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Intermolecular ForcesIntermolecular Forces

Intermolecular forces are attractive forcesbetween molecules.

The type of intermolecular forces presentdependent upon:

the types of bonds within the molecule itself.

the geometry or the three dimensional shape of the molecule.

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the resulting polarity of the molecule.

Three types of intermolecular forces are:

dispersion forces dipole forces hydrogen bonding

These three forces are relatively weakcompared to the forces found in a covalentbond.

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van der Waals Forcesvan der Waals Forces

Also known as London dispersion forces.

van der Waals forces are found in allmolecular (covalent) compounds which

resultfrom temporary (induced) dipoles.

Temporary dipoles are caused by thelocalization of electrons in an atom ormolecule.

Nonpolar covalent have only dispersionforces.

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Two nonpolar molecules are shown below.

These two molecules at an instant of time are

not polarized and therefore experience noattraction.

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Two nonpolar molecules are shown below.

These two molecules at an instant of time are

polarized and therefore experience anattraction.

Molecule A induces a similar dipole to resultin molecule B.

δ- δ+ δ- δ+

A B

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This polarization is caused by the shifting ofthe electron charge density due to

electronsrepelling each other and an electron and anucleus attracting each other.

van der Waals forces are weak forces of attraction between nonpolar molecules.

The strength of these induced dipole forcesdepend upon:

The number of electrons making up the

molecule.

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The relative ease of localizing the electrons.

Both of these factors increase with increasing molecular size.

Larger atoms have a weaker attraction for their valence electrons making it easier to displace these electrons.

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Dipole ForcesDipole Forces

Dipole forces exist between polar molecules.

A molecule is polar as a result of theunequal sharing of electron pairs betweencovalently bonded atoms.

The major distinction between dipole forcesand van der Waals forces is that they arepermanent forces.

δ- δ+ δ- δ+

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Polar covalent molecules align themselvessuch that the positive end of one molecule

isclose to the negative end of anothermolecule.

Dipole forces are dependent on thedifference of electronegativities between

theatoms.

Polar covalent molecules have bothdispersion and dipole forces and generallyhave higher melting/boiling points thannonpolar compounds.

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Hydrogen BondingHydrogen Bonding

Hydrogen bonding among water molecules.

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The previous slide illustrates the attractiveforces between the highly polarizedmolecules containing hydrogen.

Hydrogen bonding results when a hydrogenatom is bonded to nitrogen, oxygen, orfluorine and there is/are unshared pair(s)of electrons of neighboring molecules.

When hydrogen is bonded to oxygen, thesmall size of the hydrogen atom allows it todraw very closely to the unshared pair ofelectrons oxygen.