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IUnit 2

How do we determine structure?

M4. Inferring Charge Distribution Analyzing the distribution of electrons in molecules.

M3. Predicting Geometry Predicting the three dimensional geometry of molecules.

M2. Looking for Patterns Deducing atom connectivity based on atomic structure .

M1. Analyzing Light-Matter InteractionsUsing spectroscopy to derive

structural information.

The central goal of this unit is to help you develop ways of thinking that can be used to predict the atomic and molecular structure of substances.

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Unit 2

How do we determine structure?

Module 3: Predicting Geometry

Central goal: To deduce the Lewis

structure of molecules and predict their three

dimensional geometry based on the analysis of the number and type of valence electron pairs

surrounding each atom.

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The Challenge

The properties of a substance are determined by the structure of its molecules.

How can we predict molecular geometry given information about atomic composition and

atom connectivity ?

Molecular structure depends on:

Atomic CompositionAtom Connectivity

Molecular geometry

Aspirin

C9H8O4

ModelingHow do I predict it?

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We have seen that when two atoms of nonmetallic elements combine, their valence

electrons are reorganized. The number of covalent bonds that are formed are determined by the most stable electron configurations (full valence shell).

Electron Distribution

We can use the octet rule

(or full valence shell rule) to make predictions

about how electrons will distribute among the different atoms in a

molecule.

O2

N2

O O O O

N NN N

Useful Tool:Lewis Electron-dot

Structures

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2. Count valence electrons: H = 1 and O = 6

Total = (2 x 1) + 6 = 8 valence electrons

This electrons will organize in 4 pairs (spin pairing to minimize energy)

There are some simple rules that facilitate the creation of Lewis structures. Let’s illustrate them

with the molecule of water H2O.

Lewis Structures

1. Choose the central atom; never H (it forms only one bond). The central atom tends to be the one with the lowest ionization potential.

O is central in this case

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Lewis Structures

3. Use as many pairs as needed to form single bonds between the central atom and the surrounding atoms.

Each bond line represents a pair of electrons

4. Use the remaining pairs to satisfy the full valence shell rule in each atom as needed. Start with terminal or outside atoms, but not if H; place any leftover electrons on the central atom. 8 valence e-

Lone e- pairs

Bond e- pairs

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Lewis StructuresLet’s consider another case: Carbon dioxide CO2.

1. What is the central atom?

2. How many valence e-? How many pairs?

4 + 2 x 6 = 16 valence e- 8 e- pairs

3. What is the backbone?

4. How do we distribute the e- pairs left?

5. How do we satisfy the octet rule for all atoms?

Form double bonds

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ILet’s Think

A variety of substances contribute to indoor air pollution. Among the most common we find:

Build the Lewis structures of the

following greenhouse gases:CH4, CO ,NH3, CH2O

1. What is the central atom?2. How many valence e-?

How many pairs?

3. What is the backbone?4. How do we distribute the e- pairs left?5. How do we satisfy the octet rule for all atoms?

STRATEGY

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Experimental data indicates that both bonds in the O3 molecule have the same length, but the value is

intermediate between those of single and double bonds.

Interesting Cases

Bond Length (pm)

148

O3 127.8121

O O

O O

For some molecules, the derivation of their actual Lewis structure is not so straightforward.

Consider for example the ozone molecule, O3, which plays a central role in our atmosphere.

How do we explain it?

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Let’s Think

Build the Lewis structure of O3.

This molecule illustrates a structural feature that we need to take into account

when deciding how to distribute electrons among atoms in a

molecule.What is it?

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IMolecular Hybrids

The structure is a hybrid of:

Resonance Structures

Resonance structures are drawn when a single Lewis structure cannot represent the actual

electron distribution in a molecule.

3 e- pairs / 2 bondsIntermediate

between single and double

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Resonance

In molecules that exhibit resonance the electrons are “delocalized” over the entire system.

This delocalization tends to stabilize the molecule (reduces its potential energy).

Resonance Hybrid

Benzene C6H6

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ILet’s Think

SO3

CH2O

Which of these pollutants exhibits

resonance stabilization?

How many resonance structures do they have?

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Electron Repulsion

Once the Lewis structure of a molecule is derived, its geometry can be predicted applying a

simple principle:

Regions of high electron density around any single atom will be located as far as possible due

to electron repulsions.

Valence Shell Electron Pair Repulsion (VSEPR)Theory

Minimizing repulsions allows us to find the most stable shape (lower energy).

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ILet’s Think

Consider the following Lewis structures for these molecules in our atmosphere:

F F

Cl

Cl

How many regions of high electron density

do you identify around each central

atom?

How will these regions be located in space due to electron

repulsions?

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Molecular Geometry

2

Molecular geometry

e- pair geometry

Example# e- regions

3

3

Linear (180o)

Bent or Angular

Trigonal Planar

(~120o)

Trigonal

Planar (< 120o)

Linear

Trigonal planar

118o

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4

Molecular geometry

e- pair geometry

Example# e- regions

F F

Cl

Cl Tetrahedral

Tetrahedral(109o)

Molecular Geometry

Tetrahedral(< 109o)

H

HO

Bent or Angular

104.5o

4

4 Tetrahedral (< 109o)

H

H

H

lone pair of electronsin tetrahedral position

N

Trigonal Pyramid

107.8o

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ILet’s Think

Apply VSEPR theory to derive the molecular geometry of the following atmospheric molecules:

SO2, SO3, CH4, N2OEstimate the bond angles in these molecules.

Follow the

sequence

STEP 1 STEP 2 STEP 3

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4 bond pairs, 0 lone pairs

3 bond pairs, 1 lone pair

Larger MoleculesThe same ideas can be applied to deduce the

molecular geometry of larger molecules. The task is simplified by recognizing the following patterns

for some of the most common central atoms:

2 bond pairs, 2 lone pairs

CTetrahedral

CTrigonal planar

Linear

O Bent

N

Trigonal Pyramid

Trigonal planar

N

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Larger Molecules

Consider the molecule of ethanol C2H6O:

The molecule has three main “centers”:

The overall geometry is

determined by the geometry around

each of these centers.

Tetrahedral

109o Bent

~105o

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ILet’s Think

Consider the molecule of acetone C3H6O:

How many centers are in this molecule?

What is the geometry around each of these

centers?

What bond angle characterizes each

center?

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I Assess what you know

Let′s apply!

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Functionality

A central idea in chemistry is that the chemical properties of many molecules are determined by

the presence of “distinctive arrangements of atoms” that tend to behave as a single chemical

entity during a reaction.

This distinctive arrangements of atoms are called “functional groups”

and their properties are determined by their atomic composition, connectivity

and geometry. RHydroxyl group

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Functional GroupsLet′s apply!

Determine the geometry around the atomic centers of the following “functional groups”:

Chemical Class

Functional group

Structural formula Molecular geometry

Alcohol hydroxyl

Ketone carbonyl

Carboxylic acid

carboxyl

Amine Primary amine

Aromatic phenyl

R

R

R

R1

R2

R

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C

CC

C

C C

C C C

HH

H

H H

H

H

H O

O H

N

H

H

2 3

4

5

1

Let′s apply!

Phenylalanine is an essential aminoacid needed by our body to biochemically synthesize a wide

variety of proteins

Predict

What functional groups are

present in this molecule?

Estimate the value of the marked bond angles and make an sketch of the geometry of this molecule.

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Summarize in once sentence the basic principle that determines

molecular geometry.

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Summary

Predicting Geometry

The octet rule can be used to deduce the distribution of valence electrons among the different atoms in a

molecule.

The distribution of electrons is

represented through the Lewis structure

of the molecule. 8 valence e-

Lone e- pairs

Bond e- pairs

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Once the Lewis structure of a molecule is derived, its geometry can be predicted applying

a simple principle: Regions of high electron density around any single atom will be located as far as possible due to electron repulsions

(VSEPR Theory).

Summary

Predicting Geometry

We can deduce the entire molecular geometry of a complex

molecule by analyzing the electron pair distribution around

each of its atoms.

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For next class,

Investigate how molecular composition and geometry affect the distribution of electrons

within a molecule.

What is the difference between a polar and a non-polar molecule?