Strategy to Determine Hybridization

Hybridization by Counting Regions of Electron Density

Hybridization can be determined by analyzing the number of regions of electron density (bonds or lone pairs) around a central atom and then counting up throught the available orbitals.  Each second-row element can only have (1) s orbitals and (3) p orbitals (i.e. p_x, p_y and p_z).  This is four orbitals total, and if each can hold two electrons then the maximum is 8 electrons total (i.e. the octet rule).  Each type of hybridization corresponds to a specific molecular geometry and number of regions of electron density:

1. sp³ Hybridization: 4 regions of electron density.

   • Geometry: Tetrahedral
   • Bond Angle: 109.5°
   • Number of Regions of Electron Density: 4 (four single bonds, or any combination of bonds and lone pairs that add up to 4)
   • How to Identify: If the central atom is surrounded by four regions of electron density (single bonds or lone pairs), it adopts a tetrahedral geometry, and its orbitals hybridize as sp³. For example, in methane (CH₄), the carbon atom forms four single bonds with hydrogen, giving it sp³ hybridization.

2. sp² Hybridization: 3 regions of electron density.

   • Geometry: Trigonal planar
   • Bond Angle: 120°
   • Number of Regions of Electron Density: 3 (typically a double bond and two single bonds, or lone pairs)
   • How to Identify: If the central atom is surrounded by three regions of electron density, it adopts a trigonal planar geometry, and its orbitals hybridize as sp². For instance, in ethene (C₂H₄), each carbon is involved in a double bond and two single bonds, resulting in sp² hybridization.

3. sp Hybridization: 2 regions of electron density.

   • Geometry: Linear
   • Bond Angle: 180°
   • Number of Regions of Electron Density: 2 (two double bonds, or one triple bond and one single bond, or lone pairs)
   • How to Identify: When the central atom has two regions of electron density, it adopts a linear geometry and its orbitals hybridize as sp. For example, in ethyne (C₂H₂), each carbon forms a triple bond with the other carbon and a single bond with hydrogen, indicating sp hybridization.

Summary of Steps to Determine Hybridization:

1. Count the number of regions of electron density (bonds or lone pairs) around the central atom.
2. Determine the number of orbitals involved by counting through the available orbitals (1s, 2px, 2py, 2pz, etc)
3. Assign hybridization:
   • 2 regions of electron density → sp hybridization (linear)
   • 3 regions of electron density → sp² hybridization (trigonal planar)
   • 4 regions of electron density → sp³ hybridization (tetrahedral)

Understanding the relationship between electron density, geometry, and hybridization allows you to quickly determine the hybridization of atoms in various organic molecules.

Example: Methane

Example: NH₃

Example: BH3

Example: Acetylene (HCCH)