Chemistry 101 : Chap' 8

1
Chemistry 101 Chap. 8
Basic Concepts of Chemical Bonding

• Chemical Bonds, Lewis Symbols and the Octet Rule
• (2) Ionic Bonding
• (3) Covalent Bonding
• (4) Bond Polarity and Electronegativity
• (5) Drawing Lewis Structures
• (6) Resonance Structures
• (7) Exceptions to the Octet Rule
• (8) Strengths of Covalent Bonds

2
Chemical Bonds
Chemical bond is formed when two atoms or ions
are held together by the attractive force
between them.
? Ionic Bond a chemical bond formed between
cation and anion
? Covalent Bond a chemical bond formed between
two nonmetallic atoms by sharing one or more
pairs of electrons.
? Metallic Bond a chemical bond formed when
valence electrons of metal atom are attracted
by the nuclei of surrounding atoms (electrons
are free to move throughout the metal)
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Lewis Symbols
? Lewis electron dot structure (or Lewis symbol)
Symbol of element surrounded by dots
representing the valence electrons in the atom
Lewis symbol for sulfur Ne3s23p4

S
Maximum 2 electrons on each side
? This works only for representative elements
(main group)
Gilbert N. Lewis (1875-1946)
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Lewis Symbols
Elements Group e- Configuration
Lewis Symbol
Hydrogen 1A 1s1
H
Helium 8A 1s2
He
Lithium 1A
He2s1 Li
Berylium 2A He2s2
Be
Boron 3A
He2s22p1 B
Carbon 4A
He2s22p2 C
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Lewis Symbols
Elements Group e- Configuration
Lewis Symbol
Nitrogen 5A
He2s22p3 N
Oxygen 6A
He2s22p4 O
Fluorine 7A
He2s22p5 F
Neon 8A
He2s22p6 Ne
Note All four sides of the symbol are equivalent
O
O

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Lewis Symbols
? Elements in the same group of periodic table
have the same Lewis symbols
F Cl Br I
Elements in the same group have the same valence
electron configurations
For halogen atoms ns2np5
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Octet Rule
? Only the valence electrons are involved in
chemical bonding.
? Octet Rule When forming chemical bond, atoms
tend to gain, loose or share electrons in
order to achieve a complete octet of valence
electrons (ns2np6)
? same electron configuration as noble gas atom

K

Ar
Ar
electron configuration
Both ions have an octet of electrons !
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Ionic bonding

• Ionic Bonding Cations (metals) and anions
  (non-metal)
• combine to
  form ionic bonds

NaCl
Alternating positive and negative charges
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Ionic bonding
? NaCl formation Na(s) ½ Cl2(g) ? NaCl
(s) ?Hof -490 kJ
Metal small ionization energy
Na(g) ? Na(g) e- IE 496 kJ
Non-metal large electron affinity
Cl(g) e- ? Cl-(g) EA -349 kJ
Removing an electron from Na and transferring it
to Cl is NOT exothermic ! Then, why NaCl
formation is an exothermic process?
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Ionic bonding
The main driving force to form ionic bonds is the
electrostatic interaction between positive and
negative ions.
charges of ions
distance between ions
? Strength of ionic bond depends on Eel
? the larger Eel, the stronger the bond
? the greater the charges, the stronger the bond
? the smaller the distance between the charges,
the stronger the bond
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Ionic bonding
The stronger the ionic bond the the
melting point
higher
66, 133
1261oC
66, 140
2852oC
SrF2
2, -2
113, 133
1473oC
r1
r2
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Covalent bonding
? Covalent bond is formed when two atoms share
electrons in order to achieve the electron
configuration of the nearest noble gas.
? satisfy octet rule
Each hydrogen has the electron configuration of He
H
H

Each fluorine has the electron configuration of Ne
F
F

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Covalent bonding
? Lewis dot structure for covalent bonds

H
H
H H
H
H
single covalent bond
F
F

F
F
A shared electron pair is drawn as a dash (two
bonding electrons)
Unshared electrons are drawn as dots (lone-pair
electrons)
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Covalent bonding
? Example Draw the Lewis dot structures of H2O
and NH3
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Covalent bonding
? Multiple bond
F
F
F
F

F
F
or
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Covalent bonding
? Single and Multiple bond
X X
Bond strength increases
X X
X X
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Drawing Lewis Structure
? Things to know before you start to draw Lewis
structure
? Chemical formulas are often written in the
order in which the atoms are connected
ex) HCN
? Hydrogen has only two electrons (shared) and
always has only one covalent bond
? The central atom is usually written first
ex) NH3, CCl4, CHCl3, PCl3
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Drawing Lewis Structure
? Rules for drawing Lewis structure
(1) sum the number of valence electrons from
all atoms
(2) write the symbols for the atoms and connect
them with a single bond
(3) complete the "octet rule" for the atoms
bonded to central atom
(4) place any left over electrons on the central
atom
(5) If there are not enough electrons to give the
central atom 8 electrons, try multiple
bonds.
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Drawing Lewis Structure
Lewis Structure of NH3
(1) Total number of valence electrons 5 3 ? 1
8
(2) Connect atoms with a single bond
H N H H
and count the number electrons used for single
bond 6
(3) Complete the octets on the atoms bonded to
the central atom done
(4) Place remaining electrons (8-62) on
the central atom
H N H H
(5) All atoms are satisfying octet. No need to
consider multiple bonds
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Drawing Lewis Structure
Lewis Structure of CO
(1) Total number of valence electrons 4 6 10
(2) Connect atoms with a single bond
C O
and count the number electrons used for single
bond 2
(3) Complete the octets on the atoms bonded
to the central atom (6 electrons are used)
C O
(4) Place remaining electrons (10-2-6 2)
on the central atom
C O
(5) Carbon is NOT satisfying octet rule.
Need to have multiple bonds
C O
C O
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Drawing Lewis Structure
? Example Determine the Lewis structure of HCN
(1) Total number of valence electrons
(2) Connect atoms with a single bond and
count the number electrons used for single
bond
(3) Complete the octets on the atoms bonded
to the central atom
(4) Place remaining electrons on the central atom.
(5) Carbon is NOT satisfying octet rule.
Need to have multiple bonds
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Drawing Lewis Structure
? Example Determined the Lewis structure of
CH2O
(1) Total number of valence electrons
(2) Connect atoms with a single bond and
count the number electrons used for single
bond
(3) Complete the octets on the atoms bonded
to the central atom
(4) Place remaining electrons on the central atom.
(5) Carbon is NOT satisfying octet rule.
Need to have multiple bonds
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Drawing Lewis Structure
? Example Determined the Lewis structure of H2O2
(1) Total number of valence electrons
(2) Connect atoms with a single bond and
count the number electrons used for single
bond
(3) Complete the octets on the atoms bonded
to the central atom
(4) Place remaining electrons on the central atom.
(5) All atoms are satisfying octet. No
need to consider multiple bonds
What happens if you choose a different geometry
in step (2)?
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Drawing Lewis Structure
Lewis Structure of ClO3- ion
(1) Total number of valence electrons 7 6?3
1 26
(2) Connect atoms with a single bond and
count the number electrons used for single
bond 6
O Cl O O
(3) Complete the octets on the atoms bonded
to the central atom (18 electrons are used)
O Cl O O
(4) Place remaining electrons (26-18-6 2)
on the central atom
O Cl O O
O Cl O O
(5) All atoms are satisfying octet. No
need to consider multiple bonds
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Drawing Lewis Structure
? Example Determined the Lewis structure of
ClO2-
(1) Total number of valence electrons
(2) Connect atoms with a single bond and
count the number electrons used for single
bond
(3) Complete the octets on the atoms bonded
to the central atom
(4) Place remaining electrons on the
central atom
(5) All atoms are satisfying octet. No
need to consider multiple bonds
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Drawing Lewis Structure Exceptions

• Atoms having fewer than 8 valence electrons
• Group IIA and IIIA
  (mostly Be, B).
• Example BeCl2

(1) Total number of valence electrons 2 2 ? 7
16
(2) Connect atoms with a single bond and
count the number electrons used for single
bond 4
Cl Be Cl
(3) Complete the octets on the atoms bonded
to the central atom (12 electrons are used)
Cl Be Cl
(4) Place remaining electrons (16 - 4 -12 0)
on the central atom None left
(5) Be is not satisfying the octet rule, but no
electron is available
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Drawing Lewis Structure Exceptions

• Atoms having more than 8 valence electrons
• central atom with n? 3, which can use
  d-orbitals for bonding
• Example SF4

(1) Total number of valence electrons 6 4 ? 7
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(2) Connect atoms with a single bond and
count the number electrons used for single
bond 8
F F
S
F F
F F
(3) Complete the octets on the atoms bonded
to the central atom (24 electrons are used)
S
F F
(4) Place remaining electrons (34-8-24 2)
on the central atom
F F
(5) S is not satisfying the octet rule (10
electrons)
S
F F
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Drawing Lewis Structure Exceptions

• Molecule having an odd number of valence
  electrons
• Example NO2

(1) Total number of valence electrons 5 6 ? 2
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(2) Connect atoms with a single bond and
count the number electrons used for single
bond 4
O N O
(3) Complete the octets on the atoms bonded
to the central atom (12 electrons are used)
O N O
(4) Place remaining electrons (17-4-12 1)
on the central atom
O N O
(5) Nitrogen has only 5 electrons. Need to
have multiple bonds
Free radical
O N O
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Drawing Lewis Structure Exceptions
? Example Determine the Lewis structure of BF3,
BrF5 and OH
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Drawing Lewis Structure Resonance
Lewis Structure of SO3
(1) Total number of valence electrons 6 3 ? 6
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O
(2) Connect atoms with a single bond and
count the number electrons used for single
bond 6
S
O
O
O
(3) Complete the octets on the atoms bonded
to the central atom (18 electrons are used)
S
O
O
(4) Place remaining electrons on the central
atom. No more electron is left (24-6-180)
O
O
O
(5) Sulfur is NOT satisfying octet rule.
Need to have multiple bonds
S
S
S
O
O
O
O
O
O
Resonance structures
All three S-O bonds have the same length
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Drawing Lewis Structure Resonance
? Example Determine the Lewis structure of O3
and HCO2-
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Properties of Covalent Bond
? Bond length The distance between two bonded
atoms
? ?
bond length
Bond length depends on the size of two atoms and
the number of covalent bond (single, double or
triple) between them.
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Properties of Covalent Bond
? Example Predict which member of each set
would have the shortest
bond length
S
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Properties of Covalent Bond

• Bond Enthalpy Energy required to completely
  separate two
• bonded atoms in gas phase. A short bond is
  usually harder to break.

C (g) 4 H (g)
DH 1660 kJ/mol
(g)
per C-H bond
C (g) H (g)
D (C-H) 1660/4 kJ/mol 415 kJ/mol
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Properties of Covalent Bond
Bond enthalpy can be used to estimate the
enthalpy change of chemical reactions, ?Hrxn
H2(g) Cl2(g) ? 2HCl(g) ?H ?
?H1
?H2
?Hrxn
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Properties of Covalent Bond
?H1 D(H-H) D(Cl-Cl) 436kJ/mol
243kJ/mol 679 kJ/mol
?H2 2 ? ? D(H-Cl) 2 ? - 431 kJ/mol
- 862 kJ/mol
?Hrxn ?H1 ?H2 697kJ/mol 862 kJ/mol -183
kJ/mol
DHorxn S n x Dbroken S m x Dformed
moles of bonds
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Properties of Covalent Bond


?Hrxn
bonds broken H H 1 Cl Cl 1

bonds formed H Cl 2
Bond Enthalpy (kJ/mol) H H
436 Cl Cl 243 H Cl
431
?Hrxn 1 ? 436 1 ? 243 2 ? 431 -
183 kJ/mol
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Properties of Covalent Bond
? Example Estimate the ?Hrxn of following
reaction
CH4 (g) 2 O2 (g) ? CO2 (g)
2 H2O (g)
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Electronegativity
? Electronegativity A measure of the attraction
an atom has for
the electron in a bond
Metals ? low electronegativity Nonmetals ? high
electronegativity
electronegativity scale
Fluorine 4 (most electronegative) ?
most strongly attracting electron Cesium
0.7 (least electronegative) ? most easily
giving up electron
Linus Carl Pauling (1901-1994)
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Electronegativity
Pauling scale of electronegativity
Element EN F 4.0 O
3.5 Cl 3.0 N 3.0
C 2.5 H 2.1
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Bond Polarity
? Nonpolar covalent bond
When two atoms of same element are bonded
together, there is equal sharing of the
electrons in the bond
non-polar covalent bond equal sharing of
electrons
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Bond Polarity
? Polar covalent bond
When two different elements are bonded together,
there is unequal sharing of the electrons in the
bond
?
? -
The bonding pair of electrons is pulled
toward the chlorine atom (partial charge)
polar covalent bond unequal sharing of electrons
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Bond Polarity and Electronegativity
?
? -
ionic bond electrons are not shared
polar covalent bond unequal sharing of electrons
?EN 3.0 0.9 2.1
?EN 3.0 2.1 0.9
?EN lt 0.5 non-polar bond
0.5 ? ?EN lt 2.0 polar bond
?EN ? 2.0 ionic bond
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Bond Polarity and Electronegativity
? Example For each pair of bonds, predict which
bond is more polar and the
partial charge on the atoms
(a) Cl Br Br F
(c) C H C O
(b) O F S F
(d) H O Na O