Polarity and Intermolecular Forces

ReviewWe know how to draw Lewis structures for

simple molecules and polyatomic ions.We also know how to predict the 3-D

geometry of these molecules and ions, if we apply the VSEPR Theory.

Electronegativity (EN) is an atom's tendency to attract electrons in chemical bonds.EN increases to the right and up on the periodic

table, excluding the noble gases.

Bond PolarityWhen two nonmetal atoms bond, they share

electrons.They may or may not share electrons evenly.

Consider the following molecules:F2

HF

Bond PolarityH vs. F

The EN of H is 2.2.The EN of F is 4.0.

In F2, both atoms pull with equal strength on the bonding e-.The e- are shared evenly between them.

In HF, the F atom pulls harder than the H atom.The e- are drawn more toward the F atom.

HEN = 2.2

FEN = 4.0

F F F H

Bond Polarity

EN is same for both atoms.

e- density is spread evenly around molecule.

Highest e- density occurs between atoms.

Bond is nonpolar.

EN is much higher for F than for H.

e- density is drawn toward F side.

F atom acquires partial negative charge.

H atom acquires partial positive charge.

Bond is polar.

F F F H

Bond PolarityIn general, a covalent bond is:

polar if it occurs between two different atoms.nonpolar if it occurs between two identical

atoms.

Dipole MomentsDipole Moment - a measure of the polarity

of a bond.Is often represented by a special arrow.

FHArrow points toward more EN atom.

Polarity of Diatomic MoleculesDiatomic Molecules - molecules made of

only two atoms.If atoms are the same, molecule is nonpolar.If atoms are diff., molecule is polar.

NOTE: Polar does not mean charged.Is Cl2 polar or nonpolar?Is CO polar or nonpolar?

Molecules With 3 or More AtomsA molecule with 3 or more atoms is:

Polar if its central atom has lone pairs ORIf the outer atoms are not all the same.

Nonpolar if its central atom has no lone pairs ANDAll the outer atoms are identical.

CO2 vs. H2O

Consider the Lewis structure of CO2:

This molecule is nonpolar.

CO2 vs. H2O

Consider the Lewis structure of H2O:

This molecule is polar.

CH4 vs. CH3ClNeither CH4 nor CH3Cl has any lone pairs

on the central carbon atom.Is CH4 polar or nonpolar?

Is CH3Cl polar or nonpolar?

C CHHH

HClH

H

H

“Like Dissolves Like”Polar molecules mix with each other.Nonpolar molecules mix with each other.Polar and nonpolar molecules do not easily

mix.

Amphipathic MoleculesAmphipathic - has a hydrophobic region and

a hydrophilic region.Hydrophobic - “water-fearing”

Nonpolar.Hydrophilic - “water-loving”

Polar or charged.

Dish detergents contain amphipathic molecules.Why?

Grease and Water Don't Mix!

Amphipathic Molecules

Hydrophilic head

Hydrophobic tails

Amphipathic Molecules

Intermolecular ForcesIntermolecular force - a force between two

molecules that does not result from chemical bonding.Dipole-dipole interaction.Hydrogen bonding.London force.

Dipole-Dipole InteractionsDipole - polar molecule.

Like magnets, except poles are + and - instead of N and S.

Polar molecules generally have higher melting and boiling points than similar nonpolar molecules.EXAMPLE: O2 (nonpolar) boils at -183ºC.EXAMPLE: NO (polar) boils at -152ºC.NO has a higher boiling point due to its polarity.

Still far below the boiling point of any ionic cmpd.

Dipole-Dipole Interactions

Hydrogen BondingHydrogen bond - a stronger form of dipole-

dipole interaction.Occurs in molecules that have H atoms bonded

to O, N, or F atoms.The small size of the H atom allows these

molecules to get closer together.Closer together = stronger forces.EXAMPLE: H2O has a boiling point of 100ºC.EXAMPLE: H2S has a boiling point of -60ºC.The b.p. of H2O is higher b/c of hydrogen

bonding.

Hydrogen BondingBoiling Points of Several Compounds

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H2O H2S H2Se H2Te

Com pound

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London ForceLondon force - attraction between

temporary dipoles.e- move randomly around molecules.Nonpolar molecules become temporarily polar.Allows for very weak attractions between

nonpolar molecules.Named for Fritz London.

London Forces

London ForcesThe more e- a molecule has, the greater its

London forces are.Large molecules tend to have higher

melting/boiling points than small molecules.London forces apply to all molecules.

London Forces

Boiling Points of the Noble Gases

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He Ne Ar Kr Xe Rn

Noble G a s

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