Shapes of molecules & ions

Describe diagrams

A normal line shows a bond in the plane of the paper.

• A bold wedge shows the bond coming out of the plane of the paper towards you.

• A dotted wedge shows the bond going into the plane of the paper away from you.

Describe electron-repulsion theory

• Electron clouds are all negatively charged and therefore repel each other.

• Electron pairs repel to a position of maximum separation to avoid repulsion

• The amount of repulsion determines the bond angles and therefore the shape of a molecule/ion.

• Lone-pair electron clouds occupy more space than bonding pairs, so lone pairs repel more strongly.

Repulsion strength:
Lone pair–Lone pair > Bonding pair–Lone pair > Bonding pair–Bonding pair

Describe molecules with lone pairs

• A lone pair is slightly more electron dense than a bonded pair so a lone pair repels more than a bonded pair

• Each lone pair reduces bond angle by about 2.5 degrees - this is the result of the extra repulsive effect of each lone pair

Which element is the exception to the ‘filling in outer shell’ rule?

Boron

BF3

• 3 electron pairs around the central atom

• Bond angle: 120°

• Shape: Trigonal planar

CO2

• 2 bonding regions

• The two double bonds repel each other and move as far apart as possible.

• Bond angle: 180°

• Shape: Linear

SF6

• 6 electron pairs around the central atom

• Bond angle: 90°

• Shape: Octahedral

CH4

• 4 electron pairs around the central atom

• Bond angle: 109.5°

• Shape: Tetrahedral

Describe molecules with double bonds

Each double bond is treated as a bonded region, in the same way as a bonded pair.

E.g. CO₂ has two double bonds which repel one another to be as far apart as possible, resulting in a linear molecule with a bond angle of 180°.

When is a molecule bent instead of linear?

2 lone pairs