Valence Shell Electron Pair Repulsion (VSEPR) Theory Chemistry 11 November 7, 2008

Vsepr Theory Remember… What is it? Model developed to predict the shapes of molecules Remember… Atoms are bound together by electron pairs called bonding pairs These can be single (one pair e- = single bond) or multiple (2 pair e- = double bond; 3 pair e- = triple bond) Some atoms in a molecule can also have pairs of electrons not involved in bonding called lone pairs or non-bonded pairs

VSEPR Theory Why are we concerned with the shape of molecules? Will help us later when we discuss polarity What causes molecules to take on a different shape? Electron pairs (bonding pairs and lone pairs) are negatively charged and will repel each other These pairs of electrons tend to occupy positions around the atom that minimize repulsions and maximize the distance of separation between them

Three types of repulsions take place in an atom Lone Pair – Lone Pair (LP-LP) Lone Pair – Bonding Pair (LP-BP) Bonding Pair – Bonding Pair (BP-BP) Lone pairs occupy more space than bonding electron pairs Double bonds occupy more space than a single bond

LP-LP > LP-BP > BP-BP A molecule must avoid these repulsions to remain stable. When repulsion cannot be avoided, the weaker repulsion (i.e. the one that causes the smallest deviation from the ideal shape) is preferred. Lone pair-lone pair (LP-LP) repulsion is considered to be stronger than the lone pair- bonding pair (LP-BP) repulsion, which in turn is stronger than the bonding pair-bonding pair (BP- BP) repulsion. LP-LP > LP-BP > BP-BP

Things to Remember When assigning a VSEPR Shape to a molecule, we focus on the central atom and the bonding pairs or lone pairs associated with it Ex. CO2 Electron pairs are considered to exist in a domain Domains can be made up of: A lone pair A single bond A double bond A triple bond Thus, all are considered to be one electron pair Domain

Steps to Determine VSEPR Shape Draw the Lewis structure of the molecule Determine the central atom (the least electronegative) Determine the number of bonding pairs Determine the number of lone pairs Consult the VSEPR chart to find the shape

VSEPR Chart

Number of Electron Groups Name of Molecular Shape Type of Electron Pairs Shape Example 2 Linear All Bonding Pairs CO2 3 Trigonal Planar CH2O 4 Tetrahedral CH4 Pyramidal 3 Bonding Pairs 1 Lone Pair NH3 Bent 2 Bonding Pairs 2 Lone Pairs H20

Carbon Dioxide CO2 As a Lewis Dot diagram: Has two electron pairs - two sets of double bonds – two bonding pairs Creates a LINEAR shape according to VESPR

Methanal CH2O As a Lewis Dot diagram: Has three electron pairs - three bonding pairs and no lone pairs Note: Only has three bonding pairs because the double bond is considered to be one bonding pair! Creates a TRIGONAL PLANAR shape according to VESPR

Methane CH4 As a Lewis Dot diagram: Has four electron pairs - four bonding pairs and no lone pairs Creates a TETRAHEDRAL shape according to VESPR

Ammonia NH3 As a Lewis Dot diagram: Has four electron pairs - three bonding pairs and one lone pair Creates a PYRAMIDAL shape according to VESPR

Water H2O As a Lewis Dot diagram: Has four electron pairs - two bonding pairs and two lone pairs Creates a BENT shape according to VESPR

Practice Draw the VSEPR shape for each of these molecules HCN CCl4 SI2 CH2F2 AsCl3 NO2+ CH3F NF3 CCl2F2 NCl3 SO42- COCl2 NO2- NH4+ Finished? Assign VSEPR shapes to the homework sheet (questions 1-5) from last class. 