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Molecular Geometry VSEPR
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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.
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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).
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Possible 3D Shapes # of groups connected to the central atom
# of lone pairs on the central atom Shape Bond Angles Example - Linear Diatomic HCl 2 Linear Triatomic 180 º CO2 Bent (angular) 104.5 º H2O 3 Trigonal Planar 120 º BF3 1 Trigonal Pyramidal 107 º NH3 4 Tetrahedral 109.5 º CH4, CCl4 5 Trigonal Bi-pyramidal 120 º, 90 º PCl5 6 Octahedral 90 º PCl6-
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Linear Bond angle of 180 º minimizes all e- - e- interactions, giving maximum separation between all substituents (things connected to) on the central atom.
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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.
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Tetrahedral Lattice of Carbon Atoms
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.
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What about Silicon Dioxide?
Quartz (crystalline SiO2) We know it’s NOT linear triatomic, so what’s the deal? Sand (crushed crystalline SiO2)
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Structure and Bonding in Silicon Dioxide
Every Si centre is tetrahedral Every O centre is bent Remarkably strong when a crystalline solid (quartz)
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Structure and Bonding in Silicon Dioxide
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Amorphous and Everywhere
SiO2 Amorphous and Everywhere SiO2 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.
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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.
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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.
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BENT Only 90 º between H and lone pair. 104.5 º between the 2 H atoms
Water V shaped or BENT Derived from a tetrahedral arrangement, with 2 atoms replaced by lone pairs.
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Practice Time What are the shapes and bond angles in each of the molecules from yesterday? HCN, N2H4, CO2, CO, NI3, SCl2, AsCl3, PCl3, CH4, NH4+, HCl, BH3, O3, CCl4, SiCl4, SiO2, CH2Cl2, IF, AlCl3, CH2O, CH2S # of groups connected to the central atom # of lone pairs on the central atom Shape Bond Angles Example - Linear Diatomic HCl 2 Linear Triatomic 180 º CO2 Bent (angular) 104.5 º H2O 3 Trigonal Planar 120 º BF3 1 Trigonal Pyramidal 107 º NH3 4 Tetrahedral 109.5 º CH4, CCl4
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