What does bond angle depend on?

Bond angle primarily depends on the following factors:

• Electronegativity of the central atom: Generally, a more electronegative central atom leads to a larger bond angle.

  • A higher electronegativity of the central atom pulls the bonding electron pairs closer, increasing electron density around the central atom. This increased electron density results in greater repulsion between the bonding pairs, leading to a larger bond angle.

• Lone pairs of electrons: Lone pairs of electrons around the central atom exert a greater repulsive force than bonding pairs. This repulsion compresses the bond angles, making them smaller than expected.

  • The order of repulsive forces is: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.

• Size of the surrounding atoms: Larger surrounding atoms cause greater repulsion between the bonding pairs, leading to larger bond angles.

  • The size of the atoms attached to the central atom can sterically hinder each other, forcing a widening of the bond angle.

• Hybridization of the central atom: The hybridization state of the central atom influences the geometry and, consequently, the bond angles.

  • For example, sp hybridized central atoms have a bond angle of 180°, sp² hybridized central atoms have a bond angle of approximately 120°, and sp³ hybridized central atoms have a bond angle of approximately 109.5°.

• Bent's Rule: This rule states that more electronegative substituents prefer to be attached to p-rich orbitals. Thus, if a molecule has multiple substituents with differing electronegativities, the bond angles can be affected based on their orbital preferences.

  • For instance, in PF3, the Fluorine atoms are very electronegative, resulting in smaller bond angles than in PH3 because the more electronegative F atoms prefer p-rich orbitals, decreasing the s character in the bonding orbitals towards the F atoms and increasing the p character.

In summary, bond angles are determined by a complex interplay of electronegativity, lone pair repulsion, substituent size, hybridization, and Bent's rule.