lecture 9. chemical bonding
TRANSCRIPT
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CHEMICAL BONDING
GENERAL CHEMISTRY
LECTURE
9
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Skills that you should learn:
A. Draw the Lewis Structures of molecules:
Organic molecules
Polyatomic Ions
C. Determine the Formal Charge atoms
Ionic compound
Covalent compound
D. Determine the Polarity of bonds
B. Draw the Resonance Structures
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Concept Check
• What is meant by the term “chemical bond?”
• How do atoms bond with each other to form molecules?
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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
Types of Chemical Bonding
- attractive forces that hold atoms together in compounds.
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The three models of chemical bonding
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Lewis Electron-Dot Symbols
Example:
N : .
. . :
N . . .
. N : .
. : N . . .
The group number gives the number of valence electrons.
Depicts the element and its valence electrons
Nitrogen, N
The dots represents the number of valence electrons
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Lewis electron-dot symbols for elements in Periods 2 and 3.
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Chemical Bonding
When atoms bound, they lose, gain, or share electrons to attain a filled outer level of eight (or two) electrons.
Octet Rule
Representative elements usually attain stable noble gas electron configurations in most of their compounds
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Ionic Bonding
Present in IONIC COMPOUNDS
Bonding between a METAL + NONMETAL
Metal loses an electron CATION (+ charge)
Nonmetal gains an electron ANION (- charge)
e.g. NaCl, KBr, MgS
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Ionic Bonding
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Three ways to represent the formation of Li+ and F- through
electron transfer.
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Formation of Ionic Compounds
Example: Na2O
2 Na . :
O . . .. O : + 2 Na+ :
: : [ ]2-
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1. Write the Lewis-dot structure of the following compounds:
Exercise 1
a. Li3N b. AlCl3 c. MgO
a. Li3N Li
:
N . N : + 3Li+ : :
: [ ]3-
Solution:
: 3
b. AlCl3 Al
:
Cl . .. Cl : + Al3+ :
: : [ ]- : : 3 3
.
c. MgO Mg :
O .. O : + Mg2+ :
: : [ ]2- : :
.
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Covalent Bonding
Present in COVALENT COMPOUNDS
Bonding between a NONMETAL + NONMETAL
* Atoms share electrons (outer e-/valence e-)
e.g. H2O, O2, NH3, PCl5
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Covalent bonds are formed when atoms share electrons.
4 electrons - double bond formed.
6 electrons - triple bond formed.
2 electrons shared - single covalent bond formed.
Covalent Bonding
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• The OCTET rule
• Bonding (or shared) electrons
• Nonbonding (or unshared or lone pairs) of electrons.
Writing Lewis Structure
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N = number of needed electrons – N usually has a value of 8 for representative elements. – N has a value of 2 for H atoms.
A = number of available electrons in valence shells
– Correspond to the valence electron. – A is equal to the periodic group number for each element.
S = number of shared electrons in bonds. A-S = number of unshared electrons, lone pairs.
N - A = S rule
Writing Lewis Structure: The Octet Rule
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• For ions adjust the number of electrons available, A.
– Add one e- to A for each negative charge.
– Subtract one e- from A for each positive charge.
• The central atom in a molecule or polyatomic ion is determined by:
– The atom with the least number in its molecular formula
– The less electronegative element
– Capable of forming multiple bonds
Writing Lewis Structure: The Octet Rule
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-> 4 bonds
Example 2: Write the Lewis structure of hydrogen cyanide, HCN.
N = 2 (H) + 8 (C) + 8 (N) = 18
A = 1 (H) + 4 (C) + 5 (N) = 10
S = 8 A-S = 2
H N C
Lone pairs
Writing Lewis Structure: The Octet Rule
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-> 4 bonds
Example 3: Write the Lewis structure of carbon dioxide, CO2
N = 8 (C) + 2 x 8 (O) = 24
A = 4 (C) + 2 x 6 (O) = 16
S = 8 A-S = 8
O O C
Lone pairs
Writing Lewis Structure: The Octet Rule
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The formal charge is the hypothetical charge on an atom in a molecule or polyatomic ion.
The formal charge determines the correct Lewis structures.
The best Lewis structures have formal charges on the atoms that are zero or nearly zero.
Writing Lewis Structure: Formal Charge
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Rules for Assigning Formal Charge
F.C= valence e- – (# of bonds + # of unshared e-)
• Molecules: The sum of the formal charges of all atoms is equal to zero.
• Polyatomic ion: The sum of the formal charges is equal to the ion’s charge.
Writing Lewis Structure: Formal Charge
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-> 3 bonds
Example 4: Consider nitrosyl chloride, NOCl. Draw the most stable Lewis structure of the compound.
N = 8 (N) + 8 (O) + 8 (Cl) = 24
A = 5 (N) + 6 (0) + 7 (Cl) = 18_ S = 6 A-S = 12 lone pairs
Cl O N Cl O N
Writing Lewis Structure: Formal Charge
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Cl 7-(2+4) = +1
N 5-(3+2) = 0
O 6-(1+6) = -1
Cl 7-(1+6) = 0
N 5-(3+2) = 0
O 6-(2+4) = 0
Cl O N Cl O N
Writing Lewis Structure: Formal Charge
+1 0 -1 0 0 0
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1. The covalent compounds of Be. 2. The covalent compounds of the IIIA Group. 3. Species which contain an odd number of electrons. 4. Species in which the central element must have a
share of more than 8 valence electrons to accommodate all of the substituents.
5. Compounds of the d- and f-transition metals.
Writing Lewis Structure:
Limitations of the Octet Rule
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The outer atoms attached to the central atom nearly always attain noble gas configurations.
The central atom does not have a noble gas configuration but may have fewer than 8 (exceptions 1, 2, & 3) or more than 8 (exceptions 4 & 5).
Writing Lewis Structure:
Limitations of the Octet Rule
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I. Reduced Octet Ex: BBr3; BeCl2
II. Expanded Octet Ex: PCl5; SF6
III. Odd-electron molecules Ex: NO2
Exemptions to the Octet Rule
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Br
N = 8 (B) + 3 x 8 (Br) = 32
A = 3 (B) + 3 x 7 (Br) = 24
S = 8
A-S = 18
Br
-> 6 electrons
*Reduced Octet
B Br
Example 5: Write the Lewis structure of boron tribromide, BBr3.
Exemptions to the Octet Rule
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F
N = 8 (As) + 5 x 8 (F) = 48
A = 5 (As) + 5 x 7 (F) = 40
S = 8
A-S = 30
-> 10 electrons
*Expanded Octet
Example 6: Write the Lewis structure of AsF5.
F
As F
F F
Exemptions to the Octet Rule
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1. Reduced Octet a. BBr3
Br = 7–(1+6)= 0 B = 3–(3+0)= 0
b. BeCl2
Cl = 7–(1+6)= 0 Be = 2–(2+0)= 0
2. Expanded Octet a. PCl5
b. SF6
Cl = 7–(1+6)= 0 P = 5–(5+0)= 0
F = 7–(1+6)= 0 S = 6–(6+0)= 0
Exemptions to the Octet Rule
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Exercises
Draw the Lewis structure of the following molecules and calculate the formal charges of each atom:
XeF2
SF4
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Quiz
1. Write the Lewis structure of the following ionic compounds 2pts each
a. SrF2
b. K2O
c. Ca3N2
a. ICl3
b. SbF4-
2. Draw the most stable Lewis structure and calculate the formal charge of each atom on: 4 pts each
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Answer
1. Write the Lewis structure of the following salts:
a. SrF2
b. K2O
c. Ca3N2
(2 pts each)
F : Sr2+ :
: : [ ]- 2
O : 2K+ :
: : [ ]2-
N : 3Ca2+ :
: : [ ]3- 2
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a) ICl3
N = 8 (I) + 3 x 8 (Cl) = 32
A = 7 (I) + 3 x 7 (Cl) = 28
S = 4
A-S = 28-6 =22
-> 6 electrons
Cl
Cl Cl I
2. Draw the most stable Lewis structure and calculate the formal charge of each atom on: 3 pts each
Answer
0
0
0 0
I : 7- (3+4) = 0 Cl : 7 – ( 1+6) =0
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a) SbF4-
N = 8 (Sb) + 4 x 8 (F) = 40
A = 5 (Sb) + 4 x 7 (F) + 1 = 34
S = 6
A-S = 34-8 =26
-> 8 electrons
F
F F
Sb
2. Draw the most stable Lewis structure and calculate the formal charge of each atom on: 3 pts each
Answer
F -
F : 7- (1+6) = 0 Sb :5 – (4+2)= -1
0 0
0 0
-1
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Exercises
Draw the Lewis structure of the following molecules and calculate the formal charges of each atom:
1. BiF52-
2. IBr4+
3. OCCl2
4. XeOF4
5. HONO2
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Resonance
Example 8: Write the a Lewis structure of sulfur trioxide, SO3.
N = 8 (S) + 4 x 8 (O) = 32
A = 6 (S) + 3 x 6 (O) = 24
S = 8
A-S = 16 -> 4 bonds
O
O S O
O
O S O
O
O S O
*Three possible structures for SO3.
- more than one valid Lewis structure can be written for a particular molecule.
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O
O S O
O
O S O
O
O S O
All of the bonds in SO3 are equivalent.
There are no single or double bonds in SO3.
Best representation of the Lewis structure of SO3:
Resonance
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Exercises
Draw the Lewis structure of nitrate ion, NO3-
N
O
O
O
N
O
O
O
N
O
O
O
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Atom # of bonds
C 4 Tetravelent
N 3 Trivalent
O 2 Divalent
H, X 1 Monovalent
Examples: 1. CH3CH2CH3
2. CH3CH2OH 3. CH3COCH3 4. CH3NH3
1. CH3CH2CH3
2. CH3CH2OH
Molecules with More than One Central Atom
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Examples: 1. CH3CH2CH3
2. CH2CH2OH 3. CH3COCH3
4. CH3NH2
3. CH3COCH3
4. CH3NH2
Atom # of bonds
C 4 Tetravelent
N 3 Trivalent
O 2 Divalent
H, X 1 Monovalent
Molecules with More than One Central Atom
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N = 8 x 4 (O) + 8 (S) = 40
A = 6 x 4 (O) + 6 (S) + 2 = 32
S = 8
A-S = 24
Example 1: Write the Lewis structure of (NH4)2SO4
N = 4 x 2 (H) + 8 (N) = 16
A = 4 x1 (H) + 5 (N) -1 = 8
S = 8
A-S = 0
NH4+ + SO4
2- (NH4)2SO4
H
H N H
O
O S O
H + O 2-
Lewis Structure of Polyatomic Ion
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NH4+ + SO4
2- (NH4)2SO4
H
H N H
H +
O
O S O
O 2-
2
Lewis Structure of Polyatomic Ion
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Example 2: Write the Lewis structure of ammonium chloride, NH4Cl
H
H N H
H +
Cl -
Lewis Structure of Polyatomic Ion
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O
Draw the correct Lewis Structure of H3PO4
N = 3 x 2 (H) + 8 (P) + 4 x 8 (O) = 46
A = 3 x 1 (H) + 5 (P) + 4 x 6 (O) = 32
S = 14
A-S = 18
-> 7 bonds
O
P
O
O H
H
H H : 1 –(1+0)= 0 3 O : 6- (2+4) = 0 P : 5- (4+0) = 1 O : 6- (1+6) = -1
-1
+1 0
0
0
Concept Check
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O
b) H3PO4
O
P
O
O H
H
H H : 1 –(1+0)= 0 3 O : 6- (2+4) = 0 P : 5- (5+0) = 0 O : 6- (2+4) = 0
* Expanded octet
0 0
0
0
0
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• Covalent bonds in which the electrons are shared equally are designated as nonpolar covalent bonds.
– Nonpolar covalent bonds have a symmetrical charge distribution.
Examples:
1. H2
2. N2
Bond Polarity
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• Covalent bonds in which the electrons are not shared equally are designated as polar covalent bonds
– Polar covalent bonds have an asymmetrical charge distribution
Example:
1.HX (X =F, Cl, Br, I)
bondpolar very 1.9 Difference
4.0 2.1 ativitiesElectroneg
F H
1.9
Bond Polarity
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• Polar molecules have a separation of centers of negative and positive charge, an asymmetric charge distribution.
Bond Polarity
Density map of HF
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Bond Polarity
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• Polar molecules can be attracted by magnetic and electric fields.
Bond Polarity
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• The dipole moment has the symbol . (= d x q; unit: debye)
• Polar molecules have dipole moment
• Molecules that have a small separation of charge have a small . Molecules that have a large separation of charge have a large .
• For example, HF and HI:
units Debye0.38 units Debye1.91
I- H F- H
--
Dipole Moment
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Bond Order, Bond Length, Bond Energy
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Review
1. How were covalent bonds formed?
2. When forming a bond, the atoms obey what rule?
3. Formation of one covalent bond involves how many electrons?
4. In a triple bond, how many electrons are being shared?
5. How will you determine which is the best lewis structure?
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Take home quiz
Draw the best Lewis structure of the following molecules and calculate the formal charges of each atom:
a. ICl2-
b. O3
c. Na3PO4
d. CaCO3
e. Glycine, CH3CONH2