VSEPR Theory | Chemical Bonding Class 11

VSEPR Theory

Valence Shell Electron Pair Repulsion Theory | Molecular Shapes

1. Introduction & Postulates

Proposed by Sidgwick and Powell (1940) and developed by Nyholm and Gillespie. It predicts the shape of covalent molecules based on the repulsion between electron pairs in the valence shell of the central atom.

Main Principle: Electron pairs (bonded or lone) repel each other and try to stay as far apart as possible to minimize repulsion and maximize stability.

2. Order of Repulsion

Lone pairs (lp) are localized on the central atom and occupy more space than bond pairs (bp), which are shared between two nuclei.

$$lp - lp > lp - bp > bp - bp$$

This repulsion causes distortion in the regular geometry, changing the bond angles.

3. Geometry vs Shape

• Geometry: Arrangement of all electron pairs (Bond pairs + Lone pairs).
• Shape: Arrangement of atoms only (ignoring lone pairs positions).

4. Regular Geometries (No Lone Pairs)

Type	Pairs	Geometry/Shape	Angle	Example
$AB_2$	2	Linear	$180^\circ$	$BeCl_2, CO_2$
$AB_3$	3	Trigonal Planar	$120^\circ$	$BF_3$
$AB_4$	4	Tetrahedral	$109.5^\circ$	$CH_4$
$AB_5$	5	Trigonal Bipyramidal	$90^\circ, 120^\circ$	$PCl_5$
$AB_6$	6	Octahedral	$90^\circ$	$SF_6$

5. Effect of Lone Pairs (Distorted Shapes)

When lone pairs are present, the shape deviates from the regular geometry.

Molecule	Pairs ($bp + lp$)	Geometry	Shape	Angle
$NH_3$	$3 + 1 = 4$	Tetrahedral	Pyramidal	$107^\circ$
$H_2O$	$2 + 2 = 4$	Tetrahedral	Bent / V-shape	$104.5^\circ$
$SF_4$	$4 + 1 = 5$	Trig. Bipyramidal	See-Saw	$< 120^\circ$
$ClF_3$	$3 + 2 = 5$	Trig. Bipyramidal	T-Shape	$< 90^\circ$
$XeF_4$	$4 + 2 = 6$	Octahedral	Square Planar	$90^\circ$

Rule for $AB_5$ type: Lone pairs always occupy Equatorial positions to minimize repulsion.

Practice Quiz

Test your ability to predict shapes.

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