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Chapter 8 Covalent Bonding
Section 8.1 The Covalent Bond *
Section 8.2 Naming Molecules
Section 8.3 Molecular Structures *
Section 8.4 Molecular Shapes
Section 8.5 Electronegativity and Polarity
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• The stability of an atom, ion or compound is related to
its energy: lower energy states are more stable.
• Atoms achieve noble gas configuration by forming
compounds (substances composed of two or more
different elements chemically combined). They can be
either ionic compounds or molecular compounds.
• A covalent bond results from sharing electrons (usually
between nonmetallic elements.
• A molecule is formed when two or more atoms bond
covalently and is lower in potential energy than its
constituent atoms.
A molecule is the smallest electrically neutral unit of a
substance that still has the properties of the substance
and is held together by covalent bonds.
Molecules of one compound are identical, but different
from molecules of other compounds.
water
H2 O
2 Hydrogen
atoms and
1 oxygen
atom
carbon dioxide
CO2
1 carbon
atom and
2 oxygen
atoms
Seven nonmetallic elements that exist as diatomic molecules
Hydrogen H2
Fluorine F2
Oxygen O2
Nitrogen N2
Chlorine Cl2
“start at 7, make a 7, plus hydrogen”
Bromine Br2
Iodine I2
forget At2, astatine! because its
radioactive, unstable, half life of 8 hrs
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Different ways to display molecular formulas
Perspective Drawing
Molecular Formula
Lewis structure
Structural Formula
Space-filled
Molecular
Model
Ball-and Stick
Molecular Model
Strategy to draw Lewis structure see also p. 254
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A single covalent bond
is one shared pair of valence electrons
or
H H
“Lewis electron dot structure”
Pairs of valence electrons that are not shared between atoms are
called unshared, free, lone or nonbonding electron pairs.
Every single dot is
looking for one
other single dot of
another atom
(like holding hands)
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s
Sigma bonds are single covalent bonds.
Sigma bonds occur when the pair of shared electrons is
in an area centered between the two atoms.
s
s = Sigma (lower case “s” in Greek alphabet)
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Multiple Covalent Bonds
Double bonds form when two pairs of electrons are
shared between two atoms.
Triple bonds form when three pairs of electrons are
shared between two atoms.
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A multiple covalent bond consists of
one sigma bond and at least one p bond.
The pi (p) bond is formed when parallel orbitals overlap and
share electrons, and it occupies the space above and below the
line that represents where the two atoms are joined together.
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pi (p) bond,
(Sigma (s) bond along the line not shown)
Two sigma and two pi bonds
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Ex. C6H6
1)
2)
3)
4)
Drawing Lewis dot structures:
Put atoms with many single dots in the
center, every dot can make one bond.
Atoms with only one single dot are usually
on the outside, because they form only one
“regular” covalent bond (H, F, Cl, Br, I).
Leftover single dots should be used for
double or triple bonds.
Often more than one formula is possible
(isomers).
C2H4O2
Can you find a different way
to “connect the dots”?
Example acetic acid
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Lewis structure of polyatomic ions
Atoms within a polyatomic ion are covalently bonded.
 same procedure, but first add or remove dots
corresponding to charge (-) add, (+) remove
Symmetry often stabilizes a molecule or particle
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Coordinate covalent bond is a bond in which one atom
contributes both bonding electrons.
also known as dative bond (latin: to give),
dipolar bond or semipolar bond
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Resonance is a condition that occurs when more
than one valid Lewis structure can be written for a
molecule or ion. (move electrons but not atoms!!)
The molecule or ion behaves as
though it has only one average
structure. The bond lengths are
identical to each other and
intermediate between single and
double covalent bonds.
Resonance structures of NO3Electrons that can be pushed around in a molecular structure
are called delocalized and stabilize the molecule or ion. 15
carbonate CO3
2-
The double arrow is used between
resonance structures (one line but with
arrow heads on both end)
Benzene C6H6
Draw the
Lewis resonance structures for
NO2SO2
O3
Sulfur trioxide SO3
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Three Exceptions to the Octet Rule
1) A small group of molecules might have an odd number
of valence electrons, for example NO2 (5 + 6 + 6 = 17)
Resonance structures help to stabilize the molecule
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Exceptions to the Octet Rule
2) Few compounds form stable configurations with less
than 8 electrons around the atom a suboctet.
Boron trihydride is reactive and will form coordinate bonds or “dimerize”
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Exceptions to the Octet Rule
3) Elements in period 3 or higher can use d-orbitals to form
more than 4 covalent bonds (more than 8 valence electrons),
called an expanded octet. (most common exception)
Draw the expanded octet Lewis structure for ClF3
A rare compound, containing the noble gas xenon
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The Strength of Covalent Bonds
depends on the distance between the two nuclei,
or bond length. The shorter the bond the stronger it
is, the greater the energy required to break it.
The amount of energy required to break a bond is called
the bond dissociation energy
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Every chemical reactions involves a change in energy
Exothermic Reaction:
Energy is being released (test tube getting hot)
more energy is released than is required to break the bonds in
the initial reactants
CH4 (g) + 2 O2 (g) -- CO2 (g) + 2 H2O (g) + energy
Endothermic Reaction:
Energy is being absorbed (test tube getting cold)
2 H2O (l) + energy -- 2H2 (g) + O2 (g)
a greater amount of energy is required to break a bond in
reactants than is released when the new bonds form in the
products.
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Exothermic!
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What is it called when one or more
correct Lewis structures can be drawn
for a molecule?
A. suboctet
B. expanded octet
C. expanded structure
D. resonance
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Where do atoms with expanded octets
occur?
A. transition metals
B. noble gases
C. elements in period 3 or higher
D. elements in group 3 or higher
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