shapes of molecules. electron pair repulsions electron pairs are negatively charged, they repel each...
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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’.