VSEPR and Molecular Geometry
Predicting shapes from bond pairs and lone pairs. (Chemistry › Chemical Bonding and Molecular Structure, NEET UG syllabus.)
What is VSEPR and Molecular Geometry?
Valence Shell Electron Pair Repulsion theory, a model used to predict the geometry of molecules based on minimizing repulsion between electron pairs around the central atom.
Key formula / rule: Steric Number (Number of Electron Domains)
Key points
- Draw accurate Lewis structures for simple molecules and polyatomic ions.
- Determine the number of electron domains around a central atom.
- Predict the electron domain geometry based on the number of electron domains.
- Identify the number of lone pairs and bond pairs on the central atom.
Common exam trap
Confusing electron geometry with molecular geometry.
Definitions
- Term
VSEPR Theory
- Meaning
Valence Shell Electron Pair Repulsion theory, a model used to predict the geometry of molecules based on minimizing repulsion between electron pairs around the central atom.
- Term
Electron Domain
- Meaning
A region in space around a central atom where electrons are concentrated. It can be a single bond, a multiple bond (double or triple), or a lone pair of electrons.
- Term
Electron Geometry
- Meaning
The arrangement of all electron domains (bonding and non-bonding) around the central atom, determined solely by the steric number.
- Term
Molecular Geometry
- Meaning
The three-dimensional arrangement of only the atoms in a molecule, determined by the positions of the bonding electron domains (atoms) and influenced by the repulsive forces of lone pairs.
- Term
Steric Number
- Meaning
The total number of electron domains (bond pairs + lone pairs) around the central atom in a molecule.
Learning objectives
Draw accurate Lewis structures for simple molecules and polyatomic ions.
Determine the number of electron domains around a central atom.
Predict the electron domain geometry based on the number of electron domains.
Identify the number of lone pairs and bond pairs on the central atom.
Predict the molecular geometry of a molecule using VSEPR theory.
Explain how lone pairs influence bond angles and molecular shape.
Relate molecular geometry to the polarity of a molecule (though not directly covered, it's a subsequent application).
Formulae
- Name
Steric Number (Number of Electron Domains)
- Note
This formula helps determine the electron geometry. Multiple bonds to an atom count as one 'bonded atom' for this calculation if considering the number of atoms, but as one electron domain for VSEPR.
- Expression
Steric Number = (Number of atoms bonded to central atom) + (Number of lone pairs on central atom)
Prerequisites
Ability to draw correct Lewis structures for molecules and polyatomic ions.
Understanding of covalent bonding (single, double, triple bonds).
Knowledge of octet rule and exceptions to it.
Basic understanding of electron pairs (bonding and non-bonding).
Common mistakes
Confusing electron geometry with molecular geometry.
Incorrectly counting electron domains (e.g., counting a double bond as two domains).
Ignoring the presence of lone pairs on the central atom when determining molecular geometry.
Not accounting for the greater repulsive force of lone pairs, leading to incorrect bond angles.
Incorrectly drawing Lewis structures, which is the foundation for VSEPR.
Keywords
VSEPR
Molecular Geometry
Electron Geometry
Lone Pair
Bond Pair
Steric Number
Repulsion
Lewis Structure
Bond Angle
Shape
Practice preview
What is the fundamental principle of the Valence Shell Electron Pair Repulsion (VSEPR) theory?…
easy
What is the molecular shape of a water molecule (H2O)?…
easy
Which of the following pairs of molecules has the same molecular geometry?…
medium