Chapter 8: Basic Concepts of Chemical Bonding
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Transcript of Chapter 8: Basic Concepts of Chemical Bonding
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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Chemical Bonds
• ionic bond
• covalent bond
• metallic bond
electrons are transferred from oneatom to another atom
electrons are shared between two atoms
electrons are free to move between all atoms
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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The formation of chemical bonds involves valence electrons
Lewis Symbols: “shorthand” for showing valence electrons
Gilbert Lewis (1875-1946)
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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Lewis dot structures:
H
Li
He
Be B C N O F Ne
Na Mg Al Si P S Cl Ar
For representative (main group) elements:group number = number of valence electrons
1A 2A 3A 4A 5A 6A 7A 8A
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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Noble gases, except helium, have 8 valence electrons
ns2np6
When atoms react, they tend to lose, gain, or share the number of electrons required to achieve eight valence
electrons (an “octet” of electrons)
K Cl+ K+ + Cl
[Ar] [Ar]electron configuration:
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Octet Rule:
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Na+ Cl-
Cl- Na+
Na+ Cl-
Formation of an ionic lattice
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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Na+ (s) + Cl- (g) NaCl (s) very exothermicH<0
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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dQQ
Eel
charge on ions
distance between charges
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
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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The stronger the ionic bond the the melting pointhigher
SrI2 +2, -1
66, 133
66, 140
113, 220
1261oC
2852oC
538°C
r1 r2
Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
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Chapter 8: Basic Concepts of Chemical BondingcChapter 8: Basic Concepts of Chemical Bondingc
Covalent Bonds …
… are formed when two atoms share electrons
… in order to achieve “noble gas configuration”
H H+ H H Each hydrogen has the electron configuration of He
F F+ F F Each fluorine has the electron configuration of Ne
… of the nearest noble gas
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Covalent Bonds
H H+ H H
F F+ F F
H H
F F
• A shared electron pair is drawn as a dash (two electrons!)
• Unshared electrons are drawn as dots
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Single and Multiple Bonds
F F+ F FF F or
O O+ C + O C O O C Oor
N NN + N N Nor
Single bond
Double bond
Triple bond
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Single and Multiple Bonds
X X
X X
X X
Distance between atoms (bond
length) decreases
Bond strength increases
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Polarity and Electronegativity
Cl Cl non-polar covalent bond:equal sharing of electrons
• When both atoms attract bond electrons equally, electrons are shared equally
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Polarity and Electronegativity
H Cl polar covalent bond:unequal sharing of electrons
+
• If one of the atoms attracts bond electrons more strongly, electrons are shared unequally
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Bond Polarity and Electronegativity
H Cl polar covalent bond:unequal sharing of electrons
+
For comparison:
ionic bond:electrons are not sharedNa+ Cl -
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
How do we know when a non-polar, polar, or ionic bond is formed?
The tendency of atom to attract electrons when forming a molecule is
summarized in the concept of
electronegativity
Incr
ease
in E
lectro
nega
tivity
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
How do we know when a non-polar, polar, or ionic bond is formed?
If the difference in Electronegativity, EN, is…
…smaller than 0.5EN < 0.5 => nonpolar bond
…greater than or equal to 0.5 AND smaller than 2.00.5 EN < 2.0 => polar bond
…greater than or equal to 2.0EN 2.0 => ionic bond
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Among the following examples, which bond is most polar?
S
all equally non-polar
C-F (C-H is non-polar)
P-Cl (furthest apart in P.T. => largest EN)
all equally non-polar
e) O-I O-N O-F O-I (furthest apart in P.T. => largest EN)
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Among the following examples, which bond is shortest?
S
H-H
C-H
Cl-Cl
C C
Bond length depends on (a) radii of the bonded atoms
(b) the number of bonds between atoms
remember that the atomic radii decrease along a period in the P.T.
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Drawing Lewis Structures of Moleculesthere are no Lewis structures for ionic compounds!!
If the compound contains more than 2 atoms:
• how are the atoms bonded and,
• if there are nonbonding electron, where are they?
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
Molecules with a central atom : NH3, PCl3, CHCl3
central atom is generally the first in the molecular formula
NH H
HC
H
Cl Cl
Cl
PCl
Cl
Cl
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
…unless the first element is Hydrogen :
H has only one valence electron => can only make one bond
H2OO
H H
HCN C NH
(same order as in formula)
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(4) place any leftovers from (1) on the central atomcheck that central atom has octet
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
NH3
8
NH H
H
6
n/a
+ 2
n/a
Rules for Drawing Lewis Structures
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Chapter 8: Basic Concepts of Chemical BondingChapter 8: Basic Concepts of Chemical Bonding
(1) sum valence electrons from all atoms: these are the ones that need to be distributed
(3) complete "octets" of atoms bound tocentral atom
(2) connect atoms by covalent bonds: count electrons used up
(5) If there are not enough electrons to give the central atom an octet, try multiple bonds
CO10
2
+ 6
+ 2
triple bond!
C O
C O
(4) place any leftovers from (1) on the central atomcheck that central atom has octet