Chemical Bonding (Advanced)

In SEE chemistry, you learned about ionic and covalent bonds. Now we go deeper to understand why molecules have specific shapes and how atomic orbitals combine. This knowledge is essential for predicting chemical properties and reactivity.

VSEPR Theory

Valence Shell Electron Pair Repulsion theory predicts molecular geometry based on the idea that electron pairs around a central atom repel each other and arrange themselves as far apart as possible.

| Electron Pairs | Arrangement | Bond Angle | Example | |---------------|-------------|------------|---------| | 2 | Linear | 180 degrees | BeCl₂, CO₂ | | 3 | Trigonal planar | 120 degrees | BF₃, SO₃ | | 4 | Tetrahedral | 109.5 degrees | CH₄, NH₄⁺ | | 5 | Trigonal bipyramidal | 90/120 degrees | PCl₅ | | 6 | Octahedral | 90 degrees | SF₆ |

Lone pairs take up more space than bonding pairs, so they compress bond angles. For example:

• NH₃: 3 bonding pairs + 1 lone pair = pyramidal (107 degrees, not 109.5)
• H₂O: 2 bonding pairs + 2 lone pairs = bent/V-shape (104.5 degrees)

Hybridization

Hybridization is the mixing of atomic orbitals to form new hybrid orbitals of equal energy.

| Hybridization | Orbitals Mixed | Shape | Example | |--------------|----------------|-------|---------| | sp | s + p | Linear | BeCl₂, C₂H₂ | | sp² | s + 2p | Trigonal planar | BF₃, C₂H₄ | | sp³ | s + 3p | Tetrahedral | CH₄, H₂O | | sp³d | s + 3p + d | Trigonal bipyramidal | PCl₅ | | sp³d² | s + 3p + 2d | Octahedral | SF₆ |

How to determine hybridization: Count the total number of sigma bonds and lone pairs on the central atom. This equals the number of hybrid orbitals.

Worked Example

Determine the hybridization and shape of H₂O.

Solution:

• Central atom: O
• Bonding pairs: 2 (O-H bonds)
• Lone pairs: 2
• Total electron pairs: 4
• Hybridization: sp³
• Geometry (considering lone pairs): Bent/V-shaped
• Bond angle: approximately 104.5 degrees

Molecular Orbital Theory (Basics)

MOT treats bonding as the formation of molecular orbitals from atomic orbitals:

• Bonding molecular orbital (lower energy): Formed by constructive overlap. Electrons here stabilize the molecule.
• Antibonding molecular orbital (higher energy): Formed by destructive overlap. Electrons here destabilize the molecule.

Bond order = (Bonding electrons - Antibonding electrons) / 2

A bond order of zero means the molecule does not exist (e.g., He₂).

Key Takeaways

• VSEPR predicts shape based on electron pair repulsion
• Lone pairs reduce bond angles compared to bonding pairs only
• Hybridization: count sigma bonds + lone pairs on central atom
• Bond order from MOT predicts molecular stability