Valence Shell Electron Pair Repulsion Theory (VSEPR)

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Presentation transcript:

Valence Shell Electron Pair Repulsion Theory (VSEPR) Chemistry 11 Ms. McGrath

VSEPR Theory Why are we concerned with the shape of molecules? This will help us later when we discuss polarity. What causes molecules to take on different shapes? Electron pairs (bonding and lone pairs) are negatively charged and 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.

VSEPR Theory Three type of repulsions take place in atoms: 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 pairs. Double bonds occupy more space than single bonds.

VSEPR Theory A molecule must avoid these repulsions to remain stable. When repulsions cannot be avoided, the weaker repulsion (ie. the one that causes the smallest deviation from the ideal shape) is preferred.

VSEPR Theory Lone pair – Lone pair repulsion is considered to be the strongest. Next is the Lone pair – Bonding pair repulsion. Bonding pair – Bonding pair is the weakest repulsion. LP – LP > LP – BP > BP -BP

VSEPR Theory Things to remember When assigning a VSEPR shape to a molecule, we focus on the central atom and the lone pairs or bonding pairs around it. Electron pairs are considered to exist in domains. Domains can be made up of: Lone pairs A single bond A double bond A triple bond Thus, all are considered to be one electron pair.

VSEPR Theory Steps to determine VSEPR shape 1. Draw the Lewis Structure of the molecule 2. Determine the central atom 3. Determine the number of bonding pairs around the central atom 4. Determine the number of lone pairs around the central atom 5. Consult the VSEPR chart to determine shape

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

VSEPR Theory Example Carbon dioxide, CO2 Example Methanol, CH2O Example Methane, CH4 Example Ammonia, NH3 Example Water, H2O

VSEPR Theory Practice page 193, #6 and 7