Chapter 10 Chemical Bonding II Chemistry II. Chemical Bonding II Molecular Shapes

Chapter 10Chemical Bonding II

Chemistry II

Chemical Bonding IIMolecular Shapes

Chemical Bonding IIVSEPR Theory

• e- groups (lone pairs and bonds) are most stable when they are as far apart as possible –

v________ s____ e_______ p_____ r_________ theory

• Maximum separation

• 3-D representation allows us to predict the shapes and bond angles in the molecule

Chemical Bonding IIVSEPR Theory

e.g. draw the 2 possible Lewis dot structures for NO2- and

discuss the behavior of the associated e- groups

there are _____ e- groups on N____ lone pair____ single bond____ double bond (counted as 1 group)

Chemical Bonding II2 e- Groups: Linear Geometry

• 5 basic shapes of molecules:

linear,

trigonal planar,

tetrahedral,

trigonal bipyramidal,

octahedral

Chemical Bonding II2 e- Groups: Linear Geometry

• Draw both 2-dimensional and 3-dimensional pictures of the molecules in the following slides

Chemical Bonding II2 e- Groups: Linear Geometry

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

e.g. CO2

Chemical Bonding II3 e- Groups: Trigonal Geometry

• occupy triangular positionstrigonal planar geometry - bond angle is __________

e.g. BF3

Chemical Bonding II3 e- Groups: Trigonal Geometry

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

e.g. Formaldehyde, CH2O

Chemical Bonding II4 e- Groups: Tetrahedral Geometry

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

e.g. CH4

Chemical Bonding II5 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

Chemical Bonding II 6 e- Groups: Octahedral Geometry

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

e.g. SF6

Chemical Bonding II 3 e- Groups with Lone Pairs: Derivative 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

Chemical Bonding II 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

Chemical Bonding II 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 shape – bond angle is 104.5 °

e.g. H2O

Chemical Bonding II Tetrahedral-Bent Shape

Chemical Bonding II 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

Chemical Bonding II Replacing Atoms with Lone Pairsin the Trigonal Bipyramid System

Chemical Bonding II See-Saw Shape

F S F

F

F

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Chemical Bonding II T-Shape

Tro, Chemistry: A Molecular Approach 25

Chemical Bonding II Linear Shape

Br

FF

FF

F••

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•

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• 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°

Chemical Bonding II 6 e- Groups with Lone Pairs: Derivatives of Octahedral Geometry

F Xe F

F

F

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• 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

Chemical Bonding II 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 typeremember, multiple bonds count as 1 group

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

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

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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Chemical Bonding II 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

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

Describing the Geometryof Methanol

Describing the Geometryof Glycine

Tro, Chemistry: A Molecular Approach 41

Practice – Predict the Molecular Geometries in H3BO3

42

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••

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•• 4 Electron Groups on O

O has2 Bonding Groups2 Lone Pairs

Shape on O = Bent

Tro, Chemistry: A Molecular Approach 43

Practice – Predict the Molecular Geometries in C2H4

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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

Practice – Predict the Molecular Geometries in CH3OCH3

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

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

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

Molecule Polarity

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

Net result is a polar molecule.

Vector Addition

Molecule Polarity

The O-C bond is polarThe bonding e- are pulled equally toward both O’sSymmetrical 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 polarAll the sets of bonding electrons are pulled toward the NNot symmetrical

Net result is a polar molecule

Molecule Polarity

The C-H bond is polarFour 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 waterand ionic compounds as well

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

Molecular Polarity Affects Solubility in Water

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

Practice - Decide Whether the Following Are Polar

O N Cl ••

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O S

O

O

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••ENO = 3.5N = 3.0Cl = 3.0S = 2.5

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 ••

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O S

O

O

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TrigonalBent Trigonal

PlanarCl

N

O

3.0

3.0

3.5

O

O

OS

3.5

3.5 3.52.5