Concepts of Chemical Bonding Brown, LeMay Ch 8 AP Chemistry.
Bonding!!!!!!!!!!!!!!!!!!!!!!!!!! Lesson in AP Chemistry.
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Transcript of Bonding!!!!!!!!!!!!!!!!!!!!!!!!!! Lesson in AP Chemistry.
Chemical Bonding III:
Bond Characteristics
& Molecular Polarity
“I believe this movie. A dude could jump off a
mountain and not hurt himself, 'cause he did brace
himself, and knew something about the levels of
gravitivity and polarity.”
- Bobby Taylor, Hollywood Shuffle
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Electronegativity is the ability of an atom to
attract toward itself the electrons in a
chemical bond.
3
Electronegativity - relative, F is highest
Polar Covalent Bonds and
Electronegativity
• Polar bonds are bonds where the electron
density is not shared equally between the two
bonded atoms.
• In polar bonds there is a positive and a
negative end to the bond
•Refered to as a dipole
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Polar covalent bond or polar bond is a
covalent bond with greater electron density
around one of the two atoms
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H F F H
electron rich
region
electron poor
region e- rich e- poor
d+ d-
H − F d+ d- dipole
Dipole points toward e− rich region
Electronegativity
difference
Bond
Type
Zero
Intermediate
Large
Covalent
Polar
Covalent
Ionic
Co
vale
nt C
hara
cte
r decre
ases
Ion
ic C
hara
cte
r incre
ases
Bond Character
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Classification of bonds by difference in
electronegativity
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Covalent
share e-
Polar Covalent
partial transfer of e-
Ionic
transfer e-
Increasing difference in electronegativity
Difference Bond Type
0 Covalent
2 Ionic
0 < and <2 Polar Covalent
Bond Polarity
NaCl Cl-Cl HCl
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Classify the following bonds as ionic, polar covalent,
or covalent: The bond in CsCl; the bond in H2S; and
the NN bond in H2NNH2.
Cs – 0.7 Cl – 3.0 3.0 – 0.7 = 2.3 Ionic
H – 2.1 S – 2.5 2.5 – 2.1 = 0.4 Polar Covalent
N – 3.0 N – 3.0 3.0 – 3.0 = 0 Covalent
Dipole Moment
• Dipole –pull of e- towards one direction in a bond
• Dipole moment – the measure of the net molecular polarity (overall pull of e- towards one direction in a molecule)
• Dipole moments are vectors
H - F
d+ d-
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Dipole Moments and Polar Molecules
H F
electron rich
region electron poor
region
d+ d-
Polar molecules have a
permanent dipole moment
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H - F
d+ d-
+ -
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Which Molecules Have a Dipole Moment?
• Dipole moment forms when all dipoles do not cancel each other completely
• Any two atom molecule with a polar bond
• With three or more atoms there are three considerations.
1) There must be at least one dipole (polar bond)
2) There are significant strength differences among the dipoles
3) There is molecular asymmetrical geometry
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Cl is more electronegative than Be
Polar covalent bonds dipoles
Molecule is symmetrical dipole forces cancel
Molecule has no dipole moment nonpolar
Cl Be Cl
Non-polar Molecules w/ Polar Bonds
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H Be F F & H are more electronegative than Be
Polar covalent bonds dipoles
F is more electronegative than H
stronger dipole
Molecule is symmetrical but dipole forces do not completely cancel
Molecule has a dipole moment polar
dipole moment
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• Electronegativity of F > B
• Dipoles (but of equal strength)
• Trigonal planar is symmetrical
• All dipoles completely cancel
• No dipole moment
not polar
Ex: Is BF3 polar?
F
F F
B
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• Electronegativity of Cl > C
• Dipoles (but of equal strength)
• Tetrahedron is symmetrical
• All dipoles completely cancel
• No dipole moment
not polar
Ex: Is CCl4 polar?
Cl
Cl Cl
Cl
C
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• Electronegativity of H < N
• Dipoles
• Trigonal pyramidal
is asymmetrical
• All dipoles are additive
• Dipole moment
polar
Ex: Is NH3 polar ?
H H
H
N
d+
d+
d+
d-
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• Electronegativity of F > N
• Dipoles
• Trigonal pyramidal
is asymmetrical
• All dipoles do not cancel
• Dipole moment
polar
Ex: Is NF3 polar?
F F
F
N
d-
d-
d-
d+
Resultant dipole = 0.24 D
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Ex: NH3 is more polar than NF3
H H
H
N
d+
d+
d+
d-
F F
F
N
d-
d-
d+
d-
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Bond moments and resultant dipole moments in NH3 and NF3.
Water is even more polar
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Lone e− pairs
DO NOT
make a
molecule polar
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• Electronegativity of F > Br > C
• Dipoles (not of equal strength)
• Tetrahedron is symmetrical
• But all dipoles do not completely cancel
• Dipole moment
polar
Ex: Is CBr3F polar?
F
Br Br
Br
C
d+
d+
d+
d-
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Asymmetrical
dipole forces
make a
molecule polar
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Symmetrical Molecules
Linear Trigonal
planar Tetrahedral
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Asymmetrical Molecules
Bent or V-shaped Trigonal
pyramidal
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Ex: Is CH3Cl polar?
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