Valence Shell Electron Pair Repulsion Theory (VSEPR)

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How is VSEPR theory used to predict molecular structure?

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Presentation transcript:

Valence Shell Electron Pair Repulsion Theory (VSEPR) Shapes of Molecules 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

N NH3 Ammonia N has Atomic Number 7 Lone Pair Electron pattern 2,5 Bonding Pairs H

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

C Methane CH4 Tetrahedral Bond Angle HCH =109.50 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

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.

Repulsion Lone Pair Lone Pair Lone Pair Bonding Pair Bonding Pair

Ammonia NH3 3 bonding : 1 lone H N

In the following slides a three dimensional representation methane will be left in the top left hand corner for purposes of comparison

Methane Bond angle 109.50 Tetrahedral Lone Pair Repels more H N

Pyramidal Methane Bond angle 109.50 Tetrahedral Lone Pair Repels more Group V element at centre

F BF3 B F F

F BF3 B F F

F BF3 Does not obey the octet rule 3 bonding pairs B F F

Planar or Trigonal Planar Bond Angle = 120o Top View Side View Group III element at centre

C 3 bonding 0 lone Trigonal Planar O Bonding Pairs HCHO methanal One bonding pair Bonding Pairs HCHO methanal

Water H2O O H H

Methane Bond angle 109.50 Tetrahedral Water H2O O H H

Water H2O O Methane Bond angle 109.50 Tetrahedral 2 lone pairs push hard Bond angle = 104.5 O H H V shaped or Angular

BeCl2 Beryllium Atomic Number 4 Electron pattern 2,2 Cl Cl Be

BeCl2 Cl Cl Be

BeCl2 Cl Cl Be

BeCl2 2 bonding Pairs Cl Cl Be

BeCl2 2 bonding Pairs Bond Angle = 180o Be Cl Cl Linear

2 bonding pairs CO2 C O O = C = O Bond Angle 180o Linear

Examples of Molecular Shapes Tetrahedral [5 atoms] CCl4 CHCl3 SiCl4 CF4 NH4+ Pyramidal [4 atoms] Trigonal [4 atoms] Have 1 lone pair Have 0 lone pairs H3O+ PH3 NH3 AlH3; HCHO; BF3; BCl3 Angular [3 atoms] Linear [ 3 atoms] Have 2 lone pairs H2S H2O SO2 BeH2 CO2 [C2H2]