Contributed by:
The highlights are:
1. Covalent Compounds
2. Lewis dot structures
3. Exceptions to the octet rule
4. Resonance and Molecular shape
5. Electronegativity and bond polarity
1.
Chapter 4
Lecture
Outline
Prepared by
Ashlyn Smith
Anderson University
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Copyright © McGraw-Hill Education. Permission required for reproduction or display.
2.
4.1 Introduction to Covalent Bonding
Covalent bonds result from the sharing of electrons
between two atoms.
• A covalent bond is a two-electron bond in which
the bonding atoms share valence electrons.
• A molecule is a discrete group of atoms held
together by covalent bonds. 2
3.
4.1 Introduction to Covalent Bonding
Unshared electron pairs are called nonbonded
electron pairs or lone pairs.
Atoms share electrons to attain the electronic
configuration of the noble gas closest to them
in the periodic table.
• H shares 2 e−.
• Other main group elements share e− until they
reach an octet of e− in their outer shell. 3
4.
4.1 Introduction to Covalent Bonding
A. Covalent Bonding and the Periodic Table
Lewis structures are electron-dot structures for
molecules. They show the location of all valence e−.
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5.
4.1 Introduction to Covalent Bonding
A. Covalent Bonding and the Periodic Table
Covalent bonds are formed when two nonmetals
combine, or when a metalloid bonds to a nonmetal.
How many covalent bonds will a particular atom form?
• Hydrogen forms one bond, with its one valence e−.
• Atoms with one, two, or three valence e− form
one, two, or three bonds, respectively.
• Atoms with four or more valence electrons form
enough bonds to give an octet. For these
atoms, the following formula is used:
predicted
predicted 8 – number of valence e−
=
bonds
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6.
4.1 Covalent Compounds
A. Covalent Bonding and the Periodic Table
General rule for bonding elements (except for hydrogen, H)
Number
Numberof
of bonds
bonds + Number
Numberof
oflone pairs = 44
lonepairs
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7.
4.1 Introduction to Covalent Bonding
B. Focus on the Human Body
There are many covalent compounds related to the
chemistry of the heart.
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8.
4.2 Lewis Structures
• A molecular formula shows the number and identity
of all of the atoms in a compound, but not which
atoms are bonded to each other.
• A Lewis structure shows the connectivity between
atoms, as well as the location of all bonding and
nonbonding valence electrons.
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9.
4.2 Lewis Structures
A. Drawing Lewis Structures
• General rules for drawing Lewis structures:
1) Draw only valence electrons.
2) Give every main group element (except H) an
octet of e−.
3) Give each hydrogen 2 e−.
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10.
4.2 Lewis Structures
HOW TO Draw a Lewis Structure
Arrange the atoms next to each other that
Step [1]
you think are bonded together.
• Place H and halogens on the periphery, since
they can only form one bond.
H H
For CH4: H C H not H C H H
H
This H cannot form
two bonds.
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11.
4.2 Lewis Structures
HOW TO Draw a Lewis Structure
• Use the common bonding patterns from Figure 4.1
to arrange the atoms (Slide 6).
H H H
For CH5N: H C N H not H C N H
H H H
Place four atoms Place three atoms
around C, since C around N, since N
generally forms generally forms
four bonds. three bonds.
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12.
4.2 Lewis Structures
HOW TO Draw a Lewis Structure
Step [2] Count the valence electrons.
• For main group elements, the number of valence
e− is equal to the group number.
• The sum gives the total number of e− that must
be used in the Lewis structure.
For CH3Cl: 1 C x 4e− = 4e−
3 H x 1e− = 3e−
1 Cl x 7e− = 7e−
14 total valence e− 12
13.
4.2 Lewis Structures
HOW TO Draw a Lewis Structure
Step [3] Arrange the electrons around the atoms.
• Place one bond (two e−) between every two atoms.
• For main group elements, give no more than 8 e−.
• For H, give no more than 2 e−.
• Use all remaining electrons to fill octets with lone
pairs, beginning with atoms on the periphery.
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14.
4.2 Lewis Structures
HOW TO Draw a Lewis Structure
For CH3Cl: H
4 bonds x 2e− = 8 e−
H C Cl
+ 3 lone pairs x 2e− = 6 e−
H
2 e on
− 8 e− 14 e−
each H on Cl All valence e− have
been used.
• If all valence electrons are used and an atom
still does not have an octet, proceed to Step [4].
Step [4] Use multiple bonds to fill octets when
needed.
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15.
4.2 Lewis Structures
B. Multiple Bonds
• One lone pair of e− can be converted into one
bonding pair of e− for each 2 e− needed to
complete an octet on a Lewis Structure.
• A double bond contains four electrons in two 2
e− bonds.
O O
• A triple bond contains six electrons in three 2 e−
bonds.
N N
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16.
4.2 Lewis Structures
B. Multiple Bonds
Example Draw the Lewis Structure for C2H4.
Step [1] Arrange the atoms.
H C C H
H H
Step [2] Count the valence e−.
2 C x 4 e− = 8 e−
4 H x 1 e− = 4 e−
12 e− total
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17.
4.2 Lewis Structures
B. Multiple Bonds
Step [3] Add the bonds and lone pairs.
5 bonds x 2 e− = 10 e−
H C C H + 1 lone pair x 2 e− = 2 e−
H H 12 e−
C still does not All valence e− have
have an octet. been used.
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18.
4.2 Lewis Structures
B. Multiple Bonds
Step [4] Change one lone pair into one bonding
pair of e–, forming a double bond.
H C C H H C C H
H H H H
Answer
Each C now has an octet.
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19.
4.3 Exceptions to the Octet Rule
• Most of the common elements generally follow
the octet rule.
• H is a notable exception, because it needs only
2 e− in bonding.
• Elements in group 3A do not have enough
valence e− to form an octet in a neutral
molecule.
F
F B F
only 6 e− on B
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20.
4.3 Exceptions to the Octet Rule
• Elements in the third row have empty d orbitals
available to accept electrons.
• Thus, elements such as P and S may have more
than 8 e− around them.
O O
HO P OH HO S OH
OH O
10 e− on P 12 e− on S
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21.
4.4 Resonance
When drawing Lewis structures for polyatomic ions:
• Add one e− for each negative charge.
• Subtract one e− for each positive charge.
For CN– : Answer
−
C N C N C N
1 C x 4 e − = 4 e− All valence e− Each atom
are used, but has an octet.
1 N x 5 e− = 5 e−
C lacks an octet.
–1 charge = 1 e−
10 e− total
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22.
4.4 Resonance
A. Drawing Resonance Structures
• Resonance structures are two Lewis structures
having the same arrangement of atoms but a
different arrangement of electrons.
• Two resonance structures of HCO3−:
• Neither Lewis structure is the true structure of HCO3−.
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23.
4.4 Resonance
A. Drawing Resonance Structures
• The true structure is a hybrid of the two resonance
structures.
• Resonance stabilizes a molecule by spreading out
lone pairs and electron pairs in multiple bonds
over a larger region of space.
• A molecule or ion that has two or more resonance
structures is resonance-stabilized.
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24.
4.4 Resonance
B. Focus on the Environment
• Resonance structures can be drawn for neutral
molecules.
• Ozone, O3, can be drawn as two resonance
structures.
O O O
O O O
• Ozone is formed in the upper atmosphere by the
reaction of O2 and oxygen atoms.
• It acts as a shield which protects the earth’s
surface from destructive UV radiation.
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25.
4.4 Resonance
B. Focus on the Environment
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26.
4.5 Naming Covalent Compounds
HOW TO Name a Covalent Molecule
Example Name each covalent molecule:
(a) NO2 (b) N2O4
Step [1] Name the first nonmetal by its element
name and the second using the suffix
“-ide.”
(a) NO2 (b) N2O4
nitrogen oxide nitrogen oxide
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27.
4.5 Naming Covalent Compounds
HOW TO Name a Covalent Molecule
Step [2] Add prefixes to show the number of
atoms of each element.
• Use a prefix from Table 4.1 for each element.
• The prefix “mono-” is usually omitted when
only one atom of the first element is present,
but it is retained for the second element.
• If the combination would place two vowels
next to each other, omit the first vowel.
mono + oxide = monoxide (not monooxide)
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28.
4.5 Naming Covalent Compounds
HOW TO Name a Covalent Molecule
(a) NO2
nitrogen dioxide
(b) N2O4
dinitrogen tetroxide
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29.
4.6 Molecular Shape
• The Lewis structure gives information about
how the atoms are connected, but it implies
nothing of the geometry or shape.
• To determine the shape around a given atom,
first determine how many groups surround the
atom.
• A group is either an atom or a lone pair of
electrons.
• Use the VSEPR theory to determine the shape.
• The most stable arrangement keeps the
groups as far away from each other as
possible. 29
30.
4.6 Molecular Shape
A. Two Groups Around an Atom
• Any atom surrounded by only two groups is
linear and has a bond angle of 180o.
• An example is CO2:
• Ignore multiple bonds in predicting
geometry. Count only atoms and lone pairs. 30
31.
4.6 Molecular Shape
B. Three Groups Around an Atom
• Any atom surrounded by three groups is
trigonal planar and has bond angles of 120o.
• An example is H2CO (formaldehyde):
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32.
4.6 Molecular Shape
C. Four Groups Around an Atom
• Any atom surrounded by four groups is
tetrahedral and has bond angles of 109.5o.
• An example is CH4 (methane):
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33.
4.6 Molecular Shape
C. Four Groups Around an Atom
• If the four groups around the atom include one
lone pair, the geometry is a trigonal pyramid
with bond angles of 107o, close to 109.5o.
• An example is NH3 (ammonia):
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34.
4.6 Molecular Shape
C. Four Groups Around an Atom
• If the four groups around the atom include two
lone pairs, the geometry is bent and the bond
angle is 105o (i.e., close to 109.5o).
• An example is H2O:
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35.
4.6 Molecular Shape
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36.
4.7 Electronegativity and Bond Polarity
• Electronegativity is a measure of an
atom’s attraction for e− in a bond.
• It tells how much a particular atom “wants” e−.
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37.
4.7 Electronegativity and Bond Polarity
• If the electronegativities of two bonded
atoms are equal or similar, the bond is
nonpolar.
• The electrons in the bond are being shared
equally between the two atoms.
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38.
4.7 Electronegativity and Bond Polarity
• Bonding between atoms with different electro-
negativities yields a polar covalent bond or
dipole, a partial separation of charge.
• The electrons in the bond are unequally shared
between the C and the O.
• e− are pulled toward O, the more electronegative
element; this is indicated by the symbol δ−.
• e− are pulled away from C, the less
electronegative element; this is indicated by the
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symbol δ+.
39.
4.7 Electronegativity and Bond Polarity
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40.
4.8 Polarity of Molecules
The classification of a molecule as polar or nonpolar
depends on:
• The polarity of the individual bonds
• The overall shape of the molecule
Nonpolar molecules generally
o have:
• No polar bonds
• Individual bond dipoles that cancel
Polar molecules generally have:
• One or more polar bonds
• Individual bond dipoles that do not cancel
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41.
4.8 Polarity of Molecules
To determine the polarity of a molecule with
two or more polar bonds:
1. Identify all polar bonds based on
electronegativity differences.
2. Determine the shape around individual
atoms by counting groups.
o
3. Decide if individual dipoles cancel or
reinforce.
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