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Molecular Geometry VSEPR. Molecules in 3D We saw in our last class that to properly address covalent bonding we need to use a pictorial approach, so we.

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Presentation on theme: "Molecular Geometry VSEPR. Molecules in 3D We saw in our last class that to properly address covalent bonding we need to use a pictorial approach, so we."— Presentation transcript:

1 Molecular Geometry VSEPR

2 Molecules in 3D We saw in our last class that to properly address covalent bonding we need to use a pictorial approach, so we turn to Lewis Structures. These are excellent because they give us a great perspective on the electronic arrangement in a molecule. There is a drawback to a basic Lewis diagram and that is we do not get a sense for how the molecule exits in 3D.

3 VSEPR Theory Valence Shell Electron Pair Repulsion Theory... Huh?!?!  Electrons take up space! Electrons are all negatively charged, therefore they don’t want to be near other electrons. Electron pairs will repel other electron pairs. Molecules will orient themselves in 3 dimensions to minimize the interactions of all the electron pairs. A lone pair takes up more space than a bonding pair (ie they push bonding pairs further away).

4 Possible 3D Shapes # of groups connected to the central atom # of lone pairs on the central atom ShapeBond Angles Example --Linear DiatomicHCl 20Linear Triatomic180 ºCO 2 22Bent (angular)104.5 ºH2OH2O 30Trigonal Planar120 ºBF 3 31Trigonal Pyramidal107 ºNH 3 40Tetrahedral109.5 ºCH 4, CCl 4 50Trigonal Bi-pyramidal120 º, 90 ºPCl 5 60Octahedral90 ºPCl 6 -

5 Linear Bond angle of 180 º minimizes all e - - e - interactions, giving maximum separation between all substituents (things connected to) on the central atom.

6 Tetrahedral Bond angle of º minimizes all e - - e - interactions, giving maximum separation between all substituents (things connected to) on the central atom. The tetrahedral geometry is very stable and very strong.

7 Diamond Tetrahedral Lattice of Carbon Atoms A Diamond is the among the “hardest” substances in the known universe. It gains this strength from its molecular geometry, each carbon has a maximum spacing in the tetrahedral lattice, minimizing any destabilizing interactions.

8 What about Silicon Dioxide? Quartz (crystalline SiO 2 ) Sand (crushed crystalline SiO 2 ) We know it’s NOT linear triatomic, so what’s the deal?

9 Structure and Bonding in Silicon Dioxide Every Si centre is tetrahedral Every O centre is bent Remarkably strong when a crystalline solid (quartz)

10 Structure and Bonding in Silicon Dioxide

11 SiO 2 Amorphous and Everywhere SiO 2 is also common glass, it has a structure much different than Quartz. All of the bonds are the same; however typical glass “looks” like a liquid that has been flash frozen. It’s arrangement is irregular and fluid.

12 Tetrahedral Bond angle of º minimizes all e - - e - interactions, giving maximum separation between all substituents (things connected to) on the central atom. The tetrahedral geometry is very stable and very strong.

13 Trigonal Planar and Pyramidal 120 º bond angle between each F atom, maximum separation 107 º bond angle between each F atom, maximum separation Derived from a tetrahedral arrangement, with the “top” atom replaced by a lone pair.

14 BENT Only 90 º between H and lone pair º between the 2 H atoms Water  V shaped or BENT Derived from a tetrahedral arrangement, with 2 atoms replaced by lone pairs.

15 Practice Time What are the shapes and bond angles in each of the molecules from yesterday? HCN, N 2 H 4, CO 2, CO, NI 3, SCl 2, AsCl 3, PCl 3, CH 4, NH 4 +, HCl, BH 3, O 3, CCl 4, SiCl 4, SiO 2, CH 2 Cl 2, IF, AlCl 3, CH 2 O, CH 2 S # of groups connected to the central atom # of lone pairs on the central atom ShapeBond Angles Example --Linear DiatomicHCl 20Linear Triatomic180 ºCO 2 22Bent (angular)104.5 ºH2OH2O 30Trigonal Planar120 ºBF 3 31Trigonal Pyramidal107 ºNH 3 40Tetrahedral109.5 ºCH 4, CCl 4


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