Chapter 10 Chemical Bonding II

Chapter 10ChemicalBonding II

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

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Structure Determines Properties!

•properties of molecular substances depend on structures

•the structure includes many factors, including:

skeletal structure

Bonding - ionic, polar covalent, or covalent

Shape

•bonding theory should allow you to predict the shapes ofmolecules

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Molecular Geometry

•Molecules are 3-D

•Describe molecular shape using geometric terms

•Geometry has characteristic angles that we call bond angles

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Using Lewis Theory to PredictMolecular Shapes

•Lewis theory - regions of e- in an atom based on placing sharedpairs and unshared pairs of valence e-

•predicts the shapes of molecules based on negatively chargedregions which repel

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VSEPR Theory

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

•Maximum separation

•the resulting geometric arrangement will allow us to predict theshapes 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 centralatom(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

there are 3 e- groups on N

1 lone pair

1 single bond

1 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 formyou use – the molecular geometry will be the same

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•

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O S O

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•

••

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2 e- Groups: Linear Geometry

•occupy positions opposite, around the central atom

linear geometry - bond angle is 180°

e.g. CO2

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3 e- Groups: Trigonal Geometry

•occupy triangular positions

trigonal planar geometry - bond angle is 120°

e.g. BF3

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Not Quite Perfect Geometry

3 e– groups around centralatom – why not 120° ?

Because the bonds are notidentical, the observed angles areslightly different from ideal.

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4 e- Groups: Tetrahedral Geometry

•occupy tetrahedron positions around the central atom

tetrahedral 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

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6 e- Groups: Octahedral Geometry

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

octahedral geometry

e.g. SF6

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The Effect of Lone Pairs

•lone pair groups “occupy more space” on the central atom

because their e- density is exclusively on the central atom rather thanshared like bonding electron groups

•relative sizes of repulsive force interactions is:

Lone Pair – Lone Pair > Lone Pair – Bonding Pair > Bonding Pair –Bonding Pair

•this effects the bond angles, making them smaller than expected

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Effect of Lone Pairs

The bonding electrons are shared by two atoms,so some of the negative charge is removed fromthe central atom.

The nonbonding electrons are localized on thecentral atom, so area of negative charge takesmore space.

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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 havedistorted bond angles and different bond lengths, but the shape will be aderivative 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

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4 e- Groups with Lone PairsDerivatives of Tetrahedral Geometry

•when there are 4 e- groups around the central atom, and 1 is a lone pairtrigonal pyramidal shape – bond angle is 107 °e.g. NH3

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Bond Angle Distortion from Lone Pairs

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HW: Which species has the smaller bond angle,Perchlorate (ClO4-) or Chlorate (ClO3-)?

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4 e- Groups with Lone PairsDerivatives 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 aresmaller

104.5°

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Tetrahedral-Bent Shape

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Bond Angle Distortion from Lone Pairs

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5 e- Groups with Lone PairsDerivatives of Trigonal Bipyramidal Geometry

•when there are 5 e- groups around the central atom, and some are lonepairs, 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, theresult is called see-saw shape

aka distorted tetrahedron

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

•when there are 5 e- groups around the central atom, and 3 are lone pairs, theresult 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

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T-Shape

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Linear Shape

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•when there are 6 e- groups around the central atom, and 1 is a lone pair, theresult is called a square pyramid shape

the bond angles between axial and equatorial positions is < 90°

6 e- Groups with Lone PairsDerivatives of Octahedral Geometry

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•when there are 6 e- groups around the central atom, and 2 are lone pairs, theresult is called a square planar shape

the bond angles between equatorial positions is 90°

6 e- Groups with Lone PairsDerivatives of Octahedral Geometry

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Predicting the ShapesAround Central Atoms

1)Draw the Lewis Structure

2)Determine the Number of Electron Groups around the CentralAtom

3)Classify Each Electron Group as Bonding or Lone pair, andCount 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 andBond Angles in SiF5-

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Practice – Predict the Molecular Geometry andBond Angles in SiF5─

Si = 4e─

F5 = 5(7e─) = 35e─

(─) = 1e─

total = 40e─

5 Electron Groups on Si

5 Bonding Groups

0 Lone Pairs

Shape = Trigonal Bipyramid

Bond Angles

Feq-Si-Feq = 120°

Feq-Si-Fax = 90°

Si Least Electronegative

Si Is Central Atom

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

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Practice – Predict the Molecular Geometry andBond Angles in ClO2F

Cl = 7e─

O2 = 2(6e─) = 12e─

F = 7e─

Total = 26e─

4 Electron Groups on Cl

3 Bonding Groups

1 Lone Pair

Shape = Trigonal Pyramidal

Bond Angles

O-Cl-O < 109.5°

O-Cl-F < 109.5°

Cl Least Electronegative

Cl Is Central Atom

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Representing 3-Dimensional Shapes on a 2-Dimensional Surface

•one of the problems with drawing molecules is trying to showtheir 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 andindicate 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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SF6

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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 sequencee.g. acetic acid

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 inH3BO3

Practice – Predict the Molecular Geometries inH3BO3

B = 3e─

O3 = 3(6e─) = 18e─

H3 = 3(1e─) = 3e─

Total = 24e─

3 Electron Groups on B

B has

3 Bonding Groups

0 Lone Pairs

Shape on B = Trigonal Planar

B Least Electronegative

B Is Central Atom

oxyacid, so H attached to O

4 Electron Groups on O

O has

2 Bonding Groups

2 Lone Pairs

Shape on O = Bent

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Practice – Predict the Molecular Geometries inC2H4

Practice – Predict the Molecular Geometries inC2H4

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 inCH3OCH3

Practice – Predict the Molecular Geometries inDimethyl 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 anatom 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 polar

Bonding 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 polar

The bonding e- are pulled equally toward both O’s

Symmetrical 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 polar

All the sets of bonding electrons are pulled toward the N

Not symmetrical

Net result is a polar molecule

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Molecule Polarity

The C-H bond is polar

Four equal dipoles cancel each other out due to symmetry

Net result is a non-polar molecule

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Molecular Polarity AffectsSolubility in Water

•polar molecules are attracted to other polarmolecules

•since water is a polar molecule, other polarmolecules dissolve well in water

and ionic compounds as well

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Molecular Polarity AffectsSolubility in Water

•Oil and water do not mix!

Mutual attractioncauses polarmolecules to clumptogether

•Water shrinks on melting (ice floatson water)

•Unusually high melting point

•Unusually high boiling point

•Unusually high surface tension

•Unusually high viscosity

•Unusually high heat of vaporization

•Unusually high specific heatcapacity

•And more…

Unique Properties

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Molecular Polarity AffectsSolubility in Water

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

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Practice - Decide Whether the Following Are Polar

O = 3.5

N = 3.0

Cl = 3.0

S = 2.5

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Practice - Decide Whether the Following Are Polar

polar

nonpolar

1) polar bonds, N-O

2) asymmetrical shape

1) polar bonds, all S-O

2) symmetrical shape

Trigonal

Bent

Trigonal

Planar

3.0

3.5

2.5