VSEPR & Geometry

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 nucleiof the atoms in a molecule

H

O

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 atomB = outer atomn = # of “B” atoms

Examples: CO2, H2O, BF3, NH3, CCl4, CHCl3

VSEPR & Geometry The shape of ABn molecules depends, in

part, on the value of n.

AB2 molecules can either be linear or bent. O C O

H

O

H

CO2

H2O

linear

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

H

H

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.

NH H

HNH3

VSEPR & Geometry T-shaped:

F Cl

F

FClF3

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

CCl

Cl

Cl

Cl

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

H N HH

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

H N HH H O H

O C O

O N OSF

FF

F

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

3 electron domains

2 electron domains

Trigonal planare- domain geometry

Linearelectron domain geometry


4 electron domains

5 electron domains

Tetrahedral electron domain geometry

Trigonal bipyramidale- domain geometry


6 electron domains

Octahedralelectron domain geometry

Electron Domain Geometry Drawing electron domain geometries in

3-d:

AB

B

B

A

B

BB

B

Trigonal planarTetrahedral

Electron Domain Geometry Drawing electron domain geometries in

3-d:

A

B

BBB

B

A

B

BB

B

B

B

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 GeometryExample: 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 GeometryExample: 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 GeometryExample: 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.

H C

H

H

C

O

C

H

H

H

Molecular Geometry Water has tetrahedral electron domain

geometry:

The shape of the molecule itself, however, is not tetrahedral.

Water has a bent molecular geometry.

O

HH

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.

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.

C

H

HH

HTetrahedral 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 GeometryExample: Determine the name of the molecular geometry for each of the following and draw an appropriate 3-dimensional structure for it.

NO2-

ClF3


CH4

XeF4


I3-

SF4

VSEPR & Geometry

Documents