Different molecules having same hybridization but acquire different shape. To explain these phenomena in 1957 Gillespie and Nyholm provided a theory that is known as Valence Shell Electron Pair Repulsion Theory (VSEPR theory).

Valence Shell Electron Pair Repulsion (VSEPR) Theory

(i) Lone pairs of electrons (lp) repel each other more strongly than that of bond pair (bp) of electrons. The decreasing order of repulsion is lp – lp > lp – bp > bp – bp.

Examples:

• In CH₄ only bp-bp repulsion exist while, in NH₃ two types of repulsive force present, lp – bp repulsion and bp – bp repulsion. lp – bp repulsion dominates over bp – bp repulsion. Because of this the bonds in NH₃ is more compressed than that in CH₄ which provides lower band angle in NH 3 .

• In CO₂ the central C atom does not occupy any lone pair of electron, hence in CO₂ only bp – bp repulsion exists. The central S atom in SO₂ contains one lone pair of electron. Therefore, in SO₂ lp – bp repulsion and bp – bp repulsion present. As we know that lp – bp repulsion dominates over bp – bp repulsion, the O-S-O bonds is compressed by dominating lp – bp.
• NO₂⁺ does not contain any paired or unpaired electron on the central N atom. Here, only bp – bp repulsion exists. In NO₂ the central N atom contains one unpaired electron, while in NO₂⁻ the N atom contains one lone pair of electrons. As in NO₂ only one unpaired electron present, the lp – bp repulsion is lower compared to lp – bp repulsion in NO₂⁻ . Hence, the dominating lp – bp is more pronounced in NO₂⁻ compared to NO_2.

 

(ii) Repulsion between bond pairs decreases as the electronegativity of the atom bound to the central atom increases.

A bond is formed by the sharing of electrons. When the difference in electronegativity between the two connecting atoms increases the electron pair is shifted towards the more electronegative atom. With increases of electronegativity this shift is more to the electronegative atom. This decreases the repulsion effect between two bond pairs.

• F is more electronegative that that of H. Therefore, compared to NH₃ , in NF₃ the bond forming lone pair of electron is more shifted to the F atom. This decreases bp – bp repulsion. Therefore, in NF₃ the lp – bp repulsion is more predominating that that of NH₃ and makes F-N-F bond angle 102° compared to H-N-H bond angle 107.3°
• Same (above) argument is valid for having higher H-O-H bond angle in H₂O compared to F-O-F bond angle in OF_2.

 

(iii) Electronpairs in filled shell repel stronger that electron pairs in incomplete shell.

When the electron pair containing atom contains low-energy lying suitable vacant orbitals the electron pair of electrons can undergoes diffusion to the vacant orbitals. Hence, the lp – bp repulsion diminishes dramatically and bp – bp repulsion dominates. In this case, the bond angles around the central metal increase.

Example:

• On going from O to Te the size of the central atom increases. Therefore, diffusion is more predominating which further decreases lp – bp repulsion.

 

(IV) Lone pair of electron can be transferred from a filled shell to a energetically suitable empty shell of the other bonded atom.

Due to transfer of lone pair of electron the bond accumulate some sort of double bond character. This enhances the bp – bp repulsion and consequently, causing a bigger bond angle.

Example:

• P and Cl contain low-lying vacant d orbitals. Lone pair of electron from the filled orbitals of F or O can be transferred to the vacant orbital. This causes a partial – multiple bond that enhances the bp – bp repulsion. Hence, the bond angle increases.

 

(V) Multiple bond orbitals repel each other more strongly than single bond orbitals.

Examples:

O = PF₃ (F-P-F bond angle = 103°), while in O = PBr₃ (Br-P-Br bond angle = 108°)