Molecular Shapes, VSEPR Theory, and Hybridization: A Study Guide

Chemical Bonding II: Molecular Shapes, VSEPR Theory, and Hybridization

Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

VSEPR theory is a model used to predict the geometry of individual molecules based on the repulsion between electron groups around a central atom. Electron groups include bonds (single, double, or triple), lone pairs, and single electrons. The arrangement of these groups determines the overall shape of the molecule.

• Electron Group: Any region of electron density (bond or lone pair) around a central atom.

• Basic Principle: Electron groups repel each other and arrange themselves as far apart as possible.

Five Basic Electron Geometries

The five fundamental arrangements of electron groups around a central atom are:

• Linear (2 groups, 180°)

• Trigonal Planar (3 groups, 120°)

• Tetrahedral (4 groups, 109.5°)

• Trigonal Bipyramidal (5 groups, 120° and 90°)

• Octahedral (6 groups, 90°)

Electron-Group Arrangement vs. Molecular Shape

The electron-group arrangement considers all electron groups (bonding and nonbonding), while the molecular geometry considers only the positions of the atoms (bonding groups). The notation AXmEn is used, where A is the central atom, X is a surrounding atom, and E is a nonbonding electron group.

Effect of Lone Pairs and Multiple Bonds

• Lone pairs repel more strongly than bonding pairs, reducing bond angles between atoms.

• Double and triple bonds have greater electron density and also cause bond angle distortions.

Common Molecular Geometries and Bond Angles

Electron Groups	Bonding Groups	Lone Pairs	Electron Geometry	Molecular Geometry	Bond Angles
2	2	0	Linear	Linear	180°
3	3	0	Trigonal Planar	Trigonal Planar	120°
3	2	1	Trigonal Planar	Bent	<120°
4	4	0	Tetrahedral	Tetrahedral	109.5°
4	3	1	Tetrahedral	Trigonal Pyramidal	<109.5°
4	2	2	Tetrahedral	Bent	<109.5°
5	5	0	Trigonal Bipyramidal	Trigonal Bipyramidal	120°, 90°
6	6	0	Octahedral	Octahedral	90°

Examples of Molecular Shapes

• CO2: Linear (AX2), 180°

• BF3: Trigonal planar (AX3), 120°

• CH4: Tetrahedral (AX4), 109.5°

• NH3: Trigonal pyramidal (AX3E), <109.5°

• H2O: Bent (AX2E2), <109.5°

Representing 3D Shapes on Paper

To depict three-dimensional molecular shapes, use:

• Straight lines for bonds in the plane

• Solid wedges for bonds coming out of the plane

• Hashed wedges for bonds going into the plane

Molecular Shape and Polarity

A molecule is polar if it contains polar bonds and the bond dipoles do not cancel due to the molecular shape. The overall polarity is measured by the dipole moment (μ, in debye).

• CO2: Nonpolar (linear, dipoles cancel)

• H2O: Polar (bent, dipoles do not cancel)

Valence Bond Theory and Hybridization

Valence Bond (VB) Theory explains bonding as the overlap of atomic orbitals. Hybridization describes the mixing of atomic orbitals to form new, equivalent hybrid orbitals that match observed molecular shapes.

• sp: Linear (2 groups, 180°)

• sp2: Trigonal planar (3 groups, 120°)

• sp3: Tetrahedral (4 groups, 109.5°)

• sp3d: Trigonal bipyramidal (5 groups)

• sp3d2: Octahedral (6 groups)

Types of Covalent Bonds

• Sigma (σ) bond: End-to-end overlap, all single bonds are σ bonds.

• Pi (π) bond: Sideways overlap, present in double (one σ, one π) and triple bonds (one σ, two π).

Summary Table: Electron and Molecular Geometries

Electron Groups	Bonding Groups	Lone Pairs	Electron Geometry	Molecular Geometry	Bond Angles
2	2	0	Linear	Linear	180°
3	3	0	Trigonal Planar	Trigonal Planar	120°
3	2	1	Trigonal Planar	Bent	<120°
4	4	0	Tetrahedral	Tetrahedral	109.5°
4	3	1	Tetrahedral	Trigonal Pyramidal	<109.5°
4	2	2	Tetrahedral	Bent	<109.5°
5	5	0	Trigonal Bipyramidal	Trigonal Bipyramidal	120°, 90°
6	6	0	Octahedral	Octahedral	90°

Example: Methanol (CH3OH)

• The C atom is sp3 hybridized (tetrahedral geometry).

• The O atom is also sp3 hybridized (bent geometry for the O-H group).

Example: Acetone ((CH3)2CO)

• Each CH3 group: C is sp3 hybridized (tetrahedral).

• Central C (in CO): sp2 hybridized (trigonal planar).

Practice Problems

• Predict the electron geometry, molecular geometry, and bond angles for NF3, OF2, BrF5, IF4+, IF2-.

• Determine the polarity of SiCl4, IF5, CHCl3, OF2, I3-, BCl3, SeF6.

Key Equations

• Bond Angle (Linear): $180^\circ$

• Bond Angle (Trigonal Planar): $120^\circ$

• Bond Angle (Tetrahedral): $109.5^\circ$

• Bond Angle (Trigonal Bipyramidal): $120^\circ, 90^\circ$

• Bond Angle (Octahedral): $90^\circ$

Additional info: For more complex molecules with multiple central atoms, describe the geometry around each central atom separately. The overall molecular shape is a combination of these local geometries.