakdown:
- Sigma Bonds: These are single bonds, representing direct overlap between atomic orbitals.
- Lone Pairs: These are pairs of valence electrons that are not involved in bonding but still occupy space around the atom.
The sum of sigma bonds and lone pairs is called the steric number. The steric number directly correlates to the hybridization:
| Steric Number | Hybridization | Geometry (around central atom) | Example |
|---|---|---|---|
| 2 | sp | Linear | BeCl2 |
| 3 | sp2 | Trigonal Planar | BF3 |
| 4 | sp3 | Tetrahedral | CH4 |
| 5 | sp3d | Trigonal Bipyramidal | PCl5 |
| 6 | sp3d2 | Octahedral | SF6 |
Why do lone pairs count?
Lone pairs, although not directly involved in bonding other atoms, still occupy space and influence the shape of the molecule. They repel bonding pairs more strongly than bonding pairs repel each other. This is why, for example, water (H2O) has a bent shape despite having four electron groups (2 bonding pairs and 2 lone pairs) around the central oxygen atom. The oxygen atom in water is sp3 hybridized.
Example:
Consider ammonia (NH3). The nitrogen atom has three single bonds to hydrogen atoms (3 sigma bonds) and one lone pair.
- Sigma bonds: 3
- Lone pairs: 1
- Steric number: 3 + 1 = 4
A steric number of 4 corresponds to sp3 hybridization.
In summary, lone pairs must be included when determining the hybridization of an atom because they contribute to the overall electron density distribution and molecular geometry around the atom.
