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Chapter 10Chemical Bonding II

Chemistry A Molecular Approach, 1st Ed.Nivaldo

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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 of molecules

Molecular Geometry

- Molecules are 3-D
- Describe molecular shape using geometric terms
- Geometry has characteristic angles that we call

bond angles

Using Lewis Theory to PredictMolecular Shapes

- Lewis theory - regions of e- in an atom based on

placing shared pairs and unshared pairs of

valence e- - predicts the shapes of molecules based on

negatively charged regions which repel

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

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

there are 3 e- groups on N 1 lone pair 1 single

bond 1 double bond (counted as 1 group)

5 Basic Molecular Geometries

- 5 arrangements of e- groups
- for molecules that exhibit resonance, it doesnt

matter which resonance form you use the

molecular geometry will be the same

2 e- Groups Linear Geometry

- occupy positions opposite, around the central

atom - linear geometry - bond angle is 180
- e.g. CO2

3 e- Groups Trigonal Geometry

- occupy triangular positions
- trigonal planar geometry - bond angle is 120
- e.g. BF3

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.

4 e- Groups Tetrahedral Geometry

- occupy tetrahedron positions around the central

atom - tetrahedral geometry - bond angle is 109.5
- e.g. CH4

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

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

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 than shared like bonding

electron groups - relative sizes of repulsive force interactions

is - Lone Pair Lone Pair gt Lone Pair Bonding Pair

gt Bonding Pair Bonding Pair - this effects the bond angles, making them smaller

than expected

Effect of Lone Pairs

The bonding electrons are shared by two atoms, so

some of the negative charge is removed from the

central atom.

The nonbonding electrons are localized on the

central atom, so area of negative charge takes

more space.

Effect of Lone Pairs Derivative Shapes

- the molecules 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

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 lt 120e.g. SO2

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 pairtrigonal pyramidal

shape bond angle is 107 e.g. NH3

Bond Angle Distortion from Lone Pairs

HW Which species has the smaller bond angle,

Perchlorate (ClO4-) or Chlorate (ClO3-)?

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

Tetrahedral-Bent Shape

Bond Angle Distortion from Lone Pairs

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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

lt 120 - the bond angles between axial and equatorial

positions is lt 90 - linear 180 axial-to-axial

Replacing Atoms with Lone Pairsin the Trigonal

Bipyramid System

See-Saw Shape

T-Shape

Linear Shape

6 e- Groups with Lone Pairs Derivatives of

Octahedral Geometry

- 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 lt 90

6 e- Groups with Lone Pairs Derivatives of

Octahedral Geometry

- 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

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Predicting the Shapes Around Central Atoms

- Draw the Lewis Structure
- Determine the Number of Electron Groups around

the Central Atom - Classify Each Electron Group as Bonding or Lone

pair, and Count each type - remember, multiple bonds count as 1 group
- Use Table 10.1 to Determine the Shape and Bond

Angles

Practice Predict the Molecular Geometry and

Bond Angles in SiF5-

Practice Predict the Molecular Geometry and

Bond Angles in SiF5-

Si Least Electronegative

5 Electron Groups on Si

Si Is Central Atom

5 Bonding Groups 0 Lone Pairs

Si 4e- F5 5(7e-) 35e- (-) 1e- total 40e-

Shape Trigonal Bipyramid

Bond Angles Feq-Si-Feq 120 Feq-Si-Fax 90

Practice Predict the Molecular Geometry and

Bond Angles in ClO2F (Chloryl Fluoride)

Practice Predict the Molecular Geometry and

Bond Angles in ClO2F

Cl Least Electronegative

4 Electron Groups on Cl

Cl Is Central Atom

3 Bonding Groups 1 Lone Pair

Cl 7e- O2 2(6e-) 12e- F 7e- Total 26e-

Shape Trigonal Pyramidal

Bond Angles O-Cl-O lt 109.5 O-Cl-F lt 109.5

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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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

Describing the Geometryof Methanol

Describing the Geometryof Glycine

Practice Predict the Molecular Geometries in

H3BO3

Practice Predict the Molecular Geometries in

H3BO3

oxyacid, so H attached to O

3 Electron Groups on B

4 Electron Groups on O

B Least Electronegative

B has 3 Bonding Groups 0 Lone Pairs

O has 2 Bonding Groups 2 Lone Pairs

B Is Central Atom

B 3e- O3 3(6e-) 18e- H3 3(1e-)

3e- Total 24e-

Shape on B Trigonal Planar

Shape on O Bent

Practice Predict the Molecular Geometries in

C2H4

Practice Predict the Molecular Geometries in

C2H4

3 Electron Groups on C

C 2(4e-) 8e - H 4(1e-) 4e- Total 12e-

0 Lone Pairs

Shape on each C Trigonal Planar

Practice Predict the Molecular Geometries in

CH3OCH3

Practice Predict the Molecular Geometries in

Dimethyl Ether (CH3OCH3)

4 Electron Groups on C

C 2(4e-) 8e - H 6(1e-) 6e- O 6(1e-)

6e- Total 20e-

2 Lone Pairs on O

Shape on each C Tetrahedral

Shape on O Bent

Reminder about Eletronegativity!

- Electronegativity, is a chemical property that

describes the tendency of an atom to e- towards

itself

Polarity of Molecules

- in order for a molecule to be polar it must
- have polar bonds
- electronegativity difference
- dipole moments (charge x distance)
- 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

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.

Vector Addition

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Molecule Polarity

The O-C bond is polar The bonding e- are pulled

equally toward both Os Symmetrical molecule Net

result is a nonpolar molecule

Molecule Polarity

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

pulled toward the O Net result is a polar

molecule

Molecule Polarity

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

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

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 water - and ionic compounds as well

Molecular Polarity Affects Solubility in Water

- Oil and water do not mix!

Mutual attraction causes polar molecules to clump

together

Unique Properties

- 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

Molecular Polarity Affects Solubility in Water

- some molecules have both polar and nonpolar

partse.g. soap

Practice - Decide Whether the Following Are Polar

EN O 3.5 N 3.0 Cl 3.0 S 2.5

Practice - Decide Whether the Following Are Polar

Trigonal Bent

Trigonal Planar

2.5

1) polar bonds, N-O 2) asymmetrical shape

1) polar bonds, all S-O 2) symmetrical shape

polar

nonpolar