Ch 9

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Chapter 9
Models of Chemical Bonding
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Models of Chemical Bonding
9.1 Atomic Properties and Chemical Bonds
9.2 The Ionic Bonding Model
9.3 The Covalent Bonding Model
9.4 Between the Extremes Electronegativity and
Bond Polarity
9.5 An Introduction to Metallic Bonding
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A general comparison of metals and nonmetals
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Types of Chemical Bonding
1. Metal with nonmetal
electron transfer and ionic bonding
2. Nonmetal with nonmetal
electron sharing and covalent bonding
3. Metal with metal
electron pooling and metallic bonding
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The three models of chemical bonding
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Lewis Electron-Dot Symbols
For main group elements -
Example
Nitrogen, N, is in Group 5A and therefore has 5
valence electrons.
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Lewis electron-dot symbols for elements in
Periods 2 and 3
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Ionic Bonding
Ionic bonding results when there is a transfer of
electrons between two atoms. Each atom achieves
a full outer level of electrons. For many atoms
in the 2nd and 3rd period, this would be 8
electrons, known as the octet rule.
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SAMPLE PROBLEM 9.1
Depicting Ion Formation
PLAN
Draw orbital diagrams for the atoms and then move
electrons to make filled outer levels. It can be
seen that 2 sodiums are needed for each oxygen.
SOLUTION
2 Na
Na2O
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Three ways to represent the formation of Li and
F- through electron transfer.
Electron configurations
Li 1s22s1

F 1s22s22p5
Li 1s2
Orbital diagrams
Lewis electron-dot symbols
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Born-Haber Cycle
Lattice Energy - Energy released when ions come
together forming an ionic solid. Remember Hesss
law states that the enthalpy change between two
states is the same as the sum of enthalpies in a
multistep process that goes between the same two
states.
?H0f ? ?H0 elements to atoms ? ?H0 ions
from atoms ?H0lattice
So from this one equation if all of the ?H0 s
except one are known, e.g. ?H0lattice , the
value can be calculated.
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The Born-Haber cycle for lithium fluoride
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Trends in lattice energy
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Covalent Bonding
Covalent bonding results when two electrons are
shared in an orbital between two atoms. Each
atom achieves a full outer level of electrons
resulting in a lower energy system. The pair of
electrons used are called the shared or bonding
pair. In terms of the octet rule, this pair of
electrons counts for both atoms in completing the
octet. The electron pairs that are not involved
in bonding belong only to the atom with which
they are associated. These are called lone
pairs.BOND ORDER - When only one pair of
electrons are shared between two atoms, its
called a single bond. If two pairs of electrons
are shared covalently between two atoms, its
called a double bond three pairs, triple bond.
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Covalent bond formation in H2.
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The attractive and repulsive forces in covalent
bonding.
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Bond Energy - The amount of energy required to
break a bond. The greater the energy, the
stronger the bond. Bond breaking is an
endothermic process, so bond breaking enthalpies
are positive.
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Bond Length - In general, the closer the
electrons are held by the atoms, the shorter the
bond length and the higher the bond
energy. Multiple bonds result in stronger,
shorter bonds.
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SAMPLE PROBLEM 9.2
Comparing Bond Length and Bond Strength
PROBLEM
Using the periodic table, but not Tables 9.2 and
9.3, rank the bonds in each set in order of
decreasing bond length and bond strength
(a) S - F, S - Br, S - Cl
PLAN
(a) The bond order is one for all and sulfur is
bonded to halogens bond length should increase
and bond strength should decrease with increasing
atomic radius. (b) The same two atoms are bonded
but the bond order changes bond length decreases
as bond order increases while bond strength
increases as bond order increases.
SOLUTION
(a) Atomic size increases going down a group.
(b) Using bond orders we get
Bond length S - Br gt S - Cl gt S - F
Bond strength S - F gt S - Cl gt S - Br
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Electronegativity and Bond Polarity
Electronegativity, (EN), is the ability of an
atom to attract electron density of shared
electrons. To the extent that an atom attracts
extra electron density away from the other atom,
it has a partial negative charge. The other atom
has a corresponding positive charge. This creates
a polar bond. The greater the difference in EN
between the two atoms, the more polar the
bond. The extreme polar bond is where the
electrons are completely with one atom from the
other. What has already been identified as an
ionic bond. On the Pauling scale of EN, fluorine
is the most electronegative with a value of 4.0.
All other elements are less electronegative with
Cesium being the least with a 0.7 value.
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The Pauling electronegativity (EN) scale.
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Electronegativity and atomic size.
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SAMPLE PROBLEM 9.3
Determining Bond Polarity from EN Values
PROBLEM
(a) Use a polar arrow to indicate the polarity
of each bond N-H, F-N, I-Cl. (b) Rank the
following bonds in order of increasing polarity
H-N, H-O, H-C.
PLAN
(a) Use Figure 9.16(button at right) to find EN
values the arrow should point toward the
negative end.
(b) Polarity increases across a period.
SOLUTION
(a) The EN of N 3.0, H 2.1 F 4.0 I
2.5, Cl 3.0
N - H
F - N
I - Cl
(b) The order of increasing EN is C lt N lt O
all have an EN larger than that of H.
H-C lt H-N lt H-O
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Boundary ranges for classifying ionic character
of chemical bonds.
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Percent ionic character of electronegativity
difference (DEN).
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Properties of the Period 3 chlorides.
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End of Chapter 9