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Page 1: 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

Shapes of MoleculesShapes of MoleculesShapes of MoleculesShapes of Molecules

Page 2: 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

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?

Page 3: 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

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:

Page 4: 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

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.

Page 5: 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

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?

Page 6: 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

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

Page 7: 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

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

Page 8: 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

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

Page 9: 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

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

Page 10: 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

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

Page 11: 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

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

Page 12: 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

Now you try…

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