Hybrid Atomic Orbitals

Learning Objectives

• Explain the concept of atomic orbital hybridization.
• Determine hybrid orbitals associated with various molecular geometries.

Introduction to Hybridization

• Atomic orbitals overlap to form chemical bonds in diatomic molecules.
• More detailed models are required for molecules with more than two atoms, like water.
• Valence bond theory sometimes does not match experimental observations (e.g., bond angles in H2O).
• Quantum mechanics and the linear combination of atomic orbitals (LCAO) lead to hybridization.
  • Hybrid orbitals are formed by combining atomic orbitals.
  • They explain the shapes and bonding observed in molecules.

Key Concepts in Hybridization

1. Hybrid orbitals form only in covalently bonded atoms.
2. Shapes and orientations differ from atomic orbitals in isolated atoms.
3. Number of hybrid orbitals equals the number of atomic orbitals combined.
4. Hybrid orbitals are equivalent in shape and energy.
5. Type of hybrid orbitals depends on electron-pair geometry (VSEPR theory).
6. Hybrid orbitals form σ bonds; unhybridized orbitals form π bonds.

Types of Hybridization

sp Hybridization

• Example: BeCl2, which is linear.
• Involves mixing of one s and one p orbital to yield two sp orbitals oriented 180° apart.
• Results in two σ bonds.

sp_2 Hybridization

• Example: BH3, which is trigonal planar.
• Involves mixing of one s and two p orbitals to produce three _sp_2 orbitals oriented 120° apart.
• Results in three σ bonds and one unhybridized p orbital._

sp_3 Hybridization

• Example: CH4, which is tetrahedral.
• Involves mixing of one s and three p orbitals to create four equivalent _sp_3 orbitals oriented at 109.5°.
• Results in four σ bonds.
• Can also accommodate lone pairs (e.g., NH3, H2O)._

sp_3d and sp_3d_2 Hybridization

• Involves d orbitals and occurs in larger atoms.
• sp_3d: Trigonal bipyramidal, as in PCl5.
• sp_3d_2: Octahedral, as in SF6.

Assigning Hybrid Orbitals

1. Determine the Lewis structure.
2. Use VSEPR theory to find regions of electron density.
3. Assign hybrid orbitals based on geometry.

Example Problems

• Example 8.2: Sulfur in SO4^2− is _sp_3 hybridized.
• Example 8.3: Carbon in urea (NH2C(O)NH2) is _sp_2 hybridized.
• Check your learning: Hybridization in acetic acid (H3C is _sp_3; C(O)OH is _sp_2).

Notes

• Hybridization rationalizes molecular geometries and works best for small central atoms.
• Not always necessary for larger atoms where electron pairs are further apart.