Question

Explain Valance Bond Theory

Answer 1

Valence Bond Theory

• In a molecule, electrons inhabit atomic orbitals rather than molecular orbitals.
• It is based on the concept of bond overlapping.
• Covalent bonding is a chemical link generated by the sharing of electrons.

Postulates:-

• When two valence orbitals present in two separate atoms overlap on one other, the covalent bonds are created.
• Due to this overlapping, the electron density between the two bonding atoms increases, increasing the stability of the resulting molecule.
• The valence shell of an atom has several unpaired electrons, allowing it to make many bonds with other atoms.
• According to the VBT, the paired electrons present in the valence shell do not participate in the creation of chemical bonds.

Geometry of water molecule:

• The electronic configuration of the Hydrogen atom is $1{\mathrm{s}}^1$ and oxygen is $1{\mathrm{s}}^{2}2{\mathrm{s}}^{2}2{\mathrm{p}}^4$.
• In water molecules, there are 2 oxygen atoms, and hence two half-filled $\mathrm{p}$-orbitals.
• Oxygen needs two more electrons to complete the octet and Hydrogen requires one more electron to acquire the configuration of $\mathrm{He}$.
• Two-half-filled orbitals of the two Hydrogen atoms separately overlap with one of the two half-filled $\mathrm{p}$-orbitals of the Oxygen atom.
• This gives rise to two single covalent bonds between the central atom and two Hydrogen atoms.
• The three $\mathrm{p}$-orbitals are along the three perpendicular axes in space.
• The valence bond theory of the water molecule describes the two $\mathrm{O}-\mathrm{H}$ bonds as resulting from the overlap of $1\mathrm{s}$ orbital of Hydrogen with the two half-filled $2\mathrm{p}$ orbitals of the oxygen atom.
• Since the two $2\mathrm{p}$ orbitals are at right angles to one another, valence bond theory predicts a bent geometry for the water molecule with a bond angle of $90°$.
• The $\mathrm{H}-\mathrm{O}-\mathrm{H}$ bond angle in water is $104.45°$and is having a bent tetrahedral geometry or V-shaped.
• The opening of the angle to a value greater than the predicted one of $90°$can be accounted as due to the repulsion between the two hydrogen nuclei.