Shapes of molecules

The shapes of molecules are determined by the way clouds of electrons are arranged around the central atom in the molecule.

A molecule containing only a single cloud of electrons must be linear.

H—H

Arrangements of electron clouds

Two clouds arrange themselves on opposite sides of the central atom.

The bond angle will be 180°.

O = C = O

Since clouds of electrons are negatively-charged, they repel each other.

Two clouds

Notice that the double bonds in CO2 each act as a single cloud of electrons.

Adding a third cloud of electrons will change the bond angle from 180° to 120°.

All the atoms still lie on a flat plane (like a sheet of paper). The shape is trigonal planar.

Three clouds

When a fourth cloud is added, the previous clouds are pushed downwards.

This shape is tetrahedral.

The bond angle is now 109°.

HH

H

H

C

Four clouds

Remember this number. A circle divided in 4 makes an angle of 90°, but a sphere divided in 4 makes an angle of 109°.

A fifth cloud enters from below, raising the lower 3 clouds back to a flat triangle.

This shape is trigonal bipyramid.

Bond angles around the central triangle are 120°, while the angle between the triangle and the top and bottom atoms is 90°.

P

ClClCl

Cl

Cl

Five clouds

A sixth cloud squeezes around the centre, making all bond angles 90°.

S

FF

FF

F

F

This shape is octahedral.

Six clouds

Shapes of molecules

The shape of a molecule is determined by the positions of atoms within that molecule.

The position of those atoms is determined by the arrangement of the electron clouds around the central atom. If there are any non-bonding (‘lone pair’) electrons around the central atom, they will squeeze the bonding pairs (with their attached atoms) closer together.Lone pairs are closer to the central atom than bonding pairs (because they are not shared between two nuclei). They push the bonding pairs slightly closer together than if all pairs were bonding. This is known as lone pair repulsion.

Three clouds of electrons around central atom

Trigonal planar

Bent Linear

Bond angle = 120°

Bond angle = 119°

OSO

O SOO

Four clouds of electrons around central atom

H

HH

HCH

HHN

HH

OH

Cl

Bond angle = 109°

Bond angle = 107°

Bond angle = 105°

Tetrahedral Trigonal pyramid

Bent Linear

Five clouds of electrons

When five clouds of electrons are around the central atom there are two different bond angles — 120° and 90°. Any lone pairs will be positioned around the central triangle first, where they can be 120° away from other clouds.

Trigonal bipyramid

See-saw T-shaped Linear

P F

F

F

FF

S F

F

FF

Cl F

F

F

I

I

I

Six clouds of electrons

Octahedral

Square pyramid

Square planar

SFFF

F

F

F

BrFFF

F

F

XeFFF

F

Polarity of molecules

Molecules are polar when there is an uneven distribution of charge throughout the molecule.

Once the shape of a molecule has been determined, we can decide whether it is polar or non-polar.

There are two ways to decide molecular polarity:

1. Look at the location and direction of the bonds within the molecule. Consider each polar bond as a pull force, and decide whether there is a net force on the molecule, or whether those forces are balanced.

2. Consider the charge on each atom of the molecule, and locate the centre of positive charge, and the centre of negative charge. If they are in the same place the molecule is non-polar, but if the centres of positive and negative charge are separated, the molecule is polar.

HH

O

The shape of the water molecule is bent, due to the two lone pairs on the oxygen molecule.

Each H—O bond is polar, because the electronegativity difference between hydrogen (2.1) and oxygen (3.5) is between 0.5 and 1.6. Oxygen is the more electronegative element, so oxygen has a slight negative charge and hydrogen a slight positive charge.

HH

Examples

O

Net dipole

HH

O

δ+

δ–

δ+

–

+Net dipole

Method one Method two

HHN

HH

HNH

δ+

δ–

δ+

–+ δ

+

Polar Polar

P F

F

F

FF Br

FFF

F

F

Polar Non-polar

XeFFF

F

Non-polar

or