Intermolecular bonding

Intermolecular forces

• Three types
  • Van der Waals forces
  • Dipole-dipole forces
  • Hydrogen bonding

Every pair of atoms that bond together in some way will have Van der Waals forces acting on them. Many will have dipole-dipole forces acting on them and few will have hydrogen bonds.

Dipole-dipole force

• between molecules that have permanent dipoles (when one atom has a higher electronegative value than the other)
• two molecules that both have dipoles will attract oneanother
• molecules with dipoles will 'flip' to give an arrangement where the two molecules attract

Dipole moments

• occurs when molecules have polar bonds
• in molecules with more than one polar bond, the effects of each bond may cancel the whole thing out if they are linear, or they may reinforce each other

Although atoms may be neutral, they will still be made up of negative and positive charges. Van der Waals forces are the very weak electrostatic attractions that occur between all atoms and molecules.

In any atom, the electrons could be anywhere, although the whole atom is always neutral. This means that where the dipole moment occurs changes constantly, and therefore this is known as a temporary dipole. 

This temporary dipole affects the electron distribution in nearby atoms - the positive part of the atom will be attracted to the negative part of the first atom. The nearby atoms have experienced 'induced dipoles' because the dipole moment in their atom was created by the dipole moment in another atom.

The electron distribution in the original atom still changes constantly (as does its dipoles) and will therefore induce new dipoles in the atoms around it due to the attraction. These forces are called instantaneous dipole-induced dipole forces, better known as Van der Waals forces.

The more electrons there are, the larger the instantaneous dipole will be.

For this reason, the size of the Van der Waals forces increases with the number of electrons present.

So atoms/molecules with large atomic/molecular masses produce stronger Van der Waals forces.

This explains why the boiling points of the noble gases increases as the atomic numbers increase and also why the boiling points of hydrocarbons increases with increased chain length.

What is it?

A type of intermolecular force with some characteristics of dipole-dipole attraction and some of a covalent bond.

A hydrogen atom is between two very electronegative atoms.

What is needed?

A very electronegative atom with a lone pair of electrons covalently bonded to a hydrogen atom.

How strong are they?

Hydrogen bonds are much stronger than dipole-dipole forces although they are much weaker than covalent bonds.

Which electronegative atoms does hydrogen bonding occur with?

Oxygen, nitrogen, and fluorine.

Why are hydrogen bonds linear when two electronegative atoms are involved?

The pair of electrons in one bond (between the hydrogen and one electronegative atom) repels the electrons in the second bond (between the hydrogen and the other electronegative atom).

Why is hydrogen bonding important?

Due to the fact that they are weaker than covalent bonds, and can break or make under conditions. They are only 10% of the strength of covalent bonds but this means that covalent bonds would be unnaffected by conditions that hydrogen bonds are.