Shapes of MoleculesShapes of MoleculesShapes of MoleculesShapes of Molecules

Electron Pair Repulsions

• Electron pairs are negatively charged, they repel each other and therefore like to get as far apart as possible.

• Using molymod kits build a structure to represent methane (CH4), what do you think is the angle between the bonding pairs?

Tetrahedral• When there are 4 bonding pairs

(BPs) of electrons around the central atom they adopt a ‘Tetrahedral’ shape.

• The bond angle is approximately 109◦

• This is represented by:

Drawing 3D Structures

» Represents a bond in the plane of the paper.

» Represents a bond in a direction behind the plane of the paper.

» Represents a bond in a direction in front of the plane of the paper.

What about lone pairs?

• Lone pairs also repel against the bonding pairs. In fact their repulsion is even stronger!

• Try making structures for Ammonia (NH3) and Water (H2O)

• How would you describe the shapes and bond angles?

Water and Ammonia• Ammonia’s 3

bonding pairs and lone pair adopt similar positions to 4 bonding pairs in methane.

• Bond Angle = ~109◦

• Name = Pyramidal

• Water has 2 lone pairs and 2 boning pairs and again they adopt positions similar to methane.

• Bond Angle = ~109◦

• Name = Bent

Linear Molecules• When there are only two groups around

the central atom they will get as far apart as possible.

• Name = Linear

• Bond Angle = 180◦

• E.g. BeCl2 and CO2

Planar Molecules• When there are three groups of electrons

around the central atom the furthest they can get apart is 120◦

• Name = Trigonal Planar

• Bond angle = 120◦

• E.g. BF3, methanal and ethene

Trigonal Bipyramidal Molecules

• This is where there are 5 groups of bonding electrons. It is like trigonal planar but with two extra groups of electrons occupying spaces above and below the plane.

• Name = Trigonal bipyramidal

• Bond angle = 120° or 90 °

• E.g. PCl5

Octahedral• This is where 6 groups of bonding

electrons surround a central atom, it is similar to trigonal bipyramidal apart from having an extra group of electrons in the central plane.

• Name = Octahedral

• Bond Angle = 90°

• E.g. SF6

Shapes of ions• You can use the same principles to

determine the shapes of ions by looking at how many electrons have been gained or lost.

• E.g. NH4+ Ammonia

NH2-

Tetrahedral Pyramidal Bent 4xBP 3xBP and 1xLP 2xBP and 2xLP

Now you try…

• Now have a go at 3.2 Problems 1-3 before attempting the ‘Balloon Challenge’.