1f. Don't confuse weak intermolecular forces (intermolecular bonding) with strong covalent bonds!

Doc Brown's Chemistry: Chemical Bonding and structure GCSE/IGCSE/O/IB/AS/A US grade 9-12 Level Revision Notes


INTERMOLECULAR FORCES – INTERMOLECULAR BONDING

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.

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 100oC and freezing–solidification to form ice crystals below 0oC.

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

Above 0oC (evaporation), and particularly at 100oC (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

 


What next?

Recommend next: Part 3 Covalent Bonding: small molecules & properties

 

Sub-index for: Part 1 Introduction to chemical bonding - why? how? and patterns

 

Index for ALL chemical bonding and structure notes

 

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