Intermolecular ForcesH

O

H

H

O

H

H

O

HH

O

H

H

O

HH

O

HH

O

HH

O

H

H

O

H

H

O

H

H

O

H

H

O

H

(given in increasing strength)

1. London dispersion force (Weakest)

Occurs between NON-POLAR molecules.

• Atoms of a NP molecule equally share bonding e-.• normally no overall charge.

(O2, Cl2, CH4, C2H8)

H

C HH

H

+ +

At any given time, e- in a molecule are unevenly distributed – e- move.

Causes temporary dipoles (charged regions) to form, which leads to attraction between molecules.

H

C HH

H

δ+

δ-

+ + + +

F F F FF2F2

δ+ δ-

non - polar molecules

H

C H

CH4CH4

δ+ δ-

non - polar molecules

H

H

C H

H

H

H

temporarydipole

2. Dipole-Dipole

One atom of a molecule pulls the shared electrons closer than the other (unequal sharing).

+ + HClδ+δ-

Causes permanent negative and positive dipoles.

The polar ends attract other polar molecules.

Cl

HCl

H

polar molecules

δ+

δ-

δ-

δ+

permanentdipole

+ +

δ+δ-

3. Hydrogen bond (special dipole-dipole force)

• Very strong force.

• Any substance containing hydrogen bonded directly to O, N, or F.

• O, N and F pull extremely hard on the bonding electrons (VERY unequal sharing) - large dipoles.

H

O

H

HOH

H

OH

N

H

H

H

H

N

H

H

H

H

N

H

H

H

H

polar molecules

δ+δ-

δ+

δ+

δ-

δ+

δ+

δ-

δ+

4. Strong ionic attraction

Positive and negatively charged IONS held together by electrostatic (attraction) forces.

These electrostatic forces are extremely strong, giving ionic compounds very large melting and boiling points.

Dispersion Forces - weakest, temporary positive and negative attraction caused by moving electrons.

Dipole / Dipole - weak, permanent dipole attraction caused by one atom pulling electrons more strongly.

Hydrogen Bonding - strong permanent dipole attraction caused by O, N or F bonded to hydrogen.

Electrostatic Attraction - strongest force of attraction between positive and negative ions in ionic compound.

Electrostatic Attraction

Dipole forces OR Hydrogen bonding

Dispersion Forces

The Special Case for Water

Hydrogen-bonding holds water molecules together strongly.

Responsible for the higher than expected melting pointand boiling point.

Because of H - bonding, ice forms unique crystal shape (six-sided), having an area greater than that liquid water.

Ice is less dense than water...most solids are more dense than their respective liquids.