Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
Discrete molecules
Simple molecular substances consist of small, separate molecules. Strong covalent bonds hold atoms within each molecule, while much weaker intermolecular forces act between molecules.
Melting and boiling
Melting or boiling overcomes intermolecular forces, not covalent bonds. Relatively little energy is required, so simple molecular substances usually have low melting and boiling points.
Conductivity
The molecules are neutral and usually contain no mobile ions or delocalised electrons. Most simple molecular substances are therefore poor electrical conductors.
Molecular size
Within a related series, larger molecules often have stronger total intermolecular attractions and higher boiling points. Specific force types are not required, but the trend may be interpreted qualitatively.
Common error
Low melting point does not mean the covalent bonds are weak. The bonds within molecules are strong; the forces between molecules are weak.
Practical or data skill
Use melting-point and conductivity data to classify unknown substances as simple molecular or another structure type.
Examination tip
Always distinguish covalent bonds within molecules from forces between molecules.
Review questions and suggested answers
Question 1
Why are boiling points often low?
Suggested answer
Only weak intermolecular forces are overcome.
Question 2
Are covalent bonds broken during boiling?
Suggested answer
No.
Question 3
Why no conduction?
Suggested answer
There are no mobile charged particles.
