Structure and Bonding - Properties of Molecular Substances

Understanding how molecules behave helps explain why water boils at 100°C100°C, why plastic bags are flexible, and why sugar dissolves in tea. This knowledge is essential for predicting the properties of different materials.

1. Small Molecular Substances

Many everyday substances like water (H2OH2​O), carbon dioxide (CO2CO2​), and oxygen (O2O2​) are made of small molecules. These molecules have a unique structure that explains their properties.

Each small molecule has:

• Strong covalent bonds holding the atoms together inside each molecule

• Weak intermolecular forces between different molecules

Think of it like this: imagine each molecule as a tightly-bound group of friends (strong covalent bonds), but different groups only wave at each other from a distance (weak intermolecular forces).

Because the forces between separate molecules are weak, it doesn't take much energy to separate them. This is why many small molecular substances are gases or liquids that evaporate easily at room temperature. Most have low melting and boiling points.

1.1 Melting and Boiling

When a molecular substance melts or boils, something important happens: the strong covalent bonds inside each molecule stay intact. Only the weak intermolecular forces are overcome.

energy supplied⟶overcome intermolecular forcesenergy supplied⟶overcome intermolecular forces

This explains two key patterns:

• Stronger intermolecular forces need more energy to overcome them

• The intermolecular forces are stronger between larger molecules

Comparison table

Notice how the boiling point increases as the molecules get bigger - this is because the intermolecular forces become stronger.

1.2 Electrical Conduction

Small molecular substances cannot conduct electricity. This is because:

• The molecules are electrically neutral overall

• There are no free ions to carry electric charge

• There are no free-moving electrons that can move through the material

This is why pure water doesn't conduct electricity, but tap water (which contains dissolved ions) does.

2. Explaining Bulk Properties

The key to understanding molecular substances is remembering the difference between two types of forces:

Key terms

Covalent bonds - Very strong forces that hold atoms together within a molecule

Intermolecular forces - Much weaker forces that exist between different molecules

Worked example

Question: Explain why iodine crystals sublime (turn directly from solid to gas) easily when gently heated.

Solution:

1. Iodine exists as I2I2​ molecules held together by strong covalent bonds

2. Between different I2I2​ molecules, there are only weak intermolecular forces

3. When heated gently, there's enough energy to overcome the weak intermolecular forces

4. The I2I2​ molecules can escape directly into the gas phase without needing to melt first

5. The covalent bonds within each I2I2​ molecule remain intact throughout

Comparing water and ethanol: Both water (H2OH2​O) and ethanol (C2H5OHC2​H5​OH) are molecular substances, but water has a higher boiling point (100°C100°C vs 78°C78°C). This indicates that the intermolecular forces between water molecules are stronger than those between ethanol molecules, so more energy is needed to overcome them.

3. Polymers – Very Large Molecules

Key terms

Polymer - A substance made of extremely long chains of atoms joined together by covalent bonds

Polymers are like molecular substances, but much, much bigger. A single polymer molecule might contain thousands or even millions of atoms all joined by covalent bonds. Poly(ethene) - the plastic used in shopping bags - is a familiar example.

Because polymer molecules are so large:

• They have much stronger intermolecular forces than small molecules

• They are usually solids at room temperature

• They often have higher melting points than small molecular substances

• They can be flexible because the long chains can bend and move past each other

Everyday polymers:

• Poly(ethene) - plastic bags, bottles

• Polystyrene - disposable cups, packaging

• PVC - pipes, window frames

• Nylon - clothing, ropes

All of these materials are made of very long molecular chains, which gives them their useful properties like flexibility and durability.

The key difference between small molecules and polymers is size - but this size difference leads to dramatically different properties that make polymers incredibly useful in everyday life.