Covalent Bond – An attraction between two atoms caused by the sharing of a pair of electrons between two atoms.

Polar Covalent – A covalent bond in which electrons are shared unequally.

Nonpolar covalent – electrons are shared equally. SYMMETRICAL arrangement of valence electrons.

Electronegativity – The tendency of an atom in a bond to attract shared electrons to itself.

Look at electronegativity difference to determine bond type:

EN Difference Bond Type

> 1.5 Ionic

0.5-1.5 Polar covalent

< 0.5 Nonpolar covalent

Partially Positive

Partially Negative

Polar Molecule – A molecule in which valence electrons (bonds and unshared pairs of electrons) are not equally distributed (asymmetrical)

To be polar, a molecule must:

1. contain polar covalent bonds

2. be asymmetrical

Examples – Is each molecule polar or nonpolar?

1. CO2

2. H2S

3. CCl4

4. NBr3

• The covalent bond holding a molecule together is an intramolecular force.

• The attraction between molecules is an intermolecular force.

• Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl).

• When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).

Intermolecular Forces• The covalent bond holding a molecule

together is an intramolecular force.

• The attraction between molecules is an intermolecular force.

• Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl).

• When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).

Intermolecular ForcesDipole-dipole forces – Attraction between

polar molecules (also called dipoles).

Molecules are aligned with the “partial positive” end of one molecule near the “partial negative” end of another.

London Dispersion Forces• Weakest of all intermolecular forces.• It is possible for two adjacent neutral molecules to affect

each other.• The nucleus of one molecule (or atom) attracts the

electrons of the adjacent molecule (or atom).• For an instant, the electron clouds become distorted.• In that instant a dipole is formed (called an instantaneous

dipole).

• One instantaneous dipole can induce another instantaneous dipole in an adjacent molecule (or atom).

• The forces between instantaneous dipoles are called London dispersion forces.

Hydrogen Bonding• Special case of dipole-dipole forces.• By experiments: boiling points of compounds with H-F,

H-O, and H-N bonds are abnormally high.• Intermolecular forces are abnormally strong.

• H-bonding requires H bonded to an electronegative element (most important for compounds of F, O, and N).– Electrons in the H-X (X = electronegative element) lie much

closer to X than H.

– H has only one electron, so in the H-X bond, the + H presents an almost bare proton to the - X.

– Therefore, H-bonds are strong.

Hydrogen Bonding

Summary of Intermolecular Forces

– Nonpolar – contains only London dispersion forces (LDF)

– Polar – contains LDF and dipole-dipole forces

– Polar with H bonded to N, O, or F (with unshared pair) – contains LDF, dipole-dipole forces, and hydrogen bonds.

– Larger molecule, stronger LDF (all other factors equal)

Property Stronger forces mean…

Viscosity

Surface tension

Melting point (freezing)

Boiling point (condensation)

ΔHfus

ΔHvap

Vapor Pressure

Property Stronger forces mean…

Viscosity higher

Surface tension higher

Melting point (freezing) higher

Boiling point (condensation)

higher

ΔHfus higher

ΔHvap higher

Vapor Pressure lower