Prediction of shapes of some molecules and ions by VSEPR rules

Prediction of Shapes of Molecules and Ions Using VSEPR Theory

Introduction: Valence Shell Electron Pair Repulsion (VSEPR) theory helps in predicting the molecular and ionic shapes by considering the repulsions between electron pairs in the valence shell of the central atom. It is based on the principle that electron pairs around a central atom repel each other and try to arrange themselves as far apart as possible to minimize repulsion.

Basic Concept of VSEPR Theory:

The geometry of a molecule is influenced by:

• Bonding electron pairs (shared between atoms)
• Lone pairs, which are valence electrons not involved in bonding and remain on the central atom

The presence of lone pairs affects the shape because lone pair–bond pair and lone pair–lone pair repulsions are stronger than bond pair–bond pair repulsions. This leads to deviations from ideal bond angles.

Key Assumptions of VSEPR Theory:

• The shape of a molecule depends on the number of valence shell electron pairs around the central atom.
• Electron pairs position themselves as far apart as possible to reduce repulsive forces.
• Lone pairs occupy more space than bonding pairs.
• Multiple bonds are treated as a single electron domain for geometry prediction.

Common Molecular Geometries Predicted by VSEPR Theory:

Application to Ions:

• NH₄⁺: The ammonium ion has four bonding pairs and no lone pairs, leading to a tetrahedral shape.
• ClO₃⁻: This ion has three bonding pairs and one lone pair, resulting in a trigonal pyramidal shape.
• SO₄²⁻: With four bonding regions and no lone pairs, the sulfate ion adopts a tetrahedral geometry.

The VSEPR theory effectively determines the geometry of molecules by considering how electron pairs are distributed around the central atom.