Reading Assignments:
Chapter 8 in
R. Chang, Chemistry, 9th Ed., McGraw-Hill,
2006. or previous editions.
Or related topics in other textbooks.
Periodic Relationships Among
the Elements
Consultation outside lecture room:
Office Hours:
Tuesday & Thursday
10 AM -12 noon,
Wednesday
1-4 PM
Chapter 8
@Room 313-3 or by appointment
Chemistry for Engineers, SCS126
Chemistry for Engineers, SCS126
Classification of the Elements
Ground State Electron Configurations of the Elements
Chemistry for Engineers, SCS126
8.2
Chemistry for Engineers, SCS126
8.2
Electron Configurations of Cations and Anions
Cations and Anions Of Representative Elements
Of Representative Elements
Na [Ne]3s1
Na+ [Ne]
Ca [Ar]4s2
Ca2+ [Ar]
Al [Ne]3s23p1
Al3+ [Ne]
Atoms gain electrons
so that anion has a
noble-gas outer
electron configuration.
Atoms lose electrons so
that cation has a noble-gas
outer electron configuration.
H 1s1
H- 1s2 or [He]
F 1s22s22p5
F- 1s22s22p6 or [Ne]
O 1s22s22p4
O2- 1s22s22p6 or [Ne]
N 1s22s22p3
N3- 1s22s22p6 or [Ne]
Chemistry for Engineers, SCS126
Na+: [Ne]
Al3+: [Ne]
O2-: 1s22s22p6 or [Ne]
8.2
Chemistry for Engineers, SCS126
8.2
8.2
Chemistry for Engineers, SCS126
8.3
F-: 1s22s22p6 or [Ne]
N3-: 1s22s22p6 or [Ne]
Na+, Al3+, F-, O2-, and N3- are all
with Ne
What neutral atom is isoelectronic with H- ?
Chemistry for Engineers, SCS126
The Radii (in pm) of Ions of Familiar Elements
Cation is always smaller than atom from
which it is formed.
Anion is always larger than atom from
which it is formed.
Chemistry for Engineers, SCS126
8.3
Chemistry for Engineers, SCS126
Reading Assignments:
Chapter 9 in
R. Chang, Chemistry, 9th Ed., McGraw-Hill,
2006. or previous editions.
Chemical Bonding I:
Basic Concepts
Or related topics in other textbooks.
Consultation outside lecture room:
Office Hours:
Tuesday & Thursday
10 AM -12 noon,
Wednesday
1-4 PM
Chapter 9
@Room 313-3 or by appointment
Chemistry for Engineers, SCS126
Chemistry for Engineers, SCS126
8.3
Valence electrons are the outer shell electrons of an
atom. The valence electrons are the electrons that
participate in chemical bonding.
Group
e- configuration
1A
ns1
1
2A
ns2
2
3A
ns2np1
3
4A
ns2np2
4
5A
ns2np3
5
6A
ns2np4
6
7A
ns2np5
7
# of valence e-
Chemistry for Engineers, SCS126
9.1
The Ionic Bond
Li + F
1 22s22p5
1s22s1s
Lewis Dot Symbols for the Representative Elements &
Noble Gases
Chemistry for Engineers, SCS126
is a chemical bond in which two or more
electrons are shared by two atoms.
Li+ F -
Why should two atoms share electrons?
[He]
1s
1s2[2Ne]
2s22p6
F
e- +
Li+ +
9.1
+ e-
Li
Li+
F
F -
F -
Li+ F -
+
7e-
F
F F
7e-
8e- 8e-
Lewis structure of F2
single covalent bond
lone pairs
F
F
lone pairs
single covalent bond
lone pairs
Chemistry for Engineers, SCS126
9.2
F F
lone pairs
Chemistry for Engineers, SCS126
9.4
Lewis structure of water
H
+
O +
H
Lengths of Covalent Bonds
single covalent bonds
H O H
or
H
O
H
2e-8e-2eDouble bond – two atoms share two pairs of electrons
O C O
or
O
O
C
double bonds
- 8e8e- 8ebonds
double
Triple bond – two atoms share three pairs of electrons
N N
triple
bond
8e-8e
or
N
N
triple bond
Chemistry for Engineers, SCS126
9.4
Chemistry for Engineers, SCS126
9.4
or polar bond is a covalent bond
with greater electron density around one of the two
atoms
electron poor
region
H
Chemistry for Engineers, SCS126
9.4
electron rich
region
F
H
F
Chemistry for Engineers, SCS126
9.5
Electronegativity is the ability of an atom to attract
toward itself the electrons in a chemical bond.
The Electronegativities of Common Elements
Electron Affinity - measurable, Cl is highest
X (g) + e-
X-(g)
Electronegativity - relative, F is highest
Chemistry for Engineers, SCS126
9.5
Classification of bonds by difference in electronegativity
9.5
Classify the following bonds as ionic, polar covalent,
or covalent: The bond in CsCl; the bond in H2S; and
the NN bond in H2NNH2.
Bond Type
Difference
Chemistry for Engineers, SCS126
Covalent
Ionic
Polar Covalent
Increasing difference in electronegativity
Covalent
Polar Covalent
share e-
partial transfer of e-
Ionic
transfer e-
Chemistry for Engineers, SCS126
9.5
Chemistry for Engineers, SCS126
9.5
Writing Lewis Structures
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – N is less electronegative than F, put N in center
1. Draw skeletal structure of compound showing
what atoms are bonded to each other. Put least
electronegative element in the center.
Step 2 – Count valence electrons N - 5 (2s22p3) and F - 7 (2s22p5)
5 + (3 x 7) =
Step 3 – Draw single bonds between N and F atoms and complete
octets on N and F atoms.
2. Count total number of valence e-. Add 1 for
each negative charge. Subtract 1 for each
positive charge.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) =
3. Complete an octet for all atoms except
hydrogen
4. If structure contains too many electrons, form
double and triple bonds on central atom as
needed.
Chemistry for Engineers, SCS126
9.6
Chemistry for Engineers, SCS126
Two possible skeletal structures of formaldehyde (CH2O)
Write the Lewis structure of the carbonate ion (CO32-).
Step 1 – C is less electronegative than O, put C in center
Step 2 – Count valence electrons C - 4 (2s22p2) and O - 6 (2s22p4)
-2 charge – 2e4 + (3 x 6) + 2 =
Step 3 – Draw single bonds between C and O atoms and complete
octet on C and O atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) =
Step 5 - Too many electrons, form double bond and re-check # of
O
C
O
e-
2 single bonds (2x2) = 4
1 double bond = 4
8 lone pairs (8x2) = 16
Total = 24
H
C
O
H
H
C
H
O
An atom’s formal charge is the difference between the
number of valence electrons in an isolated atom and the
number of electrons assigned to that atom in a Lewis
structure.
formal charge
on an atom in
a Lewis
structure
=
total number
of valence
electrons in
the free atom
-
total number
of nonbonding
electrons
-
1
2
(
total number
of bonding
electrons
)
The sum of the formal charges of the atoms in a molecule
or ion must equal the charge on the molecule or ion.
O
Chemistry for Engineers, SCS126
9.6
9.6
Chemistry for Engineers, SCS126
9.7
H
-1
+1
C
O
formal charge
on an atom in
a Lewis
structure
C – 4 eO – 6 e2H – 2x1 e12 e-
H
=
total number
of valence
electrons in
the free atom
-
2 single bonds (2x2) = 4
1 double bond = 4
2 lone pairs (2x2) = 4
Total = 12
total number
of nonbonding
electrons
-
1
2
(
0
C
H
H
total number
of bonding
electrons
)
0
O
formal charge
on an atom in
a Lewis
structure
total number
of valence
electrons in
the free atom
=
formal charge
on C
formal charge
on C
formal charge
on O
formal charge
on O
Chemistry for Engineers, SCS126
C – 4 eO – 6 e2H – 2x1 e12 e-
-
2 single bonds (2x2) = 4
1 double bond = 4
2 lone pairs (2x2) = 4
Total = 12
total number
of nonbonding
electrons
9.7
Formal Charge and Lewis Structures
1
2
-
(
total number
of bonding
electrons
)
Chemistry for Engineers, SCS126
9.7
A resonance structure is one of two or more Lewis structures
for a single molecule that cannot be represented accurately by
only one Lewis structure.
1. For neutral molecules, a Lewis structure in which there
are no formal charges is preferable to one in which
formal charges are present.
2. Lewis structures with large formal charges are less
plausible than those with small formal charges.
O
3. Among Lewis structures having similar distributions of
formal charges, the most plausible structure is the one in
which negative formal charges are placed on the more
electronegative atoms.
O
+
-
O
O
+
O
O
What are the resonance structures of the
carbonate (CO32-) ion?
Which is the most likely Lewis structure for CH2O?
H
-1
+1
C
O
H
H
H
0
C
-
0
O
O
C
O
Chemistry for Engineers, SCS126
9.7
O
-
O
C
O
O
-
-
-
O
C
O
O
-
Chemistry for Engineers, SCS126
9.8
Exceptions to the Octet Rule
Exceptions to the Octet Rule
The Incomplete Octet
Odd-Electron Molecules
Be – 2e2H – 2x1e4e-
H
Be
N – 5eO – 6e11e-
H
N
O
The Expanded Octet (central atom with principal quantum number n > 2)
B – 3e3F – 3x7e24e-
F
B
F
3 single bonds (3x2) = 6
9 lone pairs (9x2) = 18
Total = 24
F
Chemistry for Engineers, SCS126
9.9
6e-
S–
6F – 42e48e-
F
F
F
S
F
F
F
6 single bonds (6x2) = 12
18 lone pairs (18x2) = 36
Total = 48
Chemistry for Engineers, SCS126
9.9