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.

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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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