Valence Bond Theory & Hybridization Theory

Valence bond theory

• electrons in each bond act independently of the electrons in the other bonds

• covalent bond forms when valence orbitals from 2 atoms overlap, unpaired electrons pair up making shared electron density

• must have opposite spins

how valence bond theory works

• when 2 valence orbitals overlap, potential energy lowers from increased attraction

• at ideal distance (bond length) between nuclei, push/pull forces balanced

• energy released when bond forms (Bond enthalpy)

Hybridization

introduced to explain molecular structure when the valence bond theory failed to correctly predict them

process of mixing atomic orbitals to form new orbitals with different shapes and energies compared to the originals (hybrid orbitals) better suited for covalent bonding

how orbital hybridization works

An electron may be promoted to create more unpaired electrons. Then, orbitals mix to form hybrid orbitals, which allow atoms to form stable, directional bonds.

sp hybridization characteristics

• 1 s & 1 p orbital → 2 sp hybrid orbitals

• 180° linear fashion

• ½ s & ½ p character, energies exactly between s and p orbitals

sp² hybridization characteristics

• 1 s & 2 p orbitals → 3 sp² hybrid orbitals

• 120° trigonal planar

• 1/3 s & 2/3 p character

• higher energy than sp orbital, closer to original p orbital

determining the hybridization

1. Count the number of atoms bonded to the central atom in Lewis structure

2. Count the number of lone pairs on the central atom and add it to the previous number

3. If the sum is 2, then it's sp hybridization; if 3, then sp² hybridization; if 4, then sp³ hybridization, and so on.

bond axis

an imaginary line that passes through the centers of two bonded atoms, representing the path along which the chemical bond is formed

Sigma (σ) bonds

• formed by the overlap of orbitals in an end-to-end fashion, with the electron density concentrated between the nuclei of the bonding atoms

• free rotation around the bond axis

• Almost all single bonds are sigma bonds

Pi (π) bonds

• formed by the sideways overlap of unhybridized p orbitals with the electron density concentrated above and below the plane of the nuclei of the bonding atoms.

• pi bond is weaker than a sigma bond

• Pi bonds also restrict rotation around the bond axis