Shapes of Molecules and Ions

Condensed notes for Edexcel unit 2.3

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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.

Possible shapes:

  • 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 
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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. 

 

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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.
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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...
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