Shapes of molecules and ions

• Electron repulsion theory

  The shape of a molecule is determined by the electron pairs surrounding the central atom. this is only relevant for the outer shell
• linear structure 


  • two bonding areas and no lone pairs.
  • With the bond angle of 180
  • example - CO2

  
• trigonal planer 


  • 3 bonding pairs and no lone electrons
  • bond angle 120
  • BF3

  
• tetrahedral molecule 


  • 4 bonding Pairs and no lone electrons
  • bond angle of 109.5
  • example - methane

  
• trigonal bipyramidal


  when there are five bonding pairs to minimise repulsion two of the pairs have to move to opposite sides of the molecule

  • 5 bonding pairs and lone electrons
  • bond angle 90/120
  • example -PF5

  
• octahedral 


  • 6 bonding pairs and no lone electrons
  • bond angle of 90
  • example - SF6

  
• What effect do lone pairs have on bond angles compared to bonding pairs?


  Lone pairs repel more strongly than bonding pairs
• How much do lone pairs decrease other bond angles?

  Lone pairs decrease other bond angles by 2.5 degrees
• pyramidal 


  • 3 bonding pairs and 1 lone pair
  • bond angle of 107
  • example - NH3
• non linear 1 


  • 2 bonding pairs and 1 lone pair
  • bond angle of 117.5
  • 120 - 2.5 =117.5
• non linear 2

  • two bonding pairs and two lone pairs
  • bond angle of 104.5
  • 109.5 - (2x2.5) =104.5