Unit 8. Bonding
Upon successful completion of this unit, the students should be able to:
8.1 Determine the number of valence electrons for main group elements.
1.
Determine the number of valence electrons for the following elements:
a.
b.
c.
d.
e.
Al
Ar
Ag
Pb
Ca
8.2 Draw Lewis dot symbols for atoms and ions of main group elements.
1.
Draw the Lewis dot symbols for: a) Al
b) Ar
c) He
d) Pb
e) Sr
8.3 Describe the difference between ionic and covalent bonding modes.
1.
True or False. The phrase “transfer of electrons” is generally associated with covalent bonding.
8.4 Recognize that bonding is an example of a model.
1.
On page 370 of your textbook, the author states “Models are often wrong”. If this is the case, explain
why we use models in the first place.
8.5 Predict which combinations of elements tend to bond ionically, polar covalently,
and nonpolar covalently.
1.
Label the bond between the following as ionic, polar covalent, nonpolar covalent, or none (if no bond
forms).
a. K and Ne
b. S and S
c. C and I
2.
Arrange in order of increasing ionic character: Na2S, MgS, S8.
3.
Label the bond between the following as ionic, polar covalent, nonpolar covalent, or none (if no bond
forms).
a. Rb and Cl
4.
b. Br and N
Which of the following has the most ionic character: CaCl2, NiCl2, or AlCl3?
8.6 State and explain the effect of a loss or gain of electrons on atomic size.
1.
a.
b.
Which is larger in size: K or K+?
Which of the following is a partial explanation of the correct answer to part “a”:
A.
B.
C.
D.
K+ has one more electron than K therefore shielding has increased
K+ has one more electron than K therefore shielding has decreased
K+ has one less electron than K therefore shielding has increased
K+ has one less electron than K therefore shielding has decreased
2.
Using the concept of effective nuclear charge in your explanation, explain why anions are larger than
there parent atoms.
3.
a.
b.
Which is larger in size: F or F-?
Which of the following is a partial explanation of the correct answer to part “a”:
A.
B.
C.
D.
4.
a.
b.
F- has one more electron than F therefore shielding has increased
F- has one more electron than F therefore shielding has decreased
F- has one less electron than F therefore shielding has increased
F- has one less electron than F therefore shielding has decreased
Arrange the following in order of increasing size: Sr2+, Kr, Br-.
Explain in detail your answer to part “a” making sure to describe all important factors.
8.7 Describe the meaning of lattice energy and solve qualitative problems related to the
concept.
1.
Which of the following would have the larger lattice energy: CaBr2 or KBr?
2.
Which of the following would have the larger lattice energy: CaBr2 or SrBr2?
3.
Define lattice energy.
8.8 Write the electron configurations of ions.
1.
Write the noble gas abbreviated electron configuration of the following:
a.
b.
c.
d.
Mn2+
Te2Ag+
Cs+
2.
Write the noble gas abbreviated electron configuration for the stable ion of selenium (Se).
3.
Write the noble gas abbreviated electron configuration of the following:
a. Cr
b. Cr3+
c. As3-
4.
How many unpaired electrons are there in Cr3+?
8.9 Define electronegativity and apply it towards determining bond polarity.
1.
Which of the following bonds would be more polar: H-F or H-Cl?
2.
Which atom in the following bonds will be partially negative: a. C-Cl?
3.
Which of the following bonds would be more polar: N-Cl or C-Cl?
b. H-Br?
8.10 Determine the formal charge on any atom within a molecule or ion.
1.
a.
What is the formal charge on the bromine in the following ion?
O
O Br O
O
b.
What is the formal charge on each oxygen in the above ion?
8.11 Draw Lewis structures for molecules and polyatomic ions and to recognize when
to apply the octet rule model and when to apply the model which satisfies formal
charge rules.
1.
2.
Draw the best Lewis structure for the following compounds. Include any nonzero formal charges.
b.
d.
f.
h.
i.
NO2+
HNO2
PCl3
CS2
BH3
a.
c.
e.
g.
i.
H2O
CH3OCH2CH3
H2SO4
PCl5
CH3COOH (only one O is a central atom)
a.
b.
Following the usual rules for writing Lewis structures, draw the Lewis structure for O2.
Experimental evidence indicates that not all the valence electrons in O2 are paired. What does this
evidence suggest about using Lewis structure rules as our model for covalent bonding?
8.12 Draw resonance structures for molecules and ions and solve problems related to
the resonance concept.
1.
Indicate if the following structures are actually different compounds or simply resonance forms of the
same compound.
H
H C
H C
H
H
H
C
C
H
C
C
H
H
H
H C
H
C
C
C
H
C
H
C H
H
2.
Draw the three best resonance structures of the sulfite ion, SO32-. Include any nonzero formal charges.
3.
Using resonance structures, explain why acetic acid, CH3COOH, is a much stronger acid than ethanol,
CH3CH2OH. (hint: the strength of an acid is related to the stability of the anion that forms after it
loses a proton).
4.
Draw all the resonance structures of NCF (C is central atom) that satisfy the octet rule for all atoms.
Using formal charges rules, explain which is the most plausible structure and most accurately
represents the true molecule.
5.
Draw the two best resonance structures for methyl isocyanate, CH3NCO, a toxic gas that was
responsible for the deaths of at least 3000 people when it was accidentally released into the atmosphere
in December 1984 in Bhopal, India. Indicate any nonzero formal charges.
8.13 Use the Valence Shell Electron Pair Repulsion Theory (VSEPR) to predict
electron pair geometries for the following electron pair arrangements: linear, trigonal
planar, tetrahedral, trigonal bipyramidal, octahedral.
See sample questions under objective 8.14.
8.14 Use VSEPR to predict bond angles and molecular geometries, and to sketch
shapes of molecules using perspective drawings.
1.
2.
Provide the requested information for a) Cl2CO (carbon is the only central atom), and b) BrF3.
i.
ii.
iii.
iv.
v.
Lewis structure
Sketch of actual shape
Approximate bond angles
electronic geometry
molecular geometry
a.
The following is from the archives of Chemical and Engineering News:
NEWS OF THE WEEK
January 25, 1999
Volume 77, Number 4
CENEAR 77 4 p. 7
ISSN 0009-2347
[Table of Contents][Next Story]
N5+ CATION MAKES
EXPLOSIVE DEBUT
High-energy ion is first new all-nitrogen species in 100 years
Rebecca Rawls
It wasn't in his abstract, but Karl O. Christe, a chemist who studies high-energy
materials at the Air Force Research Laboratory at Edwards Air Force Base in
California, had a little something extra to offer last week in his plenary presentation
at the American Chemical Society's Winter Fluorine Conference in St. Petersburg
Beach, Fla. Along with colleague William W. Wilson, Christe has synthesized and
characterized a salt containing the N5+ cation. The cation is the first new all-nitrogen
species to be synthesized in isolable quantities in more than a century, and only the
third ever to be produced.
Suggest a reasonable Lewis structure for the newly synthesized N5+ cation and provide a sketch of
what you believe to be the actual shape of the molecule. Make sure to include all formal charges
in your Lewis structure.
b.
The article continues:
Christe synthesized the AsF6salt of N5+ by reacting N2F+AsF6 with HN3 in anhydrous
hydrogen fluoride at 78 C. He calls the resulting white powder "marginally stable" at 22
C. With careful handling, he was able to obtain mass spectroscopic data on the
compound at that temperature. However, his presentation included slides of his low
temperature laser spectroscopy apparatus "before" and "after" a sample of a few
milligrams of the N5+ salt exploded and destroyed the sample chamber.
The counter ion for the newly formed (and fairly unstable!) N5+ was AsF6-. Provide the
following information for the AsF6- ion.
i.
ii.
iii.
iv.
v.
3.
Lewis structure
Sketch of actual shape
Approximate bond angles
electronic geometry
molecular geometry
Which is the most likely bond angle for O3?
a.
b.
c.
d.
90o
109.5o
117o
120o
8.15 Predict whether molecules are polar (have a dipole moment) or nonpolar (do not
have a dipole moment).
1.
Which of the following is nonpolar?
a.
b.
c.
d.
2.
CO
SO2
CHCl3
PF5
Is ICl3 polar or nonpolar? Explain how you arrived at your answer.
3.
Which of the following molecules have a dipole moment?
a.
b.
c.
SF6
H2Se
XeF2
8.16 Use the valence bond theory (localized electron model) to predict hybridization
and solve related problems.
1.
The structure of cocaine is shown below:
H
N
H
C
1
H O
H
2
C O C H
H
H
H
O C
O
cocaine
a.
What is the electronic geometry around: C#1 _________________
C#2 __________________
b.
What is the approximate bond angle around: C#2? ________
c.
What is the hybridization of: C#1 ______ C#2 ________ O#1 ________
O#1? _________
N _______
2.
The structure of the antimalaria drug quinine is shown below. It was first obtained by extraction of the
bark of the South American Cinchona tree.
H
H
H
H
C
1
C H
H
H
C
C
H
H H
H C C
C H
H
N
C H
C
1
HO C
H H
H
C
H
C
C
H
C
C
CH 3O
C
C
C
C
N
2
H
H
a.
b.
c.
d.
What is the hybridization of nitrogen #1?
What is the hybridization of nitrogen #2?
What is the hybridization of carbon #1?
What is the bond angle around carbon #1?
3.
What hybridization on a central A atom would make an AX4 compound into a polar molecule?
Assume a polar A-X bond. (Lone pairs may be present on the central atom, A.)
a.
sp
b.
sp2
c.
sp3
d.
sp3d
8.17 Describe the difference between sigma and pi bonds and be able to sketch pi
bonds.
1.
Sketch the pi bonding in N2.
2.
List two ways in which a sigma bond differs from a pi bond.
3.
How many sigma bonds are contained in the following molecule?
H H
C
C
C
H
4.
The C=O bond in COCl2 can be described as:
a.
b.
c.
d.
5.
CH3
a σ bond and a π bond, both involving sp hybrid orbitals on C
a σ bond involving an sp hybrid orbital on C and a π bond involving a p orbital on C
a σ bond and a π bond, both involving sp2 hybrid orbitals on C
a σ bond involving an sp2 hybrid orbital on C and a π bond involving a p orbital on C
How many pi bonds are in the following molecule?
H
H O H
N C
C C
C
H
H
H
C
C H
C
C H
H C C H
H H H
8.18 State the relative bond lengths of similar single, double, and triple bonds and solve
related problems.
1.
Rank the following in order of increasing C to O bond length: CO, CO2, CO32-. Explain your choices.
2.
Hydrogen azide, HN3, is an acid in aqueous solution. The structure of hydrogen azide is indicated
here. Write a Lewis structure(s) consistent with the data given.
H
N
124 pm
N
113 pm
N
8.19 Describe basic molecular orbital theory, in particular how it contrasts with
valence bond theory.
1.
Describe the basic difference between molecular orbital theory and valence bond theory.
8.20 Define and explain the difference between bonding and antibonding orbitals.
1
Describe the difference between a bonding and antibonding orbital.
2.
Compare the energies of molecular orbitals of homonuclear diatomic molecules with the energies of
the atomic orbitals with which they correlate.
a.
b.
c.
d.
Both bonding and antibonding molecular orbitals lie lower in energy than the atomic orbitals.
Bonding orbitals are lower and antibonding orbitals are higher in energy than the atomic orbitals.
Bonding orbitals are higher and antibonding oribitals are lower in energy than the atomic orbitals.
Both bonding and antibonding molecular orbitals are higher in energy than the atomic orbitals.
8.21 Draw MO diagrams for period 1 and 2 homonuclear diatomics and use them to
evaluate bond order.
1.
Draw a MO diagram for O2 and O22-. Is either of these species paramagnetic? What is the bond
order in each?
2.
The elements N, O, and F exist as diatomic molecules. Use MO theory to explain why Ne, the next
element on the periodic table, does not exist as diatomic molecules.
Additional Unit 8 Sample Questions:
1.
2.
In the blanks to the right of the bonds, indicate what type of bond (ionic, polar covalent or nonpolar
covalent) is shown. For ALL the polar bonds given below, illustrate the bond dipole in this polar
bond by using either an arrow or partial charges.
a. N ≡ N
Type of Bond: _____________________
b. Si – O
Type of Bond: _____________________
c. C – H
Type of Bond: _____________________
d. P – Cl
Type of Bond: _____________________
e. Li – Br
Type of Bond: _____________________
Determine which of the following statements is FALSE. Then rewrite this false statement so that it
would be a TRUE statement.
a.
b.
c.
d.
e.
X-rays have a smaller wavelength than microwaves.
A photon of UV light would be higher in energy than a photon of IR light.
A lithium atom would have a higher ionization energy than a potassium atom.
A Sn atom is larger than a Sn2+ ion.
A double bond is shorter and stronger than a triple bond.
3.
Complete the following table with the requested information (table continued on next page).
Formula
Lewis structure
SeF4
NCl3
TeF6
CH2Cl2
H3PO4
RnF4
NO2-
(two best resonance structures)
Sketch
Approximate
Bond Angles
Electronic
Geometry
Formula
SeF4
NCl3
TeF6
CH2Cl2
H3PO4
RnF4
NO2-
Molecular Geometry
Hybridization of
Central Atom
Polar or Nonpolar Molecule
4.
The Lewis structure for HCN is shown below. Answer the following questions about HCN.
H―C≡N:
a.
b.
c.
d.
5.
What is the hybridization of the carbon atom?
What is the hybridization of the nitrogen atom?
What orbitals are overlapping to form the H-C bond?
List all the orbitals that are present on the nitrogen atom, and explain how they are used (σ bond?
lone pairs?, etc.). Include a sketch if necessary.
If you recall from biology, proteins are composed of many amino acid units. The linkage from one
amino acid unit to the next is called a peptide bond. An example of a peptide bond is shown below.
O
NH2CHC
CH3
O
NHCHCOH
CH3
peptide bond
One reason that proteins assume their unique three-dimensional structures is that rotation about the C
to N peptide bond is NOT possible. Provide an explanation to this apparent contradiction of the usual
rules of bond rotation. Support your explanation with a structure. (hint: resonance)
6.
Write the electron configuration for the following atoms or ions in their ground state.
a.
b.
c.
d.
e.
7.
The dichromate ion, Cr2O72-, has neither Cr-Cr nor O-O bonds.
a.
b.
8.
Cu
Ga
Ga3+
P3Mg2+
Taking both 4s and 3d electrons to be valence electrons for chromium, draw the best Lewis
structure for the ion (actually several resonance structures are equally as good, but you only need
to draw one structure).
What is the likely geometry around the Cr atoms?
True or False. Adding a π-bond between two σ-bonded atoms will decrease the bond length and
increase the bond order.