1f. Don't confuse weak intermolecular forces (intermolecular bonding) with the stronger covalent bonds within molecules!

The covalent bonds in molecules, the ion attraction in ionic compounds and the bonds between metal atoms are generally speaking very strong compared to 'intermolecular' bonds.

However, between all particles, but with particular reference to covalently bonded molecules, there always exists some very weak electrical attractive forces known as intermolecular forces or intermolecular bonding between the molecules.

These constantly acting attractive forces or intermolecular bonds are very much weaker than full covalent or ionic chemical bonds (approximately ¹/₃₀ to ¹/₂₀th in comparative attractive force).

For example, although the oxygen and hydrogen atoms are very strongly bonded in water to make a VERY stable molecule, BUT this does NOT account for the existence of liquid water and ice!

It is the weak intermolecular forces that induces condensation below 100^oC and freezing–solidification to form ice crystals below 0^oC.

In the reverse process, when ice is warmed, the intermolecular forces are weakened and at 0^oC the intermolecular bonds are weakened enough to allow melting to take place.

Above 0^oC (evaporation), and particularly at 100^oC (boiling), the intermolecular forces are weak enough for 'intact water molecules' to escape from the surface of the liquid water.

It is VERY important to realise that the chemical hydrogen–oxygen covalent bonds (O–H) in water are NOT broken and the state changes ...

solid <== freezing/melting ==> liquid <== condensing/boiling ==> gas ...

are due to the weakening of the intermolecular forces/bonds with increase in temperature OR the strengthening of the intermolecular bonds/forces with decrease in temperature.

For Advanced A Level chemistry students only:

Introduction to intermolecular forces - intermolecular bonding

[SEARCH BOX]