3.1.3.5 Shapes of simple molecules and ions

How are the shapes of molecules or ions determined?

Governed by the arrangement of electron pairs around the central atom.

Two types of electron pairs

Bonded pairs

Lone pairs

Bonded pairs

A pair of electrons that are shared in covalent bonds.

Lone pairs

A pair of electrons which are unshared.

Valence Shell Electron Theory

Pairs of electrons in the outershell arrange themselves as far apart as possible to minimise replusion.

Lone pairs held closer to the central atom have a greater repulsive effect on other pairs of electrons.

This means that lone pair to lone pair repulsion is greater than lone pair to bonded pair repulsion which is greater than bonded pair to bonded pair repulsion.

3 things which determine molecular/ion shape?

Take the shape which minimises electron pair repulsions.

Determined by:

Total amount of electron pairs around the central atom.

Amount of bonded pairs.

Amount of lone pairs.

What does a wedge mean?

Shows the bond is pointing towards you to imitate a 3D drawing of the molecule.

What does a broken line mean?

Shows the bond is pointing away from you to imitate a 3D drawing of the molecule.

Explain the hape of molecules.

There are (no.) of bonded pairs of electrons around the central atom and (no.) of lone pairs.

They repel equally resulting in a (shape name) with bond angle (angle)/ lone pairs repel more than bonded pairs so squeeze the bonds closer.

Rule for change of bond angle affected by lone pairs

-2.5° for every lone pair of electrons there are.

2 bonded pairs + 0 lone pairs

Linear shape

180°

3 bonded pairs + no lone pairs

Repel equally

Triagonal planar

120°

2 bonded pairs + 1 lone pair

Lone pair repulsions are greater than bonded pair repulsions + squeezes bond together

Bent

118°

4 bonded pairs + 0 lone pairs

Repel equally

Tetrahedral shape

109.5°

3 bonded pairs + 1 lone pair

Lone pair repulsions are greater than bonded pair repulsions + squeezes bond together

Triagonal pyramid

107°

2 bonded pairs + 2 lone pairs

Lone pair repulsions are greater than bonded pair repulsions + squeezes bond together

Bent

104.5°

5 bonded pairs + 0 lone pairs

Repel equally

Triagonal bipyramid

90° and 120°

4 bonded pairs + 1 lone pair

Lone pair repulsions are greater than bonded pair repulsions + squeezes bond together

See-saw shape (the lone pair is always on the top)

87.5° and 117.5°

3 bonded pairs + 2 lone pairs

Lone pair repulsions are greater than bonded pair repulsions + squeezes bond together

T- shape (the lone pair is always on the top)

85° and 115°

6 bonded pairs + 0 lone pairs

Repel equally

Octohedral

90°

5 bonded pairs + 1 lone pair

Lone pair repulsions are greater than bonded pair repulsions + squeezes bond together

Square pyramidal (the lone pair is always on the top/bottom)

87.5°

4 bonded pairs + 2 lone pairs

Lone pair repulsions are greater than bonded pair repulsions + squeezes bond together

Square planar (the lone pairs is always on the top and bottom)

90°

How to explain the shape of molecules

State the number of bonded pairs and lone pairs.

Electrons pairs repel and try to get as far apart as possible in a position of minimum repulsion.

If no lone pairs, electron pairs repel equally.

If there are lone pairs of electrons, then state that lone pairs repel more than bonding pairs.

State actual shape and bond angles.