# Shapes of Molecules. Electron Pair Repulsions Electron pairs are negatively charged, they repel each other and therefore like to get as far apart as possible.

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Shapes 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 (CH 4 ), 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 (NH 3 ) and Water (H 2 O) 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. BeCl 2 and CO 2

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. BF 3, 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. PCl 5

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. SF 6

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. NH 4 + AmmoniaNH 2 - TetrahedralPyramidalBent 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’.

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