SECTION II: BUILDING MODELS

Lesson 9 New Smells, New Ideas

Lesson 10 Two’s Company

Lesson 11 Let’s Build It

Lesson 12 What Shape Is That Smell?

Lesson 13 Sorting It Out

Lesson 14 How Does the Nose Know?

LEARNING OBJECTIVES

• Apply VSEPR to predict electronic

geometry and shapes of simple

molecules

• Distinguish between polar and nonpolar

bonds in molecules

• Predict polarity of simple molecules

from bond polarity and molecular shape

LESSON 9: NEW SMELLS, NEW IDEAS

Ball-and-Stick Models

A ball-and-stick model is a three-dimensional representation of a molecule that shows us how the atoms are arranged in space in relationship to one another.

TOTAL NUMBER OF GROUPS

DICTATES ELECTRONIC GEOMETRY

Octet rule:

Two – linear

Three – trigonal planar

Four – tetrahedral

Additional possibilities (expand octet):

Five – trigonal bipyramidal

Six - octahedral

Electronic geometry considers

bonded atoms only.

Molecular geometry considers

unbonded pairs as well

LESSON 10: TWO’S COMPANY

Electron Domains

YOU WILL BE ABLE TO:

determine the shapes of small

molecules

explain how lone pairs of electrons

influence molecular shape

describe electron domain theory and

how it relates to molecular shape

KEY QUESTION

How do electrons affect the shape of

a molecule?

MOLECULAR GEOMETRY

Molecular geometry is the three-dimensional

arrangement of a molecule’s atoms in space.

Linear

Bent

Trigonal-planar

Tetrahedral Trigonal-

pyramidal

Trigonal-bipyramidal Octahedral

Electron domain: The space occupied by valence

electrons in a molecule, either a bonded pair(s) or a

lone pair. Electron domains affect the overall shape

of a molecule.

Electron domain theory: The idea that every

electron domain in a molecule is as far as

possible from every other electron domain in

that molecule.

VALENCE

SHELL

ELECTRON

PAIR

REPULSION

THEORY

VSEPR theory assumes that the shape of a

molecule is determined by the repulsion of

electron pairs.

VSEPR theory states

that repulsion between

the sets of valence-level

electrons surrounding an

atom causes these sets

to be oriented as far apart as possible.

Molecular Shape

VSEPR THEORY

• VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion

• Based on Electron Dot (Lewis structures) • Theory predicts shapes of compounds based on electron pairs repelling (in bonds or by themselves)

• Electrons around central nucleus repel each other. So, structures have atoms maximally spread out

METHANE AS A MODEL

Examine a simple hydrocarbon such as methane.

Methane’s chemical formula: CH4

15

What do you predict for

it’s molecular structure?

What do you predict for

it’s geometric shape?

(remember VSEPR)

The overall geometric shape of a methane model is

tetrahedral. The H atoms are at the vertices of a

tetrahedron.

Incorrect models–electron pairs

are not equally distant. Correct models–All angles

between bonds are the same.

Bonded pairs of electrons take

up space. This space is called an

electron domain

Tetrahedral shape: The shape around an atom

with four bonded pairs of electrons. This is the

shape of a methane molecule.

ANOTHER EXAMPLE

Now examine another simple molecule such as

ammonia.

Ammonia’s chemical formula: NH3

18

What do you predict for

it’s molecular structure? What do you predict for it’s

geometric shape?

An electron domain describes the area occupied by

a set of electrons in a bond or a lone pair.

• Unshared electron pairs repel other electron

pairs more strongly than bonding pairs do. • This is why the bond angles in ammonia and water are

somewhat less than the 109.5o bond angles of a perfectly

tetrahedral molecule.

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Trigonal pyramidal

one atom at the apex and three atoms at the corners of

a trigonal base, resembling a tetrahedron, However, the three

hydrogen atoms are repelled by the electron lone pair in a way

that the geometry is distorted to a trigonal pyramid.

● ●

Use VSEPR theory to predict the molecular

geometry of boron trichloride, BCl3.

First write the Lewis structure.

Boron is in Group 13 and has 3 valence

electrons.

Chlorine is in Group 17 so each chlorine atom

has 7 valence electrons.

The three B-Cl bonds stay farthest apart by pointing to the

corners of an equilateral triangle, giving 120o angles between

the bonds.

This would be trigonal-planar geometry.

The “z” plane – 3-D

• Each shape has a name (you will have to memorize these)

• tetrahedral • trigonal pyramidal • bent • linear • trigonal planar

Note: that lone pairs of electrons effect the shape but

are ignored in the name of the geometry

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The geometry of a

molecule refers to

the positions of

atoms only.

VSEPR AND MOLECULAR GEOMETRY

VSEPR AND MOLECULAR GEOMETRY

methane, CH4

Bonds are all evenly spaced apart

Tetrahedral

109.5○

Less repulsion between the bonding pairs of electrons

.. ammonia

NH3

Trigonal Pyramidal

●●

surprise: the lone pairs occupy more space than the bonded atoms (with very few exceptions)

Two unbonded pairs of electrons make bond angles slightly less than tetrahedral due to

greater repulsion

.. water

H2O

bent

O H H

Bonded electrons can take maximum position apart – 180°

.. Carbon dioxide

CO2

linear

No lone pairs of electrons allows maximum bond angle in 1-D plane

.. Barium floride

BF3

Trigonal planar

LESSON 11: LET’S BUILD IT

Molecular Shape

CHEMCATALYST

1. What is the Lewis dot structure of

formaldehyde, CH2O?

2. Draw formaldehyde’s structural formula.

3. How many electron domains do you think this

molecule has? Explain your reasoning.

KEY QUESTION

How can you predict the shape of a molecule?

YOU WILL BE ABLE TO:

predict and explain molecular shape, including in

molecules with multiple bonds

PREPARE FOR THE ACTIVITY

Work in groups of four.

Using the gumdrop, marshmallow, and toothpick kits,

build a model of formaldehyde, CH2O.

DISCUSSION NOTES

Double or triple bonding changes the number of

electron domains around an atom, affecting the

overall shape of a molecule.

Trigonal planar shape: A flat triangular shape

found in small molecules with three electron

domains surrounding the central atom.

DISCUSSION NOTES (CONT.)

Linear shape: A geometric shape found in

small molecules with two electron domains

surrounding the central atom.

The number of electron domains is more

important in determining the structure of a

molecule than is the number of atoms.

DISCUSSION NOTES (CONT.)

The more atoms in a molecule, the more

combinations of shapes you might see

together.

WRAP UP

How can you predict the shape of a molecule?

Drawing the Lewis dot structure of a molecule

allows us to predict its three dimensional shape.

The presence of double or triple bonds changes the

number of electron domains around an atom, which

in turn affects the overall shape of the molecule.

The shape of large molecules is determined by the

smaller shapes around individual atoms.

Water, H2O, has two unshared pairs, and its

molecular geometry takes the shape of a

“bent” or angular molecule.

Bent

Molecule Lewis Structure Number of

electron pairs

CH4

NH3

SHAPE

Tetrahedral

Trigonal

Pyramidal

4

4

(3 shared

1 lone pair)

Molecule Lewis Structure Number of

electron pairs

H2O

CO2

SHAPE

Bent or V

4

(2 shared

2 lone pairs)

2

Linear

Molecule Lewis Structure Number of

electron pairs

BeCl2

BF3

SHAPE

2

3

Linear

Trigonal

Planar

LESSON 14: HOW DOES THE NOSE KNOW?

Receptor Site Theory

CHEMCATALYST

1. Suppose you needed to separate coins but

could not see them. Explain how you would

make a machine that detects and sorts coins.

2. How do you think your nose detects a smell?

KEY QUESTION

How does the nose detect and identify different smells?

YOU WILL BE ABLE TO:

come up with a plausible model to explain how

smell works in the nose, based on the evidence

thus far

describe the receptor site model

PREPARE FOR THE ACTIVITY

Work in groups of four.

DISCUSSION NOTES

Scientists have proposed many theories about how

smell works and created models corresponding to

these theories.

DISCUSSION NOTES (CONT.)

Receptor site theory: The currently accepted model

explaining how smells are detected in the nose. Molecules

fit into receptor sites that correspond to the overall shape of

the molecule. This stimulates a response in the body.

WRAP UP

How does the nose detect and identify different

smells?

The currently accepted model for smell describes

smell molecules landing in receptor sites that fit the

shape of the smell molecules.

In the receptor site model, each receptor site has a

specific shape that corresponds to the shape of just

a few smell molecules.

CHECK-IN

One of the molecules that gives coffee its smell is 2-furylmethanethiol.

1. Write down everything you know about how this

molecule is detected by the nose.

2. Draw a possible receptor site for this molecule.

The strongest intermolecular forces exist

between polar molecules.

•Polar molecules act as tiny dipoles. A dipole is created by equal but opposite charges that

are separated by a short distance. •The direction of a dipole is from the dipole’s positive pole to

its negative pole.

•A dipole is represented by an arrow with its head pointing

toward the negative pole and a crossed tail at the positive

pole.

•The dipole created by a hydrogen chloride molecule is

represented below:

MOLECULAR POLARITY AND DIPOLE-DIPOLE

FORCES

H Cl

The negative region in one

polar molecule attracts the

positive region in adjacent

molecules. So the molecules

all attract each other from

opposite sides.

The forces of attraction

between polar molecules

are known as dipole-

dipole forces.

Which atom attracts e- more?

H ― Cl δ+ δ-

electronegativities 2.1 3.0

C = O H H

2.5 3.5 2.1

2.1

O = C = O