chapter 6 - allen independent school district€¦ · chapter 6 chemical bonding ... the table...
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B O N D I N G - S T R U C T U R E C H A P T E R 5 | 1
Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
Chapter 6 CHEMICAL BONDING Zumdahl. Chemistry 5
th ed. 8.1-4, 8.6-7, 8.9-13, 9.1, 10.1, p. 466-474, 22.1-22.3
I. Introduction
Picture1
As two atoms approach one another, there may be an attraction between the electrons on one atom and the positive nucleus
on another atom. The inner, or ___________________ electrons are quite content with their attraction to their own nucleus,
but the outer, or ___________________ electrons can sometimes be kind of fickle – especially if the other nucleus is
exceedingly attractive!
Picture:2
In PreAP/IB chemistry, we focused primarily on ionic bonding and covalent bonding. We are going to expand our bonding
offerings in AP/IBHL Chemistry by including network covalent, metallic, and coordinate covalent bonding.
1 http://www.webelements.com/webelements/elements/media/nearingzero/F.gif
2 http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch02/ljpot.jpg
Notice that energy when there is an attraction
between atoms to form a bond. Energy is
released when bonds form, energy is required
to break bond
After the optimal bond length –
the most stable “ r” – the energy
rapidly increases due to nucleus-
nucleus repulsion
Notice that energy when there is an attraction
between atoms to form a bond. Energy is
released when bonds form, energy is required
to break bond
After the optimal bond length –
the most stable “ r” – the energy
rapidly increases due to nucleus-
nucleus repulsion
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
II. Network Covalent Compounds Comic
3
A. Network covalent bonds involve the ___________________ of electrons by two atoms.
B. It differs from conventional covalent by virtue of its ___________________
___________________ , ___________________ , ___________________ of covalent
bonds between atoms arranged in a ___________________, ___________________
___________________
C. These are not ___________________ molecules with a defined beginning and end.
D. These substances are represented by the lowest whole number ratio, or
___________________ ___________________.
E. The size depends on when the ___________________ is fractured.
F. Your text discusses band-gap theory – you do not need to know this.
G. The table below outlines some common examples. The ones on the left are ___________________ of carbon.
Allotropes have the same element, but different structures and hence functions.
Name Structure Name Structure
Graphite Silica (SiO2)
SAND OR GLASS!
Diamond Silicates (SiO>2, neg
charge)
Fullerenes,
nanotubes
Silicon Carbide4
3 http://www.offthemarkcartoons.com/cartoons/1997-05-06.gif
4 http://www.nature.com/nature/journal/v430/n7003/images/430974a-f1.2.jpg
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
III. METALLIC BONDING
A. Metallic bonding also involves a ___________________
___________________, ___________________, and
___________________ ___________________.
B. Metals are typically described by ___________________ electron model, sometimes called a
___________________ ___________________ ___________________.
C. Metal ions are arranged in a ___________________ and the valence electrons are ___________________
throughout the lattice structure.
D. Again, these are not ___________________ molecules with a defined beginning and end.
E. An ___________________ has a metallic ___________________ mixed with a ___________________, which is
often, but not necessarily a metal. Examples include bronze (Cu & Sn or Al) and brass (Cu & Zn).5 Alloys do not
have a constant composition and are represented by concentration units such as ppm or % by mass.
a. In ___________________ alloys, the ___________________ metal replaces the ___________________
metal at lattice points. This type of structure is common when the metal atoms are roughly the same size.
b. When the solute is smaller than the solvent, the solute molecules may squeeze into the spaces surrounding
the metal atoms. These are called ___________________ ___________________. Steel is an alloy with
carbon atoms in the interstices of an iron lattice. Stainless steel is a combination of these two types of
alloys.
c. Stainless steel is a combination of substitutional and interstitial alloy. Iron is the solvent metal, nickel and
chromium are substituted for some of the iron atoms and carbon is an interstitial solute.
5 http://en.wikipedia.org/wiki/Alloy
POSITIVE METAL ION
“SEA” (COMMUNE!) OF VALENCE ELECTRONS
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
IV. Ionic Bonding A. Ionic bonds involve the ___________________ of electrons an atom of low electronegativity (likely a metal) to an
atom of high electronegativity (likely a non-metal).
B. The metal atom ___________________ electron(s) to become a ___________________ and is said to be
___________________.
C. The non-metal ___________________ electron(s) to become an ___________________ and is said to be
___________________.
Picture6
IN GENERAL: The ___________________ atoms are away from each other on the periodic table the
___________________ polar the bond. NOTE: Although carbon is more electronegative than hydrogen, a C-H bond is
considered ___________________.
Lets do it 1. Use the periodic table to predict the rank for the following bonds from least polar
(#1) to most polar(#6).
Ca-Se Li-F S-P O-F Sn-I I-O
Lets do it 2. Use electronegativity values to rank the following bonds from least polar (#1) to
most polar(#6).
Ca-Se Li-F S-P O-F Sn-I I-O
6 http://www.chemistry.ohio-state.edu/~grandinetti/teaching/Chem121/lectures/Electronegativity/ColvanttoIonic.jpg
0.5 1.0 1.5 2.0 2.5 3.0
Increasing ionic character
Increasing covalent character
At a EN = 1.7, the bond has >50% ionic
character, so we classify it as “ionic”. When in
doubt, classify it as ionic if a metal is present,
although we know from the previous example
of Sn-I that we are not always exactly correct!
0.5 1.0 1.5 2.0 2.5 3.0
Increasing ionic character
Increasing covalent character
At a EN = 1.7, the bond has >50% ionic
character, so we classify it as “ionic”. When in
doubt, classify it as ionic if a metal is present,
although we know from the previous example
of Sn-I that we are not always exactly correct!
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
D. Formula & structure
1. Formula units – When you write the chemical formula for an ionic compound,
you are simply indicating the smallest whole number ratio that balances charges.
2. The structure of an ionic compound is actually a long-range network of
___________________ positive and negative ions that are present in that
___________________ whole number ratio. No matter how much calcium
chloride you have, the ratio of Ca2+
to Cl─ is always 1:2.
E. Lewis Dot Structure
Step One: Form the ions. Use brackets around the ions.
·Ca·
® Caé
ëê
ù
ûú
2+
····Cl ·
··
®··
··Cl ·
···
é
ë
êê
ù
û
úú
1-
Step Two: Determine the smallest whole number ratio needed to form a neutral formula unit.
x(+2) + y(-1) = 0, least common multiple is 2, therefore two chloride ions combine with one calcium
ion Step Three: Draw the appropriate number of ions, making sure to alternate positive and negative ions.
····Cl ·
···
é
ë
êê
ù
û
úú
1-
Caé
ëê
ù
ûú
2+ ····Cl ·
···
é
ë
êê
ù
û
úú
1-
Lets do it 3. Draw Lewis dot structure for Aluminum sulfide.
F. Energy and size
1. This great little formula that can help us compare the ___________________ ___________________
between different ionic compounds, which in turn will help us compare ___________________
/___________________ in our next chapter. Note that this formula assumes that the bonds are pure
___________________ in character. Deviations from this calculation would indication increasing
___________________ character to the bond!
B O N D I N G - S T R U C T U R E C H A P T E R 5 | 6
Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
Cation(g) + anion(g) ionic cmpd (s)
r
QQkLE 21
Lets do it 4. You are the head chemist on the team designing the next generation of space shuttles. Your main
roadblock is developing a new, non-fossil based, fuel. You come up with the seemingly great idea of harnessing the
energy released from the formation of ionic bonds. Below are pairs you are considering. Write the lattice energy
reactions for each substance and then circle the one in each pair that should provide the greatest amount of energy
and write a sentence supporting your choice.
PAIR LATTICE ENERGY REACTIONS WINNER!
MgCl2 and NaCl
LiF and LiCl
NaCl and SrCl2
2. BORN-HABER CYCLE.7
Another way to approach the lattice energy of a substance is a more ___________________ but typically provides closer
values. In a Born-Haber cycle the lattice energy is calculated by ___________________ the energy of a variety of processes.
Although we have not had thermochemistry yet, you should be familiar with the concept of ___________________ – the
energy involved in a constant pressure process (remember exothermic and endothermic?!)
7 Pictures from http://www.4college.co.uk/a/O/bornhaber2.gif and
http://jcsu.jesus.cam.ac.uk/~rpc25/notes/chemistry/lattice_enthalpies/Image641.gif
As “r” (ie bigger ions), Lattice energy
As Q (either one!), Lattice energy
You need to know how to write the reaction equation (with phases) for lattice energy! what a lattice energy reaction is!
k = proportionality constant
Q1 = positive charge
Q2 = negative charge
R = ionic radius (distance
between ion centers)
For some reason students keep forgetting about this handy little formula! Memorize it – keep it near and dear to your heart!
B O N D I N G - S T R U C T U R E C H A P T E R 5 | 7
Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
B O N D I N G - S T R U C T U R E C H A P T E R 5 | 8
Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
V. Covalent Compounds – INORGANIC
A. Formula & structure
1. Molecular Formulas – unlike ionic compounds, molecular formulas for covalent molecules indicate the
___________________ ___________________ ___________________ ___________________ that are
bonded to one another. They do not, however, provide much information on ___________________ the atoms are
bonded to one another. For that we need some sort of ___________________ model.
2. Structural models
a. Lewis Dot structures
Count up the total number of valence electrons
Add or subtract the appropriate number of electrons as indicated by any charge
The element with the lowest electronegativity is typically written first and is also the central
atom.
Surround the central atom with the other atoms. REMEMBER: F and H can only form one
bond, O can only form two, and B can only form three. If there is an O and an H, often the
hydrogen is bonded to the oxygen rather than the central atom.
Place 2 dots (or some other symbol!) between atoms to form bonds.
Satisfy the octet for the central atom (or the peripherals – it doesn’t matter in the end).
Attempt to satisfy the octet of peripheral atoms.
Place extra electrons on the central atom. I often draw a line to any extra non-bonded
electron pairs.
If you are short electrons, have the central atom share more electrons to form double or
triple bonds.
Draw the Lewis dot structure for CH3Cl:
Valence electrons = 4 + 3(1) + 7 = 14
Draw the Lewis Dot Structure for phosphate: Valence electrons = 5 + 4(6) + 3 = 32
Draw the Lewis Dot structure for H3PO4: Valence electrons = 3(1) + 5 + 4(6) = 32
ClH
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C ClH
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B O N D I N G - S T R U C T U R E C H A P T E R 5 | 9
Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
Lets do it 5. Draw LDS for NF3, SF6, and SO42─
. Nitrogen trifluoride is used to clean chemical vapor deposition
chambers (used to deposit material on wafers). Sulfur hexafluoride is used as a dielectric (insulating material) in
high voltage circuit breakers and also as an etchant (chemically removed material) in the semiconductor industry.
Sulfuric acid is produced more than any other chemical!
Lets do it 6. Combined ionic:covalent LDS: Draw a Lewis dot structure for Sr3(PO4)2.
b. WHAT IF MORE THAN ONE LEWIS DOT STRUCTURE SEEMS REASONABLE?!!
SCENARIO ONE: The two structures are EQUAL ENERGY. Reality is a BLEND of the possibilities.
Resonance – To resonate means to match ___________________ (or vibrations). Resonance occurs when more than one
equivalent LDS structure can be drawn for a molecule. NOTE: The molecule does not transition from one resonance
structure to the next. The actual structure is best described a ___________________ or average of the resonant structures.
The shared electrons are said to be ___________________.
Bond Order=# bonds (e-pairs)
peripheral atoms sharing
Draw the Lewis Dot structure(s) for SO2
S OO •• •• •••• ••
• •• • • •
• • S OO •••• •••• ••
• •• • • •• •
or ??
S OOBond order = 1.5
S OO •• •• •••• ••
• •• • • •
• • S OO •••• •••• ••
• •• • • •• •
or ??
S OOBond order = 1.5
S OO •• •• •••• ••
• •• • • •
• • S OO •• •• •••• ••
• •• • • •
• • S OO •••• •••• ••
• •• • • •• •S OO •••• •••• ••
• •• • • •• •
or ??
S OO S OOBond order = 1.5
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
Picture8
NOTE: As bond order ___________________________, bond length ________________________________ & bond
energy _______________________________! (see cartoon for answers!)
Lets do it 7. Draw the LDS for the carbonate ion. What is the bond order for each bond to the carbon atom?
SCENARIO TWO: The two structures are NOT EQUAL ENERGY. Reality will be a CHOICE of the possibilities.
Formal Charge – How the heck do you know which is the most likely?? This is where formal charge comes into play.
Formal charge is a method of assigning electrons. There are two criteria to be considered:
(1) The most stable structure will typically have all formal charges as close to zero as possible
(2) If there is no choice but to have a residual negative charge, then it should be on the most electronegative atom.
FC = # valence electrons – [non-bonded electrons + ½ bonded electrons]
Lets do it 8. Use formal charges to determine the best structure for N2O
N N O Evaluation
8 http://www.newspaper.unsw.edu.au/archive/05_04_27/images/panel/images/VSU.jpg
I bet the bond
order concept is
important!
Do n ’t fo rge t th e
“s tru c tu re d e te rm in e s
fu n c tio n ” %# @** s h e
m a ke s u s kn o w !
Y eah, but its not hard!
T hink about it – as
bond order , there are
more electrons to pull
atoms closer together
so bond length
Not only that, but
increased electron
density between atoms
means the bond energy
is greater as well!...this
is scary – we are
starting to think l ike
her!
I bet the bond
order concept is
important!
Do n ’t fo rge t th e
“s tru c tu re d e te rm in e s
fu n c tio n ” %# @** s h e
m a ke s u s kn o w !
Y eah, but its not hard!
T hink about it – as
bond order , there are
more electrons to pull
atoms closer together
so bond length
Not only that, but
increased electron
density between atoms
means the bond energy
is greater as well!...this
is scary – we are
starting to think l ike
her!
B O N D I N G - S T R U C T U R E C H A P T E R 5 | 11
Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
c. VSEPR – this is an acronym for VALENCE SHELL ELECTRON PAIR REPULSION. The
model is built on the premise that electrons want to get as far away from one another as possible. If
you can visualize in three dimensions and apply a little geometry, this will be easy! NOTE: the order
of repulsion is:
non-bonded pair non-bonded > non-bonded pair bonded pair > bonded pair bonded pair
Cartoon9
References for visuals:
http://intro.chem.okstate.edu/1314F97/Chapter9/VSEPR.html
d. Hybridization – This model combines, or hybridizes, atomic orbitals to form molecular orbitals. For
each bond formed on the atom in question (typically the central atom or a carbon in a chain), the model
proposes hybridizing one atomic orbital. These hybridized molecular orbitals are called sigma ()
bonds. In a sigma bond, the electron density of the bond is between the atoms. The first bond
formed between two atoms is always a sigma bond. DON’T COUNT THE SECOND OR THIRD
BOND FORMED! These bonds are formed from unhybridized “p” orbitals and are called pi ()
bonds. In pi bonds, the electron density is above and below the atoms.
References for visuals:
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/flash.mhtml
http://winter.group.shef.ac.uk/orbitron/
B + X
# MOLECULAR
ORBITALS
NEEDED
# ATOMIC ORBITALS
TO HYBRIDIZE HYBRIDIZATION
2
3
4
5
6
What is the hybridization for the sulfur in SF6? You did the LDS earlier and saw that there are six bonds to the sulfur. That means we need six atomic orbitals from S for
form the six molecular orbitals needed for SF6. That means we need: s, p, p, p, d, d atomic orbitals. We say the hybridization
is sp3d
2 (NOTE: some sources would write this as d
2sp
3)
9 http://comosr.spps.org/sites/93f37ca8-07e5-4006-a522-
e7a5c43def17/uploads/Edited_Cartoon_male_teacher_using_overhead.JPG
We’re going to do a modeling activity that incorporates all of these models. THIS IS VERY IMPORTANT! I don’t know about you, but I have trouble thinking in 3-D, so we are going to play with models. If you understand this chart, you will be well on your way to correctly answering many of the anal questions we chemists contrive!
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
What is the hybridization on the carbon in H2CO(formic acid – the sting of an ant bite That means we need s, p, & p or a hybridization of sp
2
Check out this website to see rotating molecules! http://www.molecules.org/VSEPR_table.html
In order to simplify the memorization, we are going to use the following notation to generalize molecule types:
A = central atom B = # peripheral atoms X = # non-bonded electron pairs
Use this notation to memorize the names of the shapes!
Polarity – the word polarity implies some sort of an opposition or separation. We talk about two people being “polar
opposites.” Magnets and the earth both have north/south poles.10
Politics and religion can be VERY POLARIZING topics!
In our context, we are going to be looking at the separation of charge that arises when there is an ___________________
electron distribution within a molecule. This permanent, uneven charge distribution is called a ___________________
dipole moment.
Molecules with a permanent dipole moment (ie polar molecules)
will line up when placed in an electric field. NOTE: positive with
negative and vice versa!)
10
http://www.physics.ubc.ca/~outreach/phys420/p420_01/shaun/shaun/whyit7.jpg
In the CH3Cl molecule, EN(Cl) > EN (H & C). This means the electrons have a greater probability of being found in the region of space nearer the chlorine so that end of the molecule has a partial negative charge, leaving the other end electron deficient and partially positive. CH3Cl has a permanent dipole moment.
In H2O, EN(O) > EN (H) & there a two non-bonding pairs of electrons. Electron density on oxygen end > hydrogen end, so oxygen end has partial negative and hydrogen end has partial positive charge. Water has a permanent dipole moment. The arrow indicates the DM.
In CO2, EN(O) > EN (C) BUT oxygens pull evenly on both sides – no uneven distribution of electron density, no permanent dipole moment
In CCl4, EN(Cl) > EN (C) BUT chlorines pull evenly on all sides – no uneven distribution of electron density, no permanent dipole moment
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
BUT DR. LEGGETT – WHAT ABOUT THOSE DOUBLE AND TRIPLE
BONDS WE HAVE BEEN IGNORING?
The first bond formed between two atoms is called a ___________________ () bond. The electron
density in this type of bond is centered ___________________ the atoms.
The second and third bonds formed are called ___________________ () bonds. The bond is formed
between valence electrons in non-hybridized “p” orbitals. The electron density is centered either ___________________
/___________________ OR ___________________ /___________________ of the bonding atoms.
Lets do it 9. Let’s evaluate theobromine, a key component in chocolate! Step one: Draw in “C” atoms at each “bend”.
Step two: Count all the bonds to the carbon atoms – there must be four. If there aren’t – add “H” atoms! Step
three: Count the number of sigma and pi bonds. BE CAREFUL – “CH3” represents three sigma bonds. Step four:
Add non-bonded pairs of electrons to nitrogen and oxygen so that each has an octet. Step five: indicate the
hybridization around the oxygen, nitrogen and carbon atoms.
VI. Covalent Molecules – ORGANIC
This is a HUGE area of chemistry. We are only going to learn basic naming and ___________________
groups.
CLASS FUNCTIONAL
GROUP
NAMING EXAMPLE FUNCTIONAL
GROUP
POLARITY
MLCL
FORMULA
Alkane
Alkene
Alkyne
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
Alcohol
Aldehyde
Ketone
Carboxylic
Acid
Amine
VII. Coordinate Covalent Bonding
A. Definitions:
a. Coordinate Covalent Bond: A covalent bond in which ___________________ of the bond participants
supplies ___________________ of the electrons.11
b. Complex ion:
i. A metal ion, an ion such as H+, or an electron deficient atom acts as a ___________________
___________________ and ___________________ electron pairs.
ii. Ligand: group with non-bonded pair of electrons that can ace as a ___________________
___________________ (donates electron pairs)
c. Coordination complex: Complex ion with a counter ion to balance charge to form a neutral complex
d. Coordination number: The number of bonds a metal ion is able to form with ligands. Depends on
size, oxidation state and electron configuration.
B. Common Coordination numbers
2 2 & 4 4 4 & 6 6 Ag
+ Cu+ Zn
2+ Al3+ Fe
3+
Au+ Cd
2+ Co2+ Fe
2+
Ni2+ Cr
3+
Cu2+ Co
3+
11
http://hrsbstaff.ednet.ns.ca/dawsonrj/12%20Chem/Chapter%20notes/Chapter%2019%20%20Notes_files/image006.gif
NOTE: AP will accept any reasonable coordination
number. For most of these it means that you can use a
coordination number that is simply twice the oxidation
number! Fe2+
is the only exception.
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
C. Common Ligands:
Ligand Name Charge balance
CN− Cyano must balance + and – charge I
− Iodo must balance + and – charge
OH− Hydroxo must balance + and – charge SCN
− thiocyano must balance + and – charge
F−
Fluoro must balance + and – charge
Cl− Chloro must balance + and – charge NH3 Ammine charge is the same as the + ion
Br− Bromo must balance + and – charge H2O Aqua charge is the same as the + ion
D. Structures12
VII. Sample AP Questions: Copyright College Board – for face to face teaching only
1997 - Consider the molecules PF3 and PF5.
a) Draw the Lewis electron-dot structures for PF3 and PF5 and predict the molecular geometry of each.
b) Is the PF3 molecule polar, or is it nonpolar? Explain.
c) On the basis of bonding principles, predict whether each of the following compounds exists. In each
case, explain your prediction.
(i) NF5
(ii) AsF5
12
http://www.chemguide.co.uk/inorganic/complexions/octions.gif
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Dena K. Leggett, Ph.D., Allen High School, Allen, TX 2012-2013
1992 D
NO2 NO2─ NO2
+
Nitrogen is the central atom in each of the species given above.
(a) Draw the Lewis electron-dot structure for each of the three species.
(b) List the species in order of increasing bond angle. Justify your answer.
(c) Select one of the species and give the hybridization of the nitrogen atom in it.
(d) Identify the only one of the species that dimerizes and explain what causes it to do so.