The Shapes of Simple Molecules & Ions

What determines the shape of a molecule or ion

Number of electron pairs around the central atom and the repulsion between them

Why do electron pairs repel

Like charges repel, maximising bond angles

Effect of lone pairs on bond angles

Lone pairs create extra repulsion, reducing bond angles by ~2.5° per lone pair

How to interpret solid, wedged and dotted lines in diagrams

Solid line = bond in plane, wedged = bond out of plane, dotted = bond into plane

Steps to determine molecular shape

1. Count total electron pairs 2. Identify bonding and lone pairs 3. Bonding pairs give basic shape, lone pairs adjust angles

Bonding pairs and lone pairs in linear molecule

2 bonding, 0 lone

Bond angle in linear molecule

180°

Example of linear molecule

Cl-Be-Cl

Bonding and lone pairs in bent molecule

2 bonding, 2 lone

Bond angle in bent molecule

104.5°

Example of bent molecule

H2O

Bonding and lone pairs in trigonal planar molecule

3 bonding, 0 lone

Bond angle in trigonal planar molecule

120°

Example of trigonal planar molecule

BF3

Bonding and lone pairs in trigonal pyramid molecule

3 bonding, 1 lone

Bond angle in trigonal pyramid molecule

107°

Example of trigonal pyramid molecule

NH3

Bonding and lone pairs in tetrahedral molecule

4 bonding, 0 lone

Bond angle in tetrahedral molecule

109.5°

Example of tetrahedral molecule

CH4

Bonding and lone pairs in trigonal bipyramidal molecule

5 bonding, 0 lone

Bond angles in trigonal bipyramidal molecule

120° and 90°

Example of trigonal bipyramidal molecule

PCl5

Bonding and lone pairs in octahedral molecule

6 bonding, 0 lone

Bond angle in octahedral molecule

90°

Example of octahedral molecule

SF6