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Copyright 1999, PRENTICE HALL Chapter 9 1

Molecular Shapes

Lewis structures give atomic connectivity: they tell uswhich atoms are physically connected to which.

The shape of a molecule is determined by its bond

angles.

Consider CCl4: experimentally we find all Cl-C-Cl

bond angles are 109.5.Therefore, the molecule cannot be planar.

All Cl atoms are located at the vertices of a tetrahedron with

the C at its center.


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Molecular Shapes


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Molecular ShapesIn order to predict molecular shape, we assume the

valence electrons repel each other. Therefore, themolecule adopts whichever 3D geometry minimizes this

repulsion.

We call this process Valence Shell Electron Pair

Repulsion (VSEPR) theory.


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The VSEPR ModelThe valence electrons in a molecule are the bonding

pairs of electrons as well as the lone pairs.

There are 11 shapes that are important to us:

Number of atoms, formula Shapes(3 atoms, AB2) linearor bent

(4 atoms, AB3) tr igonal planar, tr igonal bipyramidal,

or T-shaped

(5 atoms, AB4) tetrahedral, square planar, or see-saw

(6 atoms, AB5) tr igonal bipyramidalor square pyramidal(7 atoms, AB6) octahedral


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The VSEPR ModelPredicting Molecular Geometries

To determine the shape of a molecule, we distinguishbetween lone pairs (or non-bonding pairs, those not in a

bond) of electrons and bonding pairs (those found

between two atoms).

We define the electron pair geometry by the positions in

3D space ofALL electron pairs (bonding or non-

bonding).

The electrons adopt an arrangement in space to

minimize e--e- repulsion.




To determine the electron pair geometry:draw the Lewis structure

count the total number of electron pairs around the central

atom

arrange the electron pairs in one of the above geometries to

minimize e--e- repulsionmultiple bounds count as one bonding pair


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The VSEPR ModelThe Effect of Nonbonding Electrons and

Multiple Bonds on Bond AnglesWe determine the electron pair geometry only looking

at electrons.

We name the molecular geometry by the positions of

atoms.

We ignore lone pairs in the molecular geometry.

All the atoms that obey the octet rule have tetrahedral

electron pair geometries.


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Multiple Bonds on Bond AnglesBy experiment, the H-X-H bond angle decreases on

moving from C to N to O:

Since electrons in a bond are attracted by two nuclei, they do

not repel as much as lone pairs.Therefore, the bond angle decreases as the number of lone

pairs increase.

OH

H104.5

O107

O

NH

HH

C

H

HHH

109.5O


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Multiple Bonds on Bond AnglesSimilarly, electrons in multiple bonds repel more than

electrons in single bonds.

C O

Cl

Cl

111.4o

124.3

o


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The VSEPR ModelMolecules with Expanded Valence Shells

Atoms that have expanded octets have AB5 (trigonal

bipyramidal) or AB6 (octahedral) electron pairgeometries.

For trigonal bipyramidal structures there is a plane

containing three electrons pairs. The fourth and fifth

electron pairs are located above and below this plane.

Axial pairs are not equivalent to equatorial pairs

For octahedral structures, there is a plane containing

four electron pairs. Similarly, the fifth and sixth

electron pairs are located above and below this plane.

All positions are equivalent in an octahedron.


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The VSEPR Model

Molecules with Expanded

Valence ShellsTo minimize e--e- repulsion,lone pairs are always placed in

equatorial positions.


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The VSEPR ModelMolecules with More than One Central Atom

In acetic acid, CH3COOH, there are three centralatoms.

We assign the geometry about each central atom

separately.


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Polar i ty of MoleculesPolar molecules interact with electric fields.

If the centers of negative and positive charge do notcoincide, then the molecule is polar.


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Polar i ty of MoleculesIf two charges, equal in magnitude and opposite in sign,

are separated by a distance r, then a dipole isestablished.

The dipole moment, m, is given bym = Qr

where Qis the magnitude of charge.

Dipole Moments of Polyatomic MoleculesIn a polyatomic molecule, each bond can be a dipole.

The orientation of these individual dipole moments

determines whether the molecule has an overall dipole

moment.


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Polar i ty of MoleculesDipole Moments of Polyatomic Molecules

Example: in CO2, each C-O dipole is canceled becausethe molecule is linear. In H2O, the H-O dipoles do not

cancel because the molecule is bent.


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It is possible for a molecule with polar bonds to beeither polar or non-polar.

For diatomic molecules:polar bonds always result in an overall dipole moment.

For triatomic molecules:if the molecular geometry is trigonal pyramidal, there is an

overall dipole moment;

if the molecular geometry is trigonal planar and all three

bonds are identical, there is no overall dipole moment;if the molecular geometry is trigonal planar and one or two

bonds are different, there is an overall dipole moment.


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Copyright 1999 PRENTICE HALL Chapter 9 25


For tetraatomic molecules with identical bonds:if the molecular geometry is tetrahedral or square planar,

then the molecules are nonpolar;

if the molecular geometry is see-saw, the molecule is polar.

For tetraatomic molecules in which one, two, or threebonds are different:

the molecule is polar.

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