Theories of Covalent Bonding

Valence Bond (VB) Theory and Orbital Hybridization

The Mode of Orbital Overlap and the Types of Covalent Bonds

The Central Themes of VB Theory

Basic Principle

A covalent bond forms when the orbitals of two atoms overlap and the overlap region, which is between the nuclei, is occupied by a pair of electrons.

The two wave functions are in phase so the amplitude increasesbetween the nuclei.

The Central Themes of VB Theory

Themes

A set of overlapping orbitals has a maximum of two electrons that must have opposite spins.

The greater the orbital overlap, the stronger (more stable) the bond.

The valence atomic orbitals in a molecule are different from those in isolated atoms.

There is a hybridization of atomic orbitals to form molecularorbitals.

Orbital overlap and spin pairing in three diatomic molecules.

Hydrogen, H2

Hydrogen fluoride, HF

Fluorine, F2

Hybrid Orbitals

The number of hybrid orbitals obtained equals the number of atomic orbitals mixed.

The type of hybrid orbitals obtained varies with the types of atomic orbitals mixed.

Key Points

sp sp2 sp3 sp3d sp3d2

Types of Hybrid Orbitals

The sp hybrid orbitals in gaseous BeCl2.

atomic orbitals

hybrid orbitals

orbital box diagrams

The sp hybrid orbitals in gaseous BeCl2.

orbital box diagrams with orbital contours

The sp2 hybrid orbitals in BF3.

The sp3 hybrid orbitals in CH4.

The sp3 hybrid orbitals in NH3.

The sp3 hybrid orbitals in H2O.

The sp3d hybrid orbitals in PCl5.

The sp3d2 hybrid orbitals in SF6.

The bonds in ethane(C2H6).

both C are sp3 hybridizeds-sp3 overlaps to bonds

sp3-sp3 overlap to form a bondrelatively even

distribution of electron density over all bonds

The and bonds in ethylene (C2H4).

The and bonds in acetylene (C2H2).

The paramagnetic properties of O2