Post on 08-Jul-2018
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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
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
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.
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
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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
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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.
• Each shape has a name (you will have to memorize these)
• tetrahedral • trigonal pyramidal • bent • linear • trigonal planar
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
No lone pairs of electrons allows maximum bond angle in 1-D plane
.. Barium floride
BF3
Trigonal planar
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.
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
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?
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
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.