VSEPR & Geometry Lewis structures show the number and type of bonds between atoms in a molecule or polyatomic ion. Lewis structures are not intended to show the 3-dimensional structure (i.e. shape or geometry) of the molecule. The shape of a molecule is determined by the bond angles and bond lengths between the atoms.

VSEPR & Geometry Bond length: the distance between two atoms held together by a chemical bond Bond length is affected by the number of bonds between the two atoms. Single bonds are longest. Triple bonds are shortest. Bond angle: the angle made by the imaginary lies joining the nuclei of the atoms in a molecule O H H 104.5o

VSEPR & Geometry Many of the molecules we have discussed have a central atom surrounded by two or more “outer” atoms: ABn where A = central atom B = outer atom n = # of “B” atoms Examples: CO2, H2O, BF3, NH3, CCl4, CHCl3

VSEPR & Geometry CO2 linear H2O bent The shape of ABn molecules depends, in part, on the value of n. AB2 molecules can either be linear or bent. CO2 O C O linear H O H2O bent

VSEPR & Geometry AB3 molecules can be trigonal planar, trigonal pyramidal, or T-shaped. Trigonal planar: Atom “A” in the center of an equilateral triangle with “B” atoms at each corner. All atoms in the same plane. H B BH3

VSEPR & Geometry Trigonal pyramidal: The three “B” atoms are arranged at the corners of an equilateral triangle Central atom “A” is located in the center but above the plane of the triangle. N H NH3

VSEPR & Geometry T-shaped: F Cl ClF3

VSEPR & Geometry How do you decide which AB3 molecules are trigonal planar, which are trigonal pyramidal, and which are T-shaped? If the central atom “A” is a main group element, the valence shell electron-pair repulsion model (VSEPR) can be used to predict the shape of a molecule (or polyatomic ion).

VSEPR & Geometry VSEPR counts the number of electron domains around the central atom and uses this number to predict the shape. Electron domain: A region around the central atom where electrons are likely to be found Two types of electron domains: Bonding electron domains Nonbonding electron domains

VSEPR & Geometry Bonding electron domains Also called bonding electrons Electrons that are shared between two atoms CCl4 has 4 bonding pairs of electrons

VSEPR & Geometry Nonbonding electron domains Also referred to as nonbonding pairs or lone pairs of electrons: Electrons that are found principally on one atom Unshared electrons

VSEPR & Geometry Example: Count the number of electron domains around the central atom in each of the following Lewis structures.

VSEPR & Geometry Since electron domains are regions of high electron density, they tend to repel each other. According to VSEPR, the best arrangement of a specified number of electron domains is the one that minimizes repulsions between them by placing them as far away from each other as possible.

Electron Domain Geometry The electron domain geometry is found by counting the number of electrons domains and considering the arrangement that minimizes repulsions. Electron domain geometry: the arrangement of the electron domains around the central atom Linear Trigonal planar Tetrahedral Trigonal bipyramidal Octahedral

Electron Domain Geometry 2 electron domains Linear electron domain geometry 3 electron domains Trigonal planar e- domain geometry

Electron Domain Geometry 4 electron domains Tetrahedral electron domain geometry Trigonal bipyramidal e- domain geometry 5 electron domains

Electron Domain Geometry 6 electron domains Octahedral electron domain geometry

Electron Domain Geometry Drawing electron domain geometries in 3-d: Trigonal planar Tetrahedral

Electron Domain Geometry Drawing electron domain geometries in 3-d: Trigonal bipyramidal octahedral

Electron Domain Geometry To determine the name of the electron domain geometry: Draw the Lewis structure Count the number of electron domains around the central atom Double bonds and triple bonds count as 1 electron domain Assign the name of the electron domain geometry. You should also be able to draw a 3-dimensional structure for a given substance.

Electron Domain Geometry Example: Determine the name of the electron domain geometry for each of the following and draw an appropriate 3-dimensional structure for it. NO2- ClF3

Electron Domain Geometry Example: Determine the name of the electron domain geometry for each of the following and draw an appropriate 3-dimensional structure for it. CH4 XeF4

Electron Domain Geometry Example: Use the Lewis structure shown below for acetone, the major component of nail polish remover, to identify the electron domain geometry around each carbon atom.

Molecular Geometry Water has tetrahedral electron domain geometry: The shape of the molecule itself, however, is not tetrahedral. Water has a bent molecular geometry.

Molecular Geometry Molecular geometry: The arrangement in space of the atoms in a molecule or polyatomic ion Molecular geometry is a consequence of the electron domain geometry. Lone pairs of electrons take up space around the central atom. The atoms in the molecules occupy positions around the central atom that minimizes repulsion between all of the electron domains.

Tetrahedral e.d. and molecular geometry Each type of electron domain geometry gives rise to certain specific types of molecular geometries. The electron domain geometry and molecular geometry are the same only if all of the electron domains are bonding domains. Tetrahedral e.d. and molecular geometry

Molecular Geometry Tables 9.2 and 9.3 list all possible molecular geometries for each of the five electron domain geometries. You must be able to determine the name of the e.d. geometry and the molecular geometry. You must be able to draw each geometry in 3-dimensions.

Molecular Geometry To determine the name of the molecular geometry: Draw the Lewis structure Count the total # of electron domains Identify the electron domain geometry Determine the molecular geometry by considering the arrangement of the bonded atoms.

Molecular Geometry Example: Determine the name of the molecular geometry for each of the following and draw an appropriate 3-dimensional structure for it. NO2- ClF3

Molecular Geometry Example: Determine the name of the molecular geometry for each of the following and draw an appropriate 3-dimensional structure for it. CH4 XeF4

Molecular Geometry Example: Determine the name of the molecular geometry for each of the following and draw an appropriate 3-dimensional structure for it. I3- SF4