Chapter 10Chemical

Bonding II

Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro

Tro, Chemistry: A Molecular Approach 5

VSEPR Theory

• e- groups (lone pairs and bonds) are most stable when they are as far apart as possible – valence shell electron pair repulsion theory

• Maximum separation

• the resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule

• 3-D representation

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Electron Groups

• the Lewis structure predicts the arrangement of valence e- around the central atom(s)

• each lone pair of e- constitutes one e- group on a central atom

• each type of bond constitutes one electron group on a central atom

e.g. NO2

O N O ••

••

••

••

•••• there are 3 e- groups on N1 lone pair1 single bond1 double bond (counted as 1 group)

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5 Basic Molecular Geometries

• 5 arrangements of e- groups

• for molecules that exhibit resonance, it doesn’t matter which resonance form you use – the molecular geometry will be the same

•••• •• ••••••••

•• ••O S O O S O•••••• ••••

••••

••••

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2 e- Groups: Linear Geometry

• occupy positions opposite, around the central atomlinear geometry - bond angle is 180°

e.g. CO2

ClBeCl

O C O

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3 e- Groups: Trigonal Geometry

• occupy triangular positionstrigonal planar geometry - bond angle is 120°

e.g. BF3

F

F B F

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Not Quite Perfect Geometry

3 e– groups around central atom – why not 120° ?

Because the bonds are not identical, the observed angles are slightly different from ideal.

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4 e- Groups: Tetrahedral Geometry

• occupy tetrahedron positions around the central atomtetrahedral geometry - bond angle is 109.5°

e.g. CH4

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5 e- Groups: Trigonal Bipyramidal Geometry

• occupy positions in the shape of a two tetrahedra that are base-to-base

trigonal bipyramidal geometry

e.g. PCl5

P

ClCl

ClCl

Cl••

•• •

•••

•••

•

••••

••

••

••••

••

••

••

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6 e- Groups: Octahedral Geometry

• occupy positions in the shape of two square-base pyramids that are base-to-baseoctahedral geometry

e.g. SF6

S

F F

FF

F

F

••

•• •

•••

••

••

••

••

••••

••

••

••••

••

••

••••

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Effect of Lone Pairs: Derivative Shapes

• the molecule’s shape will be one of basic molecular geometries if all the e- groups are bonds and all the bonds are equivalent

• molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths, but the shape will be a derivative of one of the basic shapes

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3 e- Groups with Lone PairsDerivative of Trigonal Geometry

• when there are 3 e- groups around central atom, and 1 of them is a lone pairtrigonal planar - bent shape - bond angle < 120°e.g. SO2

O S O

O S O

O S O

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4 e- Groups with Lone Pairs Derivatives of Tetrahedral Geometry

• when there are 4 e- groups around the central atom, and 1 is a lone pair

trigonal pyramidal shape – bond angle is 107 °e.g. NH3

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4 e- Groups with Lone Pairs Derivatives of Tetrahedral Geometry

• when there are 4 e- groups around the central atom, and 2 are lone pairstetrahedral-bent shapee.g. H2O it looks similar to the trigonal planar-bent shape, except the angles are

smaller

104.5°

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Tetrahedral-Bent Shape

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5 e- Groups with Lone Pairs Derivatives of Trigonal Bipyramidal Geometry

• when there are 5 e- groups around the central atom, and some are lone pairs, they will occupy the equatorial positions because there is more room

• when there are 5 e- groups around the central atom, and 1 is a lone pair, the result is called see-saw shape aka distorted tetrahedron

• when there are 5 e- groups around the central atom, and 2 are lone pairs, the result is called T-shaped

• when there are 5 e- groups around the central atom, and 3 are lone pairs, the result is called a linear shape

• the bond angles between equatorial positions is < 120°

• the bond angles between axial and equatorial positions is < 90° linear = 180° axial-to-axial

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Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System

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See-Saw Shape

F S F

F

F

•••

•

••

••

••

••

••

••

••

••

••

••

••

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T-Shape

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Linear Shape

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Br

FF

FF

F••

•• •

•••

•••

•

••••

••

••

••••

••

••

••

••

• when there are 6 e- groups around the central atom, and 1 is a lone pair, the result is called a square pyramid shape the bond angles between axial and equatorial positions is < 90°

6 e- Groups with Lone Pairs Derivatives of Octahedral Geometry

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F Xe F

F

F

•••

•

••

••

••

••

••

••

••

••

••

••

••

••

• when there are 6 e- groups around the central atom, and 2 are lone pairs, the result is called a square planar shape the bond angles between equatorial positions is 90°

6 e- Groups with Lone Pairs Derivatives of Octahedral Geometry

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Predicting the Shapes Around Central Atoms

1) Draw the Lewis Structure2) Determine the Number of Electron Groups around the Central

Atom3) Classify Each Electron Group as Bonding or Lone pair, and

Count each type remember, multiple bonds count as 1 group

4) Use Table 10.1 to Determine the Shape and Bond Angles

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Practice – Predict the Molecular Geometry and Bond Angles in ClO2F (Chloryl Fluoride)

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Practice – Predict the Molecular Geometry and Bond Angles in ClO2F

Cl = 7e─

O2 = 2(6e─) = 12e─

F = 7e─

Total = 26e─

4 Electron Groups on Cl

3 Bonding Groups1 Lone Pair

Shape = Trigonal Pyramidal

Bond AnglesO-Cl-O < 109.5°O-Cl-F < 109.5°

Cl Least Electronegative

Cl Is Central Atom

O Cl

O

F

••

••

•• •

•••

••

••

••

••

••

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Representing 3-Dimensional Shapes on a 2-Dimensional Surface

• one of the problems with drawing molecules is trying to show their dimensionality

• by convention, the central atom is put in the plane of the paper

• put as many other atoms as possible in the same plane and indicate with a straight line

• for atoms in front of the plane, use a solid wedge

• for atoms behind the plane, use a hashed wedge

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Tro, Chemistry: A Molecular Approach 40

SF6

S

F

F

F

F F

F

S

F F

FF

F

F

••

•• •

•••

••

••

••

••

••••

••

••

••••

••

••

••••

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Multiple Central Atoms

• many molecules have larger structures with many interior atoms

• we can think of them as having multiple central atoms

• when this occurs, we describe the shape around each central atom in sequence

e.g. acetic acid

H|

HOCCH|||

OH

shape around left C is tetrahedral

shape around center C is trigonal planar

shape around right O is tetrahedral-bent

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Describing the Geometryof Methanol

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Describing the Geometryof Glycine

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Practice – Predict the Molecular Geometries in H3BO3

46

Practice – Predict the Molecular Geometries in H3BO3

B = 3e─

O3 = 3(6e─) = 18e─

H3 = 3(1e─) = 3e─

Total = 24e─

3 Electron Groups on B

B has3 Bonding Groups0 Lone Pairs

Shape on B = Trigonal Planar

B Least Electronegative

B Is Central Atom

oxyacid, so H attached to O

O B

O

OH H

H••

••

••

••

••

•• 4 Electron Groups on O

O has2 Bonding Groups2 Lone Pairs

Shape on O = Bent

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Practice – Predict the Molecular Geometries in C2H4

48

Practice – Predict the Molecular Geometries in C2H4

C = 2(4e─) = 8e ─

H = 4(1e─) = 4e─

Total = 12e─

3 Electron Groups on C

Shape on each C = Trigonal Planar

0 Lone Pairs

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Practice – Predict the Molecular Geometries in CH3OCH3

50

Practice – Predict the Molecular Geometries in Dimethyl Ether (CH3OCH3)

C = 2(4e─) = 8e ─

H = 6(1e─) = 6e─

O = 6(1e─) = 6e─

Total = 20e─

4 Electron Groups on C

Shape on each C = Tetrahedral

2 Lone Pairs on O

Shape on O = Bent

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Reminder about Eletronegativity!

• Electronegativity, is a chemical property that describes the tendency of an atom to e- towards itself

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Polarity of Molecules

• in order for a molecule to be polar it must

1) have polar bonds electronegativity difference dipole moments (charge x distance)

2) have an unsymmetrical shape vector addition

• polarity affects the intermolecular forces of attraction therefore boiling points and solubilities

like dissolves like

• nonbonding pairs strongly affect molecular polarity

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Molecule Polarity

The H-Cl bond is polarBonding e- are pulled toward the Cl end of the molecule

Net result is a polar molecule.

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Vector Addition

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Molecule Polarity

The O-C bond is polarThe bonding e- are pulled equally toward both O’sSymmetrical molecule

Net result is a nonpolar molecule

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Molecule Polarity

The H-O bond is polarBoth sets of bonding e- are pulled toward the O

Net result is a polar molecule

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Molecule Polarity

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Molecule Polarity

The H-N bond is polarAll the sets of bonding electrons are pulled toward the NNot symmetrical

Net result is a polar molecule

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Molecule Polarity

The C-H bond is polarFour equal dipoles cancel each other out due to symmetry

Net result is a non-polar molecule

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Molecular Polarity Affects Solubility in Water

• polar molecules are attracted to other polar molecules

• since water is a polar molecule, other polar molecules dissolve well in waterand ionic compounds as well

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Molecular Polarity Affects Solubility in Water

• Oil and water do not mix!

Mutual attraction causes polar molecules to clump together

• Water shrinks on melting (ice floats on water)

• Unusually high melting point

• Unusually high boiling point

• Unusually high surface tension

• Unusually high viscosity

• Unusually high heat of vaporization

• Unusually high specific heat capacity

• And more…

Unique Properties

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Molecular Polarity Affects Solubility in Water

• some molecules have both polar and nonpolar partse.g. soap

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Practice - Decide Whether the Following Are Polar

O N Cl ••

••

••

••

••••

O S

O

O

••

••

•• •

•••••

••

••ENO = 3.5N = 3.0Cl = 3.0S = 2.5

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Practice - Decide Whether the Following Are Polar

polarnonpolar

1) polar bonds, N-O2) asymmetrical shape 1) polar bonds, all S-O

2) symmetrical shape

O N Cl ••

••

••

••

••••

O S

O

O

••

••

•• •

•••••

••

••

TrigonalBent Trigonal

PlanarCl

N

O

3.0

3.0

3.5

O

O

OS

3.5

3.5 3.52.5