Intramolecular bonding versus Intermolecular forces

Intramolecular bonding is the name given to the chemical bonds inside molecules. All intramolecular bonds are strong. The following are some examples:

• A pure covalent bond is a pair of electrons equally shared between atoms.
• A polar covalent bond is a pair of electrons unequally shared between atoms.
•  A dative covalent bond is a covalent bond where both electrons come from the same atom.

Intermolecular bonding is the name given to forces of attraction between molecules. There are three types, all of which are weak:

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

1. Van der Waals forces

         Out of the three, this is the weakest intermolecular force. They form between temporary dipoles in one molecule and induced dipoles in another. They are constantly changing and the strength of attraction depends on the number of electrons in the molecule.

The shape of the molecule is also important:

A large surface area = more Van der Waals forces = strong attraction.

Below is showing Van der Waals forces occurring between 2 molecules of hydrogen:

The particle on the left has formed a temporary dipole maybe caused by one side of it being more dense than the other. For a brief moment one end of the particle has a positive charge and the other end with a negative charge, causing a nearby particle to also have a temporary dipole.

2. Dipole-dipole

         These forces are stronger than Van der Waals and occur between polar molecules meaning that they have permanent dipoles.

Remember! The greater the difference in electronegativity the bigger the dipole and the stronger the attraction is between the molecules (refer back to my previous blog 

The diagram below is showing the dipole-dipole attraction between two HCl molecules. The negative dipole on the chlorine atom is attracted to the positive dipole of another HCl molecule:

3. Hydrogen bonding

         This is the strongest intermolecular force but it only occurs between molecules in which hydrogen is bonded to nitrogen, oxygen or fluorine.

Hydrogen bonding is the attraction between an electron deficient atom on one molecule and a lone pair of electrons on a highly electronegative atom on a different molecule.

The diagram above shows hydrogen bonding in water. The positive dipole on the hydrogen atom is attracted to the lone pair of electrons on the oxygen atom in another water molecule.