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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.

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Presentation on theme: "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."— Presentation transcript:

1 Shapes of Molecules

2 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?

3 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:

4 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.

5 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?

6 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

7 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

8 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

9 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

10 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

11 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

12 Now you try… Now have a go at 3.2 Problems 1-3 before attempting the ‘Balloon Challenge’.

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