X. VSEPR Theory – Valence Shell Electron Pair Repulsion theory. [Remember: Like charges repel!] Valence Shell Electron Pair Repulsion theory. [Remember: Like charges repel!] A. A theory to predict the 3-dimensional geometry, ie. the“shape” of a molecule A. A theory to predict the 3-dimensional geometry, ie. the“shape” of a molecule

1. The theory is based on “electrostatic repulsion”: Molecules will adjust their shape to keep the negatively-charged pairs of valence electrons as far apart as possible from each other. 1. The theory is based on “electrostatic repulsion”: Molecules will adjust their shape to keep the negatively-charged pairs of valence electrons as far apart as possible from each other. B. When NOT to use VSEPR theory: When there are only 2 atoms in a molecule. These molecule’s shapes are called linear – it doesn’t matter if there are single bonds, double bonds, triple bonds, or unshared electron pairs. B. When NOT to use VSEPR theory: When there are only 2 atoms in a molecule. These molecule’s shapes are called linear – it doesn’t matter if there are single bonds, double bonds, triple bonds, or unshared electron pairs.

C. Using VSEPR theory: 1. Draw the Lewis dot structure for the molecule. 1. Draw the Lewis dot structure for the molecule. 2. Identify its central atom. 2. Identify its central atom. 3. Identify the sets of valence electrons as one of two possibilities: 3. Identify the sets of valence electrons as one of two possibilities: A. Those connecting two atoms. A. Those connecting two atoms. B. Those that do not connect two atoms. These are called “unshared pairs”. B. Those that do not connect two atoms. These are called “unshared pairs”.

4. The unshared pairs found on a central atom strongly repel each other; and molecules that would otherwise be linear, will be forced into a bent (or angular) shape. 4. The unshared pairs found on a central atom strongly repel each other; and molecules that would otherwise be linear, will be forced into a bent (or angular) shape. 5. Unshared pairs also cause a molecule that would be shaped like a flat triangle (trigonal planar), to be forced into a not flat ( trigonal pyramidal) shape. 5. Unshared pairs also cause a molecule that would be shaped like a flat triangle (trigonal planar), to be forced into a not flat ( trigonal pyramidal) shape.

6.Count the number of connections separately from the number of unshared pairs. 6.Count the number of connections separately from the number of unshared pairs. 1 single bond counts as 1 connection. 1 single bond counts as 1 connection. 1 double bond counts as 1 connection. 1 double bond counts as 1 connection. 1 triple bond counts as 1 connection. 1 triple bond counts as 1 connection. Each unshared set of 2 dots counts as 1 un- shared pair. Each unshared set of 2 dots counts as 1 un- shared pair.

D Predicting Shapes Using VSEPR Table Read horizontally across the table. Read horizontally across the table. Connections To the Central Atom Unshared Pairs of Electrons Around Central Atom Molecular Shape Around Central Atom 20Linear 30Trigonal Planar 40Tetrahedral, o 21 or 2Bent 31Pyramidal # of total atoms

Shapes: Shapes:, E. Shapes: Linear diatomic Linear triatomic Trigonal Planar Bent Pyramidal Linear diatomic Tetrahedral Linear triatomic Trigonal planar bent Trigonal pyramidal tetrahedral