Molecular Geometry: Electron Groups and Molecular Shapes

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Molecular Geometry and Electron Groups

Introduction

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. The true shape of a molecule is determined by considering the repulsion between lone pairs and surrounding elements, which is explained by the Valence Shell Electron Pair Repulsion (VSEPR) theory. Lone pairs and bonding pairs of electrons around a central atom influence the molecular geometry.

Electron Groups and Molecular Geometry

Two Electron Groups

Central atoms with two electron groups (bonding or lone pairs) have only one possible molecular geometry. Electron groups are defined as regions of electron density (bonds or lone pairs) around the central atom.

Electron Groups	Bonding Groups	Lone Pairs	Shapes	Visual	Molecular Geometry
2	2	0	BeCl2, CO2, HCN	Linear arrangement	linear

Linear Geometry: Atoms are arranged in a straight line with a bond angle of 180°.
Example: CO2 (carbon dioxide), BeCl2 (beryllium chloride)

Three Electron Groups

Central atoms with three electron groups can have zero or one lone pair, resulting in two possible molecular geometries.

Electron Groups	Bonding Groups	Lone Pairs	Shapes	Visual	Molecular Geometry
3	3	0	BF3	Trigonal planar	trigonal planar
3	2	1	SO2	Bent (V-shaped)	bent / v-shaped / angular

Trigonal Planar Geometry: Three atoms are arranged around the central atom in a flat triangle with 120° bond angles.
Bent (Angular) Geometry: Two atoms and one lone pair create a bent shape with bond angles less than 120°.
Example: BF3 (boron trifluoride), SO2 (sulfur dioxide)

Example Problem

Determine the molecular geometry for the following molecule: BCl3

BCl3 has 3 electron groups (all bonding, no lone pairs).
Geometry: trigonal planar
Calculation: 3 bonding groups, 0 lone pairs

Four Electron Groups

Central atoms with four electron groups can have zero to two lone pairs, resulting in three possible molecular geometries.

Electron Groups	Bonding Groups	Lone Pairs	Shapes	Visual	Molecular Geometry
4	4	0	CH4	Tetrahedral	tetrahedral
4	3	1	NH3	Trigonal pyramidal	trigonal pyramidal
4	2	2	H2O	Bent (V-shaped)	bent / v-shaped / angular

Tetrahedral Geometry: Four atoms arranged around the central atom with bond angles of 109.5°.
Trigonal Pyramidal Geometry: Three atoms and one lone pair form a pyramid-like shape.
Bent (Angular) Geometry: Two atoms and two lone pairs create a bent shape.
Example: CH4 (methane), NH3 (ammonia), H2O (water)

Example Problem

Determine the molecular geometry for the following ion: NH4+

NH4+ has 4 electron groups (all bonding, no lone pairs).
Geometry: tetrahedral

Practice Problem

Determine the molecular geometry for the following molecule: FSSF

FSSF has a specific arrangement of electron groups; students should apply the above tables to determine its geometry.

Key Terms and Concepts

Electron Group: A region of electron density around a central atom, including bonds and lone pairs.
Lone Pair: A pair of valence electrons not involved in bonding.
Bonding Pair: A pair of electrons shared between two atoms in a covalent bond.
VSEPR Theory: Valence Shell Electron Pair Repulsion theory, which predicts molecular shapes based on electron group repulsion.

Summary Table: Electron Groups and Molecular Geometry

Electron Groups	Lone Pairs	Molecular Geometry	Example
2	0	Linear	CO2
3	0	Trigonal planar	BF3
3	1	Bent (Angular)	SO2
4	0	Tetrahedral	CH4
4	1	Trigonal pyramidal	NH3
4	2	Bent (Angular)	H2O

Formulas and Equations

General formula for electron groups:

Bond Angles:
Linear:
Trigonal planar:
Tetrahedral:

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