VSEPR Theory: Ideal and Non-Ideal Molecular Geometries

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Ideal Molecular Geometries

Introduction to VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the shapes of molecules based on the repulsion between electron pairs around a central atom. The ideal geometries are determined by the number of terminal atoms and electron domains.

Electron domains include both bonding pairs and lone pairs of electrons.
Terminal atoms are atoms directly bonded to the central atom.

Linear Geometry: Two Terminal Atoms

When a central atom is bonded to two terminal atoms and has no lone pairs, the molecule adopts a linear geometry.

Example: BeH2
Bond angle: 180°
Lewis structure: H–Be–H

Trigonal Planar Geometry: Three Terminal Atoms

With three terminal atoms and no lone pairs, the molecule forms a trigonal planar geometry.

Example: BF3
Bond angle: 120°
Lewis structure: F–B–F (all in one plane)

Tetrahedral Geometry: Four Terminal Atoms

Four terminal atoms around a central atom with no lone pairs result in a tetrahedral geometry.

Example: CH4
Bond angle: 109.5°
Lewis structure: H–C–H (three-dimensional arrangement)

Trigonal Bipyramidal Geometry: Five Terminal Atoms

Five terminal atoms around a central atom form a trigonal bipyramidal geometry.

Example: PCl5
Bond angles: 90°, 120°, and 180°
Lewis structure: Three atoms in a plane (equatorial), two above and below (axial)

Octahedral Geometry: Six Terminal Atoms

Six terminal atoms around a central atom result in an octahedral geometry.

Example: SF6
Bond angle: 90°
Lewis structure: All six atoms symmetrically arranged around the central atom

Effect of Multiple Bonds

Multiple bonds (double or triple) affect the electron domain count but do not change the basic geometry predicted by VSEPR theory.

Example: CO2 (linear geometry due to two double bonds)

Predicting Geometry: Example Problem

To predict the geometry around the central atom for H2CO (formaldehyde):

Count electron domains: 3 (two single bonds to H, one double bond to O)
Predicted geometry: Trigonal planar

Carbon Structures: Alkanes, Cycloalkanes, and Aromatics

Carbon can form various structures, each with characteristic geometries:

Alkanes: Linear or branched chains, tetrahedral geometry at each carbon
Cycloalkanes: Ring structures, tetrahedral geometry at each carbon
Aromatic compounds: Planar ring structures, trigonal planar geometry at each carbon

Allotropes of Carbon

Carbon exists in several allotropes, each with unique molecular geometries:

Diamond: 3D tetrahedral network
Graphite: Planar sheets of trigonal planar carbon atoms
Fullerene (C60): Spherical structure composed of pentagons and hexagons

Non-Ideal Molecular Geometries

Introduction to Non-Ideal Geometries

Non-ideal geometries occur when lone pairs are present on the central atom, causing deviations from ideal bond angles due to increased electron repulsion.

Trigonal Pyramidal Geometry: NH3

Ammonia has three bonding pairs and one lone pair, resulting in a trigonal pyramidal geometry.

Example: NH3
Bond angle: ~107° (less than 109.5° due to lone pair repulsion)
Lone pair: Occupies more space, compressing bond angles

Bent Geometry: H2O

Water has two bonding pairs and two lone pairs, resulting in a bent geometry.

Example: H2O
Bond angle: ~104.5° (less than 109.5° due to two lone pairs)

Predicting Geometry: Example Problem

To predict the geometry around the central atom for H2S:

Count electron domains: 4 (two bonding pairs, two lone pairs)
Predicted geometry: Bent

Summary Table: Ideal VSEPR Geometries

Electron Domains	Terminal Atoms	Example	Geometry	Bond Angle
2	2	BeH2	Linear	180°
3	3	BF3	Trigonal Planar	120°
4	4	CH4	Tetrahedral	109.5°
5	5	PCl5	Trigonal Bipyramidal	90°, 120°, 180°
6	6	SF6	Octahedral	90°

Key Equations

Electron Domain Count:

Additional info:

Allotropes of carbon (diamond, graphite, fullerene) are included for context on molecular geometry in extended structures.
Bond angles are idealized; real molecules may deviate due to lone pairs or multiple bonds.