Transcript CHEMICAL BONDING - Yale University

CHEMICAL
BONDING
Cocaine
1
Chemical Bonding
Problems and questions —
How is a molecule or
polyatomic ion held
together?
Why are atoms distributed at
strange angles?
Why are molecules not flat?
Can we predict the structure?
How is structure related to
chemical and physical
properties?
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Forms of Chemical Bonds
• There are 2 extreme forms of
connecting or bonding atoms:
• Ionic—complete transfer of
1 or more electrons from one
atom to another
• Covalent—some valence
electrons shared between
atoms
• Most bonds are
somewhere in between.
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Essentially complete electron
transfer from an element of
low IE (metal) to an element
of high affinity for electrons
(nonmetal)
2 Na(s) + Cl2(g) --->
2 Na+ + 2 ClTherefore, ionic compds. exist
primarily between metals at
left of periodic table (Grps
1A and 2A and transition
metals) and nonmetals at
right (O and halogens).
Ionic
Bonds
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Covalent Bonding
The bond arises from the mutual attraction
of 2 nuclei for the same electrons.
Electron sharing results. (Screen 9.5)
HA + H B
HA
HB
Bond is a balance of attractive and repulsive
forces.
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Chemical Bonding:
Objectives
Objectives are to
understand:
1. valence e- distribution in
molecules and ions.
2. molecular structures
3. bond properties and their
effect on molecular
properties.
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Electron
Distribution in
Molecules
• Electron distribution
is depicted with
Lewis electron
dot structures
• Valence electrons
are distributed as
shared or BOND
PAIRS and
unshared or LONE
PAIRS.
G. N. Lewis
1875 - 1946
Bond and Lone Pairs
• Valence electrons are distributed as
shared or BOND PAIRS and
unshared or LONE PAIRS.
••
H
Cl
•
•
••
shared or
bond pair
lone pair (LP)
This is called a LEWIS
ELECTRON DOT structure.
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Bond Formation
A bond can result from a “head-to-head”
overlap of atomic orbitals on
neighboring atoms.
••
H
+
Cl
••
••
•
•
H
Cl
•
•
••
Overlap of H (1s) and Cl (2p)
Note that each atom has a single,
unpaired electron.
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Valence Electrons
Electrons are divided between core and
valence electrons
B 1s2 2s2 2p1
Core = [He] , valence = 2s2 2p1
Br [Ar] 3d10 4s2 4p5
Core = [Ar] 3d10 , valence = 4s2 4p5
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Rules of the Game
No. of valence electrons of a main group atom =
Group number
•For Groups 1A-4A (14), no. of bond pairs =
group number.
• For Groups 5A (15)-7A (17), BP’s = 8 - Grp. No.
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Rules of the Game
•No. of valence electrons of an atom = Group
number
•For Groups 1A-4A (14), no. of bond pairs =
group number
• For Groups 5A (15)-7A (17), BP’s = 8 - Grp. No.
•Except for H (and sometimes atoms of 3rd
and higher periods),
BP’s + LP’s = 4
This observation is called the
OCTET RULE
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Building a Dot Structure
Ammonia, NH3
1. Decide on the central atom (the atom
with lowest electron affinity); never H.
Hydrogen atoms are always terminal.
Therefore, N is central
2. Count valence electrons
H = 1 and N = 5
Total = (3 x 1) + 5
= 8 electrons; 4 pairs
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Building a Dot Structure
3.
Form a single bond
between the central atom and
each surrounding atom
4.
Remaining electrons form
LONE PAIRS to complete octet as
needed.
3 BOND PAIRS and 1 LONE PAIR.
H N H
H
••
H N H
H
Note that N has a share in 4 pairs (8 electrons),
while H shares 1 pair.
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Sulfite ion, SO32Step 1. Central atom = S
Step 2. Count valence electrons
S= 6
3 x O = 3 x 6 = 18
Negative charge = 2
TOTAL = 26 e- or 13 pairs
Step 3. Form bonds
O
10 pairs of electrons are
now left.
O
S
O
Sulfite ion, SO32Remaining pairs become lone pairs, first
on outside atoms and then on central
atom.
••
•
•
O
••
•
•
O
••
•
•
••
S
••
O
••
•
•
Each atom is surrounded by an octet of electrons.
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Carbon Dioxide, CO2
1. Central atom = _______
2. Valence electrons = __ or __ pairs
3. Form bonds.
This leaves 6 pairs.
4. Place lone pairs on outer atoms.
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Carbon Dioxide, CO2
4. Place lone pairs on outer atoms.
5. So that C has an octet, we shall form
DOUBLE BONDS between C and O.
The second bonding pair forms a pi
(π) bond.
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Double and
even triple
bonds are
commonly
observed for C,
N, P, O, and S
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H2CO
SO3
C2F4
Sulfur Dioxide, SO2
1. Central atom = S
2. Valence electrons = 18 or 9 pairs
3. Form double bond so that S has an octet
— but note that there are two ways of doing
this.
bring in
left pair
••
•
•
O
••
••
S
OR bring in
right pair
••
O
••
•
•
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Sulfur Dioxide, SO2
This leads to the following structures.
These equivalent structures are called
RESONANCE STRUCTURES. The true
electronic structure is a HYBRID of the two.
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Urea, (NH2)2CO
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Urea, (NH2)2CO
1. Number of valence electrons = 24 e2. Draw sigma bonds.
O
H N
H
C
N
H
H
24
Urea, (NH2)2CO
3. Place remaining electron pairs in the
molecule.
••
•
•
O
••
H N
H
•
•
••
C
N
H
H
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Urea, (NH2)2CO
4. Complete C atom octet with double
bond.
••
O
••
H N
H
•
•
••
C
N
H
H
Violations of the Octet Rule
Boron Trifluoride
• Central atom = _____________
• Valence electrons = __________ or
electron pairs = __________
• Assemble dot structure
The B atom has a
share in only 6 pairs of
electrons (or 3 pairs).
B atom in many
molecules is electron
deficient.
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Violations of the Octet Rule
Sulfur Tetrafluoride, SF4
• Central atom =
• Valence electrons = ___ or ___ pairs.
• Form sigma bonds and distribute electron
pairs.
5 pairs around the S
atom. A common
occurrence outside the
2nd period.
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Violations of the Octet Rule
Odd # of electrons, NO2
• Central atom =
• Valence electrons = ___ or ___ pairs.
• Form sigma bonds and distribute electron
pairs.
•
N
•
••
•
•
N
O
••
••
••
O
••
•
•
O
••
••
O
••
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Formal Atomic Charges
Definition of Formal Charge:
• Formal charge=
Group no. – 1/2 BEs - LPEs
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Carbon Dioxide, CO2
6 - (1/ 2)(4) - 4
••
•
•
O
••
C
4 - (1/ 2)(8) - 0
O
=
•
•
0
=
0
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Calculated Partial Charges in CO2
Yellow = negative & red = positive
Relative size = relative charge
Thiocyanate Ion,
6 - (1/2)(2) - 6 = -1
5 - (1/2)(6) - 2 = 0
••
•
•
S
SCN-
C
N
•
•
••
4 - (1/2)(8) - 0 = 0
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Thiocyanate Ion, SCN-
••
••
•
•
S
C
N
•
•
•
•
••
S
C
N
••
••
•
•
S
C
N
•
•
••
Which is the most important resonance form?
•
•
Calculated Partial Charges
in SCN-
All atoms negative, but
most on the S
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••
•
•
S
••
C
N
•
•
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Boron Trifluoride, BF3
•
•
F
•
•
+1
••
•
•
F
••
-1
B
•
•
F
•
•
••
What if we form a B—F double
bond to satisfy the B atom octet?
MOLECULAR
GEOMETRY
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MOLECULAR GEOMETRY
VSEPR
• Valence Shell Electron
Pair Repulsion theory.
• Most important factor in
determining geometry is
relative repulsion between
electron pairs.
Molecule adopts
the shape that
minimizes the
electron pair
repulsions.
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Electron Pair Geometries
Figure 9.12
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No. of e- Pairs
Around Central
Atom
2
Example
Geometry
F—Be—F
linear
180Þ
F
3
F
planar
trigonal
B
F
120Þ
H
4
C
H
109Þ
tetrahedral
H
H
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Electron Pair Geometries
Figure 9.12
Structure Determination by
VSEPR
Ammonia, NH3
There are 4 electron pairs at the corners
of a tetrahedron.
lone pair of electrons
in tetrahedral position
N
H
H
H
The ELECTRON PAIR GEOMETRY is
tetrahedral.
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Bond Properties
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bond order, bond length, bond energy, bond polarity
Buckyball in HIV-protease
Bond Order
# of bonds between a pair of atoms
Double bond
Single bond
Acrylonitrile
Triple
bond
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Bond Order
Fractional bond orders in resonance structures.
Consider NO2-
••
N
••
N
•• • •••
••
••
O
O• • O
O
••
••
••
••
The N—O bond order = 1.5
Total # of e - pairs used for a type of bond
Bond order =
Total # of bonds of that type
3 e - pairs in NO bonds
Bond order =
2 N — O bonds
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Bond Order
Bond order is proportional to two important bond
properties:
(a)
(b)
bond strength
bond length
414 kJ
123 pm
110 pm
745 kJ
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Bond Length
• Bond length is the distance between the nuclei of
two bonded atoms.
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Bond Length
Bond length depends on
bond order.
Bond distances measured
using CAChe software. In
Angstrom units where 1 A =
10-2 pm.
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Using Bond Energies
Estimate the energy of the reaction
H—H + Cl—Cl ----> 2 H—Cl
Net energy = ∆Hrxn =
= energy required to break bonds
- energy evolved when bonds are made
H—H = 436 kJ/mol
Cl—Cl = 242 kJ/mol
H—Cl = 432 kJ/mol
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Using Bond Energies
Estimate the energy of the reaction
H—H + Cl—Cl ----> 2 H—Cl
H—H = 436 kJ/mol
Cl—Cl = 242 kJ/mol
H—Cl = 432 kJ/mol
Sum of H-H + Cl-Cl bond energies = 436
kJ + 242 kJ = +678 kJ
2 mol H-Cl bond energies = 864 kJ
Net = ∆H = +678 kJ - 864 kJ = -186 kJ
Molecular Polarity
Boiling point = 100 ˚C
Boiling point = -161 ˚C
Why do water and methane
differ so much in their
boiling points?
Why do ionic compounds dissolve in
water?
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Bond Polarity
HCl is POLAR because it
has a positive end and a
negative end.
+d
-d
••
••
H Cl
••
Cl has a greater share in
bonding electrons than
does H.
Cl has slight negative charge (-d) and H has
slight positive charge (+ d)
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+d
-d
••
••
H Cl
••
Bond Polarity
Due to the bond polarity, the H—
Cl bond energy is GREATER
than expected for a “pure”
covalent bond.
BOND
“pure” bond
real bond
ENERGY
339 kJ/mol calc’d
432 kJ/mol measured
Difference = 92 kJ. This difference is
proportional to the difference in
ELECTRONEGATIVITY, .
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Electronegativity, 
 is a measure of the ability of an atom in a
molecule to attract electrons to itself.
Concept proposed by
Linus Pauling
1901-1994
Linus Pauling, 1901-1994
The only person to receive two unshared Nobel
prizes (for Peace and Chemistry).
Chemistry areas: bonding, electronegativity,
protein structure
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Electronegativity
Figure 9.9
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Electronegativity, 
See Figure 9.9
• F has maximum .
• Atom with lowest  is the center atom in
most molecules.
• Relative values of  determine BOND
POLARITY (and point of attack on a
molecule).
Bond Polarity
Which bond is more polar (or DIPOLAR)?
O—H
O—F

3.5 - 2.1
3.5 - 4.0

1.4
0.5
OH is more polar than OF
-d
O
+d
H
+d
O
-d
F
and polarity is “reversed.”
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Molecular Polarity
Molecules—such as HCl and H2O— can be POLAR (or
dipolar).
They have a DIPOLE MOMENT. The polar HCl molecule
will turn to align with an electric field.
Figure 9.15
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Molecular Polarity
The magnitude of the
dipole is given in
Debye units. Named for
Peter Debye (1884 -
1966). Rec’d 1936
Nobel prize for work on
x-ray diffraction and
dipole moments.
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Dipole Moments
Why are some molecules polar but others
are not?
Molecular Polarity
Molecules will be polar if
a)
bonds are polar
AND
b)
the molecule is NOT “symmetric”
All above are NOT polar
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Polar or Nonpolar?
Compare CO2 and H2O. Which one is polar?
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Carbon Dioxide
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• CO2 is NOT polar
even though the CO
bonds are polar.
• CO2 is symmetrical.
Positive C atom
is reason CO2 +
H2O gives
H2CO3
-0.75
+1.5
-0.75
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Microwave
oven
Consequences of H2O
Polarity
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Polar or Nonpolar?
• Consider AB3 molecules: BF3, Cl2CO, and NH3.
Molecular Polarity, BF3
F
B
F
F
B—F bonds in BF3 are polar.
But molecule is symmetrical and
NOT polar
B atom is
positive and
F atoms are
negative.
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Molecular Polarity, HBF2
H
B
F
F
B—F and B—H bonds in HBF2
are polar. But molecule is NOT
symmetrical and is polar.
B atom is
positive but H
& F atoms are
negative.
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Is Methane, CH4, Polar?
H
C
H
H
H
Methane is symmetrical and is NOT polar.
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Is CH3F Polar?
F
C
H
H
H
C—F bond is very polar.
Molecule is not symmetrical and
so is polar.
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Substituted Ethylene
• C—F bonds are MUCH more polar than
C—H bonds.
• Because both C—F bonds are on same
side of molecule, molecule is POLAR.
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Substituted Ethylene
• C—F bonds are MUCH more polar than C—H
bonds.
• Because both C—F bonds are on opposing
ends of molecule, molecule is NOT POLAR.
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