Living By Chemistry
Unit 2: SMELLS
Molecular Structure and Properties
In this unit you will learn:
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how atoms form molecules
to predict the smell of a compound
to interpret molecular models
how the nose detects different molecules
Day 15
ChemCatalyst
1. What do you think is happening when you
smell something?
2. Why do you think we have a sense of smell?
Day 16
ChemCatalyst
Will any of the molecules shown on the following
slide have similar smells? Explain your thinking.
Key Question
What does chemistry have to do with smell?
How can molecules with the same
molecular formula be different?
What are the rules for drawing structural
formulas?
You will be able to:
• detect patterns in chemical formulas and relate
these patterns to a molecular property
• create a hypothesis based on analysis of data
• describe the difference between structural
formulas and molecular formulas
• recognize isomerscreate accurate structural
formulas from molecular formulas
• identify and differentiate between isomers and
molecules oriented differently in space
• explain and utilize the HONC 1234 rule
Prepare for the Activity
Work in groups of four.
Molecular formula: The chemical formula of a
molecular substance, showing the types of atoms
in each molecule and the ratios of those atoms to
one another.
Prepare for the Activity (cont.)
Chemicals may have very strong odors or be
caustic. When smelling, don’t sniff directly from the
container. Instead, use a wafting technique (use
your hand to draw air toward you).
Discussion Notes
A possible hypothesis is “The smell of a substance
can be predicted if you know its name and/or its
chemical formula.”
Discussion Notes
Even though the molecules in vials G and H have
identical molecular formulas, they have different
smells and therefore must be different somehow.
A structural formula is a two-dimensional drawing
of a molecule showing how the atoms in a molecule
are connected.
Discussion Notes (cont.)
Structural formula: A drawing or diagram that a
chemist uses to show how the atoms in a molecule
are connected. Each line represents a covalent
bond.
There are several ways to draw the same structural
formula without changing the identity of the
molecule.
Discussion Notes (cont.)
When two molecules have the same molecular
formula but different structural formulas, they are
called isomers of each other.
Isomers: Molecules with the same molecular
formula but different structural formulas.
Discussion Notes (cont.)
Discussion Notes (cont.)
Molecules can smell different even if they have the
same molecular formula.
Molecules can smell similar even if they have
different molecular formulas.
Discussion Notes
The HONC 1234 rule is a way to remember the
bonding tendencies of hydrogen, oxygen,
nitrogen, and carbon atoms in molecules.
Hydrogen tends to form one bond, oxygen two,
nitrogen three and carbon four.
When trying to decide whether two structures
represent the same molecule, you must check
how the atoms are connected.
Wrap Up
What does chemistry have to do with smell?
• Smell appears to be related to molecular
formula and chemical name.
Wrap Up
How can molecules with the same molecular formula
be different?
• Structural formulas show how the atoms in a
molecule are connected.
• A molecular formula can be associated with more
than one structural formula.
• Isomers are molecules with identical molecular
formulas but different structural formulas.
• The smell of a molecule is a property that
appears to be related to its structure.
Wrap Up
What are the rules for drawing structural formulas?
• The HONC 1234 rule indicates how many times
hydrogen, oxygen, nitrogen, and carbon atoms
tend to bond.
• When a molecule is oriented differently in space,
it is still the same molecule.
Check-in
1. How would you expect a compound with the
molecular formula C8H16O2 to smell? Explain.
2. How sure are you of your prediction?
extra
ChemCatalyst
Examine the molecules on the following slides.
What patterns do you see in the bonding of
atoms of hydrogen, oxygen, carbon, and
nitrogen?
Day 17
ChemCatalyst
These diagrams are called Lewis dot symbols.
Look at the Lewis dot symbols and answer the
questions.
1. What is the relationship between the number
of dots, the number of valence electrons, and
the HONC 1234 rule?
2. Create a Lewis dot symbol for fluorine, F.
How many bonds will fluorine make?
Key Question
How does one atom bond to another in a
molecule?
How do atoms bond to form molecules?
You will be able to:
• create accurate structural formulas using Lewis dot
symbols
• describe the type of bonding found in molecular
substances
• explain the chemistry behind the HONC 1234 rule
• apply the octet rule to predict bonding in molecules
• draw Lewis dot structures and structural formulas for
molecules that contain double and triple bonds
Prepare for the Activity
Work in groups of four.
Lewis dot symbol: A diagram that uses dots to
show the valence electrons of a single atom.
Prepare for the Activity (cont.)
Each puzzle piece contains the correct number of
valence electrons for that atom. It also contains the
appropriate number of tabs for bonding.
Prepare for the Activity
Work in groups of four.
Octet rule: Nonmetal atoms combine so that each
atom has a total of eight valence electrons by
sharing electrons.
Prepare for the Activity (cont.)
Each atom in the molecule has an “octet” of
valence electrons. Note that the hydrogen atoms
do not follow the octet rule. They have a total of two
electrons, similar to the noble gas helium, He.
After bonding, each chlorine
atom has a total of eight valence
electrons surrounding it.
Cl2
PCl3
H2S
Discussion Notes
You can use Lewis dot symbols to create Lewis dot
structures.
Lewis dot structure: A diagram that uses dots to
show the valence electrons of a molecule.
Discussion Notes (cont.)
A covalent bond is the sharing of a pair of
electrons between two nonmetal atoms.
Bonded pair: A pair of electrons that are
shared in a covalent bond between two atoms.
Discussion Notes (cont.)
Some valence electrons are not involved in
bonding.
Lone pair: A pair of valence electrons not involved
in bonding within a molecule. The two electrons
belong to one atom.
Discussion Notes
The HONC 1234 rule and the octet rule both help
you figure out Lewis dot structures and structural
formulas.
Both the HONC 1234 rule and the octet rule can be
satisfied by using double and triple bonds
appropriately.
It is not possible to create a triple-bonded oxygen
compound, according to the HONC rule.
There are exceptions to the bonding rules laid out
here.
Wrap Up
How does one atom bond to another in a
molecule?
• A covalent bond is a bond in which two atoms
share a pair of valence electrons.
• Lewis dot symbols show the valence electrons
in an atom and are used to predict bonding in
a molecule.
Wrap Up (cont.)
• In a Lewis dot structure, a pair of electrons that
are shared in a covalent bond is called a
bonded pair. Pairs of electrons that are not
involved in bonding and belong to one atom
are referred to as lone pairs.
• The HONC 1234 rule indicates how many
electrons are available for bonding in atoms of
hydrogen, oxygen, nitrogen, and carbon.
Wrap Up
How do atoms bond to form molecules?
• Elements form covalent bonds by sharing
electrons until each atom has eight valence
electrons. This is called the octet rule. Hydrogen
is an exception. It forms bonds such that it has
two valence electrons.
• Atoms can form double and triple bonds to
satisfy the octet rule.
• When covalent bonds form, each atom
resembles a noble gas in its electron
configuration.
Day 18
ChemCatalyst
Draw the Lewis dot structure for the two
covalently bonded molecules shown here.
Explain how you arrived at your answer.
a. Cl2
b. O2
Check-in
1. One of these compounds has multiple bonds in
it. Which one is it? Explain.
C4H10
C4H6
2. Draw one possible structural formula for C4H6.
Day 19
ChemCatalyst
Consider these compounds. List at least three
differences and three similarities between the
two molecules.
Molecule 1
Molecule 2