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2.5.1 Shapes of Molecules
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Valence Shell Electron Pair Repulsion Theory (VSEPR)
Electrons can be placed in two categories Bonding pairs [involved in a bond] Lone pairs [not involved in a bond]
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N NH3 Ammonia N has Atomic Number 7 Lone Pair Electron pattern 2,5
Bonding Pairs
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Bonding pairs Involved in a bond
usually one e- from each atom in the bond Dative bond is a special type of covalent bond where one atom supplies both the electrons. This is unusual. The electrons in bonding pairs have opposite spins this is why they can come together. Single, double and triple bonds are all regarded as just one bonding pair
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C Methane CH4 Tetrahedral Bond Angle HCH =109.50 All repel equally
4 bonding pairs All repel equally Move as far apart as possible Note they are NOT on the equator but one at the pole and three below the equator Bonding Pairs H
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Lone Pairs These are pairs of electrons not involved in a bond.
Lone pairs have a greater power of repulsion than bonding pairs. They push other electron pairs further away from themselves than bonding pairs do.
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Repulsion Lone Pair Lone Pair Lone Pair Bonding Pair Bonding Pair
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Ammonia NH3 3 bonding : 1 lone H N
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In each three dimensional representation methane will be left in the top left hand corner for purposes of comparison
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Pyramidal Methane Bond angle 109.50 Tetrahedral Lone Pair Repels more
Group V element at centre
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F BF3 B F F
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F BCl3 3 bonding pairs B F F
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Planar or Trigonal Planar Bond Angle = 120o
Group III element at centre Planar or Trigonal Planar Top View Side View Bond Angle = 120o
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3 bonding 0 lone Trigonal Planar O H C H Bonding Pairs HCHO methanal
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H2O Angular O H H
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Water H2O O Methane Bond angle 109.50 Tetrahedral
2 lone pairs push hard bond angle = 104.5 O H V shaped or Angular
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BeCl2 2 bonding Pairs Beryllium Atomic Number 4 Electron pattern 2,2
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BeCl2 2 bonding Pairs Bond Angle = 180o Be Cl Cl Linear
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2 bonding pairs CO2 O O C Bond Angle 180o O = C = O Linear
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Other examples of shapes
Tetrahedral [5atoms] CCl CHCl3 SiCl4 CF4 NH4+ Pyramidal [4 atoms] Trigonal [4 atoms] Has lone pair AlH3 HCHO BF3 BCl3 H3O PH3 NH3 Angular [3 atoms] Linear [ 3 atoms] Has 2 lone pairs BeH2 CO [C2H2] H2S H2O SO2
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![Shapes of Molecules. Valence Shell Electron Pair Repulsion Theory (VSEPR) Electrons can be placed in two categories Bonding pairs [involved in a bond]](/32/10063319/big_thumb.jpg "Shapes of Molecules. Valence Shell Electron Pair Repulsion Theory (VSEPR) Electrons can be placed in two categories Bonding pairs [involved in a bond]>")
