The electron pair repulsion theory in relation to
The electron pair repusion theory- mutual repulsion between electrons around an atom resulting in the shape that minimises repulsion in covalent and ionic molecules.
A lone pair is (slightly) more repulsive than a bonding pair.
- Trigonal Planar, like a triangle. Bond angle 120'
- Tetrahedral, like a triangular based pyramid. Bond angle 109.5'
- Trigonal bipyramidal, like two triangular based pyramids joined at the base. Bond angle 90' and 120'
- Octahedral. like two suare based pyramids joined at the base. Bond anlgle 90' eg Sf6
Shapes of some simple and organic molecules
Shapes of molecules can all be explained using the EPRT:
- BeCl2 and CO2 are linear
- BCl3 is trigonal planar
- CH4 is tetrahedral
- NH3 is tetrahedral (lone pair)
- NH4+ is tetrahedral (with a +1 charge)
- H2O is bent linear (two lone pairs)
All the C atoms have a tetrahedral arrangement around them in the Alkanes and Halogenoalkanes.
In an Alkene eg Ethene, the two C atoms have 3 bonds arranged in the same plane at an angle of about 120'
In Alcohols, the O has 2 lone pairs like in H2O, so bent linear C--O--H
In a Carboxylic acid the --COOH carboxyl group is planar.
In Aldehydes and Ketones the Carbonyl group C=O is planar with 120' bond angles.
Carbon and its Allotropes- Diamond and Graphite
Diamond is a giant covalently bonded molecule mad of interlocking tetrahedral units. Each C atom is bonded to 4 others; there is uniform bonding.
- Hardest material known to man, so used in drill tips/ cutting instruments
- Inert, but will burn in air at VERY high temperatures.
- Pure Diamond is colourless, but contamination by different minerals can cause different colours
- Have been manufactured artificially since 1955
Graphite is composed of planes of trigonally bonded C atoms. Each C atom is bonded to 3 others. Weak london forces hold the layers together.
- Conducts electricity and heat
- Used in electrodes
- Used as a lubricant and in pencils as layers slip over each other
- Used in Nuclear reactors to absorb some energy from neutrons which increases their ability to cause fission.
Carbon and its Allotrpes- Amorphous carbon, Fuller
Amorphous Carbon is less well defined than Diamond or Graphite- black powder that is a componant of coal and occurs as soot,
- Most reactive form of Carbon- used as fuel
- Used as a filler in rubber/plastics
- Black pigment in paint
Fullerenes are spherical. The first is 'Buckminsterfullerene' which consists of 60 C atoms and has 32 sides.
- Black solid that dissolves in petrol to form a red solution
- Can conduct electricity
Nanotubes are elongated cage-like structures. Very complex shapes can be produced eg doughnut and corkscrew shapes.
- All contain 12 five membered rings and any number of six membered rings
- Nanotechology can be used in: cosmetics, future nanorobots used in medicine...