Hybridization in Chemistry

Overview

This lecture explains the concept of hybridization in chemistry, its necessity for covalent bonding theories, types of hybridization, how to determine hybridization and geometry using shortcut tricks, and provides example problems.

Introduction to Hybridization

• Hybridization explains why all bonds in certain molecules (e.g., methane) are identical, resolving issues in valence bond theory.
• It involves the mixing of atomic orbitals to form new, equivalent hybrid orbitals.

Atomic Orbitals and Overlapping

• Atomic orbitals: s (spherical), p (dumbbell), d (double dumbbell), f (complex).
• Covalent bonds form by overlapping half-filled atomic orbitals with opposite spins.
• Overlap types: sigma (head-on, along-axis) and pi (side-by-side, parallel).

Failure of Valence Bond Theory

• Valence bond theory predicts different bond types in methane, but experiments show identical bonds.
• Hybridization solves this by forming equal-energy orbitals.

Hybridization Concept

• Different types: sp, sp2, sp3, sp3d, sp3d2, sp3d3, depending on the number and type of orbitals mixed.
• The number of hybrid orbitals equals the number of atomic orbitals combined.
• Hybrid orbitals have identical energy and shape, and form more stable bonds than pure atomic orbitals.

Conditions for Hybridization

• Only orbitals in the valence shell participate.
• Orbitals must have similar energies.
• Promotion of electrons (excited state) is not mandatory.
• Both half-filled, fully filled (lone pairs), or even empty orbitals can participate.
• Only sigma bonds and lone pairs involve hybridization (not pi bonds).

Types and Geometries of Hybridization

• sp: 2 orbitals, linear geometry (e.g., BeCl₂).
• sp2: 3 orbitals, trigonal planar geometry (e.g., BCl₃).
• sp3: 4 orbitals, tetrahedral geometry (e.g., CH₄).
• sp3d: 5 orbitals, trigonal bipyramidal geometry (e.g., PCl₅).
• sp3d2: 6 orbitals, octahedral geometry (e.g., SF₆).
• sp3d3: 7 orbitals, pentagonal bipyramidal geometry (e.g., IF₇).

Determining Hybridization and Geometry

• Shortcut: Z = (number of sigma bonds + number of lone pairs) on the central atom.
  • Z = 2 → sp; Z = 3 → sp2; Z = 4 → sp3; Z = 5 → sp3d; Z = 6 → sp3d2; Z = 7 → sp3d3.
• Alternate shortcut: Z = ½ [valence electrons of central atom + number of monovalent atoms + (-)charge - (+)charge].
• Use Z to directly deduce hybridization type and thus the geometry.

Examples

• H₂O: Z = 4 → sp3 hybridization (tetrahedral geometry).
• NH₃: Z = 4 → sp3 hybridization (tetrahedral geometry).
• SO₃: Z = 3 → sp2 hybridization (trigonal planar).
• SF₆: Z = 6 → sp3d2 hybridization (octahedral).
• Propine (C₃H₄): carbon atoms show either sp (linear) or sp3 (tetrahedral) hybridization depending on bonding.

Key Terms & Definitions

• Hybridization — Mixing of atomic orbitals to form new, equivalent hybrid orbitals.
• Atomic Orbital — Region where finding an electron is most probable (s, p, d, f types).
• Sigma Bond (σ) — Covalent bond from head-on (axial) orbital overlap.
• Pi Bond (π) — Covalent bond from side-by-side orbital overlap.
• Geometry — Spatial arrangement of hybrid orbitals.
• Lone Pair — A pair of valence electrons not involved in bonding.

Action Items / Next Steps

• Homework: Find the hybridization in xenon dioxy difluoride (XeO₂F₂) using the shortcut methods discussed.
• Review and memorize the hybridization types, orbital combinations, and associated geometries for exams.
• Practice determining hybridization in sample molecules using the Z-number trick.