Overview
This lecture introduces hybridization theory, explaining its development, importance in visualizing molecular structures in three dimensions, and its application to predicting molecular geometry and reactivity.
Why Hybridization Theory Was Developed
- Hybridization theory explains how carbon forms four covalent bonds despite its electron configuration.
- The theory accounts for observed molecular geometries and bond angles, which cannot be explained by simple electron configurations.
- Visualizing molecules in three dimensions helps predict chemical and physical properties.
Atomic Orbitals and Hybridization
- Carbon's electron configuration: 1s² 2s² 2p² (four valence electrons).
- Only valence electrons participate in bonding; core electrons are often ignored.
- Mixing atomic orbitals creates "hybrid" orbitals that better explain bonding and geometry.
Types of Hybridization
- sp³ Hybridization: Mixing one s and three p orbitals gives four identical orbitals, producing a tetrahedral geometry with 109.5° angles (e.g., methane, ethane).
- sp² Hybridization: Mixing one s and two p orbitals gives three planar orbitals and one unhybridized p, forming trigonal planar geometry with 120° angles (e.g., ethylene).
- sp Hybridization: Mixing one s and one p orbital gives two linear orbitals and two unhybridized p orbitals, yielding 180° angles (e.g., acetylene).
Sigma and Pi Bonds
- Sigma (σ) bonds are formed by direct overlap of hybrid orbitals.
- Pi (π) bonds result from the side-by-side overlap of unhybridized p orbitals, important in double and triple bonds.
Molecular Geometry and Isomerism
- Hybridization predicts 3D arrangements: tetrahedral (sp³), trigonal planar (sp²), linear (sp).
- Geometric isomers arise due to restricted rotation around π bonds.
- Newman projections and conformational analysis help visualize rotations and strain around single bonds.
Determining Hybridization
- Count the number of "groups" (atoms or lone pairs) around the central atom: 4 = sp³, 3 = sp², 2 = sp.
- Nitrogen and oxygen hybridization is deduced by counting attached atoms and lone pairs.
Deviations from Ideal Bond Angles
- Lone pair–lone pair repulsions cause bond angles to decrease from ideal values.
- VSEPR theory helps predict deviations, e.g., H₂O has a bond angle ~104°, NH₃ ~106° due to lone pairs.
Key Terms & Definitions
- Hybridization — mixing atomic orbitals to form new orbitals for bonding.
- Sigma (σ) bond — direct overlap of orbitals between two nuclei.
- Pi (π) bond — side-by-side overlap of unhybridized p orbitals.
- sp³, sp², sp hybridization — types of orbital mixing that determine molecular geometry.
- VSEPR theory — predicts molecular shapes based on electron pair repulsion.
- Isomer — molecules with the same formula but different structures or spatial arrangements.
Action Items / Next Steps
- Practice drawing Lewis structures and deducing hybridization.
- Visualize and model molecules in three dimensions.
- Read about VSEPR theory for predicting molecular shapes.