VSEPR THEORY Molecules are not flat entities like we have drawn them so far they have 3D shapes Shape is actually very important for chemistry function Shape can be represented in drawings Molecular shapes can be predicted using the Valence Shell Electron Pair Repulsion VSEPR model Each electron pair is considered a charge cloud and the charge clouds repel each other Applies to both bonding pairs and lone pairs The electron repulsion pushes the charge clouds as far away from each other as possible Molecular shape depends on the number of charge clouds Also depends on number of lone and bonding pairs Step 1 Write an electron dot structures and count the number of electron charge clouds surrounding the atom of interest Note that multiple bonds are counted as single charge cloud Molecules with 2 charge clouds are always 180 degrees linear Molecules that are trigonal planar have angles of 120 degrees Molecules with 3 charge clouds are trigonal planar or bent 120 degrees Tetrahedron has 4 charge clouds and are approximately 109 5 degrees apart 4 charge clouds Can either be tetrahedral trigonal planar or bent Methane is tetrahedral with bond angle of 109 5 degrees Ammonia is trigonal pyramidal with bond angle of 107 degrees Water is bent with bond angle of 104 5 degrees 5 charge clouds 0 pairs of lone electrons trigonal bipyramidal Multiple bond angles 90 degrees 120 degrees 180 degrees Taking away one bond 1 pair of lone electrons seesaw Taking away two bonds 2 pairs of lone electrons T shape 180 90 degrees Taking away three bonds 3 pairs of lone electrons linear 6 charge clouds Octahedral 180 degree axis 90 degrees all around SF6 is octahedral If it has 6 in it it is octahedral DONT NEED TO DRAW IT Taking away one bond 1 pair of lone electrons square pyramidal Taking away two bonds 2 pair of lone electrons square planar The rules stated above can also be applied to individual atoms in larger molecules There is simple more than one atom attached through a bond The Cambridge Crystallographic Data Centre contains structures for a lot of structures Some of the ions are part of the compounds are the ones we have looked at in this class It is FREE Ex CH3Cl tetrahedral because there are 4 bonding pairs and 0 nonbonding pairs If it is a positive or negative ion make sure that the charge is indicated with brackets Ex IF4 sqaure planar
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