Unit 6: Covalent BondingUnit 6: Covalent Bonding

Intermolecular ForcesIntermolecular Forces

Intra- versus Inter- molecular Intra- versus Inter- molecular Forces Forces

Intra (Intra (means “withinmeans “within”) and refers to the ”) and refers to the forces that hold atoms together in a forces that hold atoms together in a molecule.molecule.– CovalentCovalent– Ionic Ionic – MetallicMetallic

Inter (Inter (means “betweenmeans “between”) and refers to the ”) and refers to the forces that hold billions of molecules forces that hold billions of molecules togethertogether– A glass of HA glass of H22O O

Definition of IMFDefinition of IMF

Attractive forces Attractive forces betweenbetween molecules. molecules.

Much Much weakerweaker than than chemical bonds chemical bonds withinwithin molecules.molecules.

InterIntermolecular Forces (IMF)molecular Forces (IMF)The connections that are broken when a solid The connections that are broken when a solid melts to a liquid and a liquid turns into a gas.melts to a liquid and a liquid turns into a gas.

They are much weaker than They are much weaker than intraintramolecular molecular bonds.bonds.

There are 3 main types of IMF’s and all There are 3 main types of IMF’s and all depend on the depend on the polaritypolarity of the molecule. of the molecule.

The more polar a molecule is the The more polar a molecule is the “stickier”“stickier” it it is, and thus it is more attracted to the is, and thus it is more attracted to the molecules near it.molecules near it.

Dipole Dipole ForcesDipole Dipole ForcesForce that holds Force that holds polar polar molecules together.molecules together.

The positive end of a molecule is attracted The positive end of a molecule is attracted to the negative end of another molecule.to the negative end of another molecule.

These forces have ~10% the strength of These forces have ~10% the strength of an ionic bond.an ionic bond.

+ - + - + - + - + -

Dipole InteractionsDipole Interactions

Hydrogen BondingHydrogen BondingA A special typespecial type of of dipole dipole dipole forcedipole force that occurs that occurs between Hydrogen in one between Hydrogen in one molecule and molecule and a Nitrogen, a Nitrogen, Oxygen or FluorineOxygen or Fluorine atom atom in another molecule.in another molecule.

H-bonding is the H-bonding is the strongest type of strongest type of intermolecular forceintermolecular force and and gives water an extremely gives water an extremely high boiling point.high boiling point.

Van Der Waals (Dispersion) ForcesVan Der Waals (Dispersion) Forces

These forces hold These forces hold non-polarnon-polar molecules molecules together.together.

At any given moment electrons distribution At any given moment electrons distribution within a molecule can be uneven causing a within a molecule can be uneven causing a momentary dipole. momentary dipole.

- +

Creates temporary dipole

London Dispersion Forces/Van Der London Dispersion Forces/Van Der WaalsWaals

Present in all atoms and moleculesPresent in all atoms and moleculesMajor type of IMF for non-polar moleculesMajor type of IMF for non-polar molecules

View animation online.

Van Der Waals (Dispersion) ForcesVan Der Waals (Dispersion) Forces

They are the They are the weakestweakest forces forces These forces account for the These forces account for the lowerlower boiling and melting points for non-polar boiling and melting points for non-polar covalentcovalent molecules because they are so molecules because they are so weak.weak.Molecular weight is also a factor for non-Molecular weight is also a factor for non-polar moleculespolar moleculesLow MW – less dispersion Low MW – less dispersion Gas! Gas!Higher MW – more dispersion Higher MW – more dispersion Liquid! Liquid!

Determining IMF – Depends on PolarityDetermining IMF – Depends on Polarity

NClNCl33

– polar polar = dipole-dipole= dipole-dipole

CHCH44

– NonpolarNonpolar= dispersion= dispersion

HFHFPolar – with hydrogen attached to small Polar – with hydrogen attached to small electronegative atomelectronegative atom

= = hydrogen bondinghydrogen bonding

IMF tutorial link