Valence Bond Theory and Hybridization: Structured Study Notes

Valence Bond Theory

Introduction to Valence Bond Theory

Valence Bond (VB) Theory is a fundamental model in chemistry used to describe how atoms bond together to form molecules. It focuses on the overlap of atomic orbitals and the localization of electrons between specific atoms.

• Localized Orbitals: In VB theory, orbitals and electrons are localized between atoms, forming distinct bonds.

• Hybridization: Central atoms often hybridize their atomic orbitals to achieve the observed molecular geometry.

• Bond Properties: For example, in methane (CH4):

  • C-H bond length: 110 pm

  • C-H bond energy: 0.7 aJ

  • Carbon valence orbitals: 2s, 2p

Hybridization of Atomic Orbitals

Hybridization is the process by which atomic orbitals mix to form new, equivalent hybrid orbitals suitable for bonding.

• sp3 Hybridization: Mixing one s and three p orbitals forms four sp3 hybrid orbitals, resulting in a tetrahedral geometry (bond angle ≈ 109.5°).

• sp2 Hybridization: Mixing one s and two p orbitals forms three sp2 hybrid orbitals, resulting in a trigonal planar geometry (bond angle ≈ 120°).

• sp Hybridization: Mixing one s and one p orbital forms two sp hybrid orbitals, resulting in a linear geometry (bond angle = 180°).

Examples of Hybridization

• Methane (CH4): Carbon undergoes sp3 hybridization, forming four equivalent bonds with hydrogen.

• Ethane (C2H6): Each carbon is sp3 hybridized, forming a sigma bond between the carbons and with hydrogens.

• Boron Trifluoride (BF3): Boron is sp2 hybridized, resulting in a trigonal planar arrangement.

• Beryllium Hydride (BeH2): Beryllium is sp hybridized, resulting in a linear arrangement.

• Water (H2O): Oxygen is sp3 hybridized, with a bent geometry (bond angle ≈ 104.5°).

Expanded Octets and Hybridization

Elements in period 3 and beyond can expand their octet by utilizing d orbitals in hybridization.

• sp3d Hybridization: Five hybrid orbitals, resulting in a trigonal bipyramidal geometry.

• sp3d2 Hybridization: Six hybrid orbitals, resulting in an octahedral geometry.

Predicting Hybridization

To predict the hybridization of a central atom, count the number of electron domains (bonding and lone pairs) around it:

• 2 domains: sp

• 3 domains: sp2

• 4 domains: sp3

• 5 domains: sp3d

• 6 domains: sp3d2

Practice Questions

• If the central atom is sp3 hybridized, what is the geometry?

  • Answer: Tetrahedral

• Predict the hybridization for the central atom of SF2:

  • Answer: sp3

• Predict the hybridization for the central atom of SeF4:

  • Answer: sp3d

Hybridization and Multiple Bonds

Multiple Bonds and Their Hybridization

Multiple bonds (double and triple) involve both sigma (σ) and pi (π) bonds. Hybridization determines the geometry and bonding in these molecules.

• Double Bond: Consists of one sigma and one pi bond.

• Triple Bond: Consists of one sigma and two pi bonds.

• Example: In ethylene (C2H4), each carbon is sp2 hybridized, forming a planar structure with a double bond between carbons.

Practice Questions

• Predict the hybridization of one of the carbons in C2H4:

  • Answer: sp2

• What is the geometry around each carbon in C2H4?

  • Answer: Trigonal planar

Key Terms and Definitions

• Hybrid Orbitals: Orbitals formed by the combination of atomic orbitals on the same atom.

• Sigma (σ) Bond: A bond formed by the head-on overlap of orbitals along the internuclear axis.

• Pi (π) Bond: A bond formed by the side-to-side overlap of p orbitals, above and below the internuclear axis.

Important Equations

• Bond Order (for simple diatomics): $\text{Bond Order} = \frac{\text{Number of bonding electrons} - \text{Number of antibonding electrons}}{2}$

Summary Table: Hybridization and Geometry

Hybridization	Electron Domains	Geometry
sp	2	Linear
sp2	3	Trigonal Planar
sp3	4	Tetrahedral
sp3d	5	Trigonal Bipyramidal
sp3d2	6	Octahedral

Example Applications

• Water (H2O): Oxygen is sp3 hybridized, resulting in a bent geometry due to two lone pairs.

• Boron Trifluoride (BF3): Boron is sp2 hybridized, resulting in a trigonal planar geometry.

• Beryllium Hydride (BeH2): Beryllium is sp hybridized, resulting in a linear geometry.

Additional info: These notes expand on the provided images and text, adding definitions, examples, and a summary table for clarity and completeness.