8.2
Lesson 8.2 The Nature of Covalent Bonding
Objectives
Lesson Links
Ch. 8 Lab Practicals: Model Building: Covalent Bonding
and Empirical Formula of a Hydrated Compound
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8.2.1 Explain the result of electron sharing in covalent
bonds.
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8.2.2 Describe how coordinate covalent bonds are
different from other covalent bonds.
Ch. 8 Core TR: Section 2 Review
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8.2.3 Identify some exceptions to the octet rule.
8.2 Chemistry Tutorial: Drawing Electron Dot Structures
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8.2.4 Explain how the strength of a covalent bond is
related to its bond dissociation energy.
Ch. 8 Concepts in Action: The Air Up There
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8.2.5 Describe how resonance structures are used.
8.2 Lesson Overview (PowerPoint file)
8.2 Kinetic Art: Covalent Bonds
Chapter 8 Online Student Edition
Study WB Chapter 8 Lesson 2
Overview/Materials
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Overview
Standard
In this lesson, you will cover the topics of the octet rule, multiple and
coordinate covalent bonds, bond dissociation energy and resonance.
Classroom Materials
3
Block
1.5
Quick Lab: 1 170-g (6-oz) can of food, 2 454-g (16-oz) cans of food, 3 No.
25 rubber bands, metric ruler, coat hanger, plastic grocery bag, paper
clip, graph paper, motion detector (optional)
Standard
There are no items.
Chemistry & You
Description
page 1 of 9
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Engage
Content
Chapter 8 Online Student
Edition
Have students read the text in the
Chemistry & You feature on p. 226.
Explain that scientists attribute the thinning
of the ozone layer, in part, to the action of
compounds called chlorofluorocarbons (
CFCs), which have been released into the
atmosphere.
Ask Why is ozone important in the
atmosphere? (It filters out radiation that
could harm living things on Earth.)
Activate Prior Knowledge
Description
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Engage
Prior to beginning this lesson, ask students
to recall what they know about electron
configuration and the rules that govern it.
Have student volunteers demonstrate the
proper use of the aufbau principle, the Pauli
exclusion principle, and Hund's rule.
The Octet Rule in Covalent Bonding
Description
Explain
Write electron configurations for carbon,
nitrogen, oxygen, fluorine, and neon on
the chalkboard.
Ask How many electrons would carbon,
nitrogen, oxygen, and fluorine need to
share in order to achieve the same electron
configuration as neon? (4, 3, 2, 1,
respectively)
Have the students work in pairs to complete
8.2 Kinetic Art: Covalent Bonds on
PearsonChem.com.
Work through Sample Problem 8.1 on p. 2
29 with the class and have the students
work together in pairs to solve the problems
at the bottom of the page. Have students
complete the 8.2 Chemistry Tutorial:
Drawing Electron Dot Structures on
PearsonChem.com for problem 8 if they
need extra reinforcement.
page 2 of 9
Content
8.2 Kinetic Art: Covalent
Bonds
Content
Chapter 8 Online Student
Edition
Content
8.2 Chemistry Tutorial:
Drawing Electron Dot
Structures
The Octet Rule in Covalent Bonding
Description
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Explore
Divide students into groups of three or four.
Have them practice drawing molecular
diagrams, structural formulas, electron-dot
structures, and orbital diagrams for
molecules such as OF2, SCl2, N2H4, CCl4,
and C2H6.
Have students draw electron dot structures
for each element in the second row of the
periodic table: Li, Be, B, C, N, O, and F.
Then have them answer the following:
•
Predict how many bonds each
atom must form to attain a noble
gas configuration. (1, 2, 3, 4, 3, 2,
1)
•
Can lithium form a covalent bond
and reach stability? (no)
•
Which elements can reach
stability by forming covalent
bonds? (C, N, O, F)
•
Can fluorine form an ionic bond?
(yes)
•
Are the bonds in nitrogen
molecules, N2, ionic or covalent?
(covalent)
The Octet Rule in Covalent Bonding
Description
Explain
Direct students to examine the molecular
and structural formulas, electron dot
structures, and orbital diagrams for
ammonia on p. 228 and compare it to the
fluorine and water examples on p. 227.
Display physical models of all of the
molecules. Call attention to the fact that
fluorine has one half-filled orbital and forms
one bond, oxygen has two half-filled orbitals
and forms two bonds, and nitrogen has
three half-filled orbitals and forms three
bonds.
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Show the students the electron configuration
of carbon. Point out that carbon has two
half-filled orbitals.
Ask How many covalent bonds do you think
carbon forms? (Students may say two.) Tell
students that when carbon bonds with
hydrogen CH2 is not a stable molecule.
Explain the concept of electron promotion,
which allows carbon to form four single
covalent bonds. Draw the electron dot
structure of methane and explain that it is
more stable.
Explain that s electrons can be promoted to
p orbitals, which increases the bonding
capacity. Have the students write boron's
electron configuration and then explain how
it is possible for BCl3 to form.
To introduce the discussion of multiple
covalent bonds, use the electron dot
structure for the nitrogen molecule.
Ask How does the structure of diatomic
nitrogen satisfy the octet rule? (The nitrogen
atoms can share six electrons.)
Have students compare the bonding in
ammonia with the bonding in nitrogen gas.
Then introduce the oxygen molecule.
Instruct students to draw a structure that
obeys the octet rule.
Ask Why doesn't oxygen form a triple bond?
(Each oxygen atom needs to share only
two electrons to achieve a stable electron
configuration.)
Help students draw the electron dot
structure and orbital diagram for carbon
dioxide.
Ask What type of bonds does carbon form
with the two oxygen atoms in CO2? (double
covalent bonds)
Have students draw diagrams for hydrogen
cyanide (HCN) and formaldehyde (H2CO).
Ask What kinds of bonds does carbon form
in each of these molecules? (HCN: one
single carbon-to-hydrogen bond and one
triple carbon-to-nitrogen bond; H2CO: two
single carbon-to-hydrogen bonds and one
double carbon-to-oxygen bond)
Ask the students to make models for the
CO2, HCN, and H2CO molecules to see the
bonding structure.
page 4 of 9
Coordinate Covalent Bonds
Description
Explain
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Content
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Chapter 8 Online Student
Edition
Explain that a coordinate covalent bond is
an exception to the rule that covalent
bonding occurs between two half-empty
orbitals of two atoms. Show students that in
a coordinate covalent bond, one atom has
an empty orbital and the other, an orbital
filled with an electron pair that is not yet
involved in a chemical bond. Note that the
bonding still involves only one pair of
electrons and one pair of orbitals, but one
atom provides both of the shared electrons.
Show the carbon monoxide example from p.
232 so the students can see where the extra
pair of electrons comes from.
Have students write the electron dot
structure for SO2. Emphasize that the
structure should satisfy the bonding
requirements of all three atoms. Students
should find that, to satisfy the octet rule for
all the atoms, they must write a structure in
which one oxygen atom is double bonded to
sulfur. The other oxygen is single bonded
by a coordinate covalent bond in which the
electrons are donated by sulfur.
Exceptions to the Octet Rule
Description
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Explain
Have students try to write the electron dot
structure for NO2. They will be unable to find
a way to arrange the 17 electrons around
the central nitrogen and the two oxygen
atoms so that the octet rule is obeyed. Ask
the students to share their structures and
draw the different options on the board.
Explain that there are several different
plausible options, called resonance
structures, but that the octet rule isn't
satisfied in any of them.
Bond Dissociation Energies
Description
Explain
Explain that it takes energy to break
covalent bonds, this energy is called bond
dissociation energy.
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Content
Table 8.3
Ask Predict if bond dissociation energy will
increase or decrease with an increase in
the number of covalent bonds between
atoms. (increase)
Have the students look at Table 8.3. Have
the students write a summary statement
using any patterns they notice between
bonds, bond energy, and bond length.
Quick Lab
Description
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Purpose
To compare and contrast the stretching of
rubber bands and the dissociation energy of
covalent bonds.
Materials
1 170-g (6-oz) can of food, 2 454-g (16-oz)
cans of food, 3 No. 25 rubber bands,
metric ruler, coat hanger, plastic grocery
bag, paper clip, graph paper, motion
detector (optional)
Prep Time
10 minutes
Class Time
30 minutes
Teaching Tip
Use tape to secure the hanger in place if
necessary. Have groups of students record
the data and then create a graph as a class,
using an overhead projector.
Expected Outcome
As the mass of the load increases, the
stretch of the rubber band or bands
increases. For a given mass, a single
rubber band stretches farther than a double,
and a double rubber band stretches farther
than a triple.
Bond Dissociation Energies
Description
page 6 of 9
Extend
Have students draw a graph of actual bond
dissociation energies versus number of
bonds for carbon-carbon single, double and
triple bonds. Have students write a
statement that relates this graph to their
conclusions in the lab activity.
Resonance
Description
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Explain
Remind students of the electron dot
structures that they drew for SO2. Draw
them on the board again and show that
there are two possible ways to draw the
structure with the double and single bonds
reversed. Describe that these are called
resonance structures. Ask them to draw the
electron dot structure of ozone, O3.
Encourage them to try to draw the two
possibilities. Invite students to share their
drawings.
Explain that the resonance drawings show
all electrons paired but that experimental
evidence shows that the two bonds in ozone
are the same length, as if the actual
bonding is the average of the two electron
dot structures.
Have students fill in the blanks in the two
following questions to summarize
exceptions to the octet rule.
1.
2.
Molecules with an ________
number of electrons (odd)
Molecules where one atom has
more or less than an ________
(octet)
Assess and Remediate
Description
Evaluate
Challenge students to write electron dot
structures for SBr2 and C3H8. Have students
work in pairs to determine whether or not
their structures are correct, and if not, how
to correct their errors. Then, have students
explain coordinate covalent bonding in their
own words. (A coordinate covalent bond is
formed when one atom contributes both
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bonding electrons.)
Ask When do resonance structures occur?
(Resonance structures occur when two or
more valid electron dot structures can be
written for a molecule.)
Then, have the students complete the 8.2
Lesson Check.
Remediate
As a class, create a concept map
interlinking each concept covered in the
lesson. Have students use their notes as
well as the textbook to add as much detail
as possible. Creating a visual relationship
of the concepts may help solidify students'
understanding.
Differentiated Instruction
Description
Special Needs Students
Recreate the electron dot structures from
the lesson on large posters or overheads.
Show students with a colored marker how to
draw the dots. Point out where on the
periodic table the element is located; circle
the element and circle the group number.
Then explain the reasoning for the
placement and the number of dots. Make
sure the noble gas group on their periodic
table is highlighted.
Less Proficient Readers
Divide students into groups of three or four.
Have them draw electron dot formulas for
oxygen and fluorine, then combine the
drawings into the electron dot formula for
OF2. Have them do the same for SCl2 and
CCl4. When students have mastered these,
continue the exercise with CHCl3, N2H4 ,
and C2H6.
Advanced Students
Have students research or discover
elements and compounds with atoms that
do not follow the octet rule. (sample
answers: Be, Al, P, NO2, PF5)
page 8 of 9
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Study WB Chapter 8
Lesson 2
Content
Ch. 8 Core TR: Section 2
Review
Focus on ELL
Description
Content and Language
Have each student prepare a set of flash
cards for the vocabulary words in this lesson.
Each card should list the vocabulary word,
its definition, a representative drawing or
model, and the page number where it can
be found. Cycle through the index cards a
few times, reading the words aloud as a
class. Point out familiar prefixes, poly- and
dis- and co-, and remind students of their
meanings. (many; not; with or together)
My Notes
page 9 of 9
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Content
Study WB Chapter 8
Lesson 2
Content
Ch. 8 Core TR: Section 2
Review
Homework