Chapter 6 - 4Intermolecular Forces or IMF

(p. 219 – 224)

Intermolecular Forces or IMFIntermolecular Forces or IMF

Attractive forces between molecules.

Much weaker than chemical bonds within molecules.

a.k.a. van der Waals forces

Intermolecular Forces

If there are no attractive forces between molecules, the substance will exist as a gas because an attractive force is required to hold molecules together as liquids and solids.

Types of Intermolecular ForcesTypes of Intermolecular Forces

1.1. London Force London Force (Dispersion forces)(Dispersion forces)

fforms when any atom orms when any atom or molecule becomes a or molecule becomes a dipole at an instant dipole at an instant when its electrons are when its electrons are unequally distributed unequally distributed around the nucleus or around the nucleus or nuclei.nuclei.

more electrons, more electrons, stronger London forcestronger London force

Types of IMFTypes of IMF

London Dispersion Forces

View animation online.

Types of Intermolecular ForcesTypes of Intermolecular Forces

2. Dipole Force2. Dipole Force forms when the positive end of one forms when the positive end of one

polar molecule attracts the negative polar molecule attracts the negative end of another polar molecule.end of another polar molecule.

slightly stronger than London forces.slightly stronger than London forces.

H F

H F

Types of IMFTypes of IMF

Dipole-Dipole Forces

+ -

View animation online.

8

Dipole Interactions

Types of Intermolecular ForcesTypes of Intermolecular Forces

3. Hydrogen bond3. Hydrogen bond forms when a hydrogen atom bonded forms when a hydrogen atom bonded

to a small strongly electronegative to a small strongly electronegative atom such as F, O, and N is attracted atom such as F, O, and N is attracted to to another electron-rich atomanother electron-rich atom

tthe hydrogen is left very electron he hydrogen is left very electron deficientdeficient

the strongest of the intermolecular the strongest of the intermolecular forces.forces.

Types of IMFTypes of IMF

Hydrogen Bonding

Hydrogen Bonding

HH

O+ -

+

H HO+-

+

Hydrogen bonding

HH

O H HO

HH

O

H

H

OH

HO

H

HO HH

O

Types of IMFTypes of IMF

Determining IMFDetermining IMF

CH4

• nonpolar = London NCl3

• polar = London, dipole-dipole HF

• H-F bond = London, hydrogen bonding