Properties of Simple Covalent Compounds

A typical simple molecular compound consists of separate molecules. Within each molecule, atoms share electrons in strong covalent bonds. Attractions also act between one molecule and another; these are called intermolecular forces. They are much weaker than the covalent bonds inside the molecules.

It is therefore important to distinguish two kinds of attraction:

• within a molecule: strong covalent bonds hold its atoms together;
• between molecules: weaker intermolecular forces attract neighbouring molecules.

Water, $\mathrm{H_2O}$, and carbon dioxide, $\mathrm{CO_2}$, are examples of compounds made from separate molecules.

Why melting and boiling points are low

In a solid simple molecular substance, the molecules are held in fixed positions by intermolecular forces. Heating transfers energy to the particles.

At the melting point, enough energy has been supplied for the molecules to leave their fixed positions and move past one another. The liquid molecules remain close together, so intermolecular attractions still act between them.

At the boiling point, enough energy has been supplied for molecules to separate much more widely and form a gas. The intermolecular forces are overcome, but the molecules themselves remain intact.

During both changes of state, it is the weak intermolecular forces that are overcome—not the strong covalent bonds within each molecule. Relatively little energy is therefore needed, giving typical simple molecular compounds low melting points and low boiling points.

This explains why many simple molecular substances are liquids or gases at room temperature. Their melting and boiling points are low compared with those of giant structures, in which strong bonding extends throughout the substance.

Why they conduct electricity poorly

An electric current requires charged particles that can move through a material. In simple molecular compounds, the molecules are usually neutral. There are no ions free to move through the substance, and the bonding electrons are localised in covalent bonds rather than delocalised throughout the material.

Consequently, there are no mobile charge carriers to transfer charge. Typical simple molecular compounds are therefore poor electrical conductors, both as solids and when molten. Melting allows whole molecules to move, but those molecules remain neutral: particles must be both charged and mobile to carry current.

Connecting structure to both properties

The two properties have different immediate causes:

• low melting and boiling points arise because the forces between molecules are weak and require little energy to overcome;
• poor electrical conductivity arises because there are no mobile ions or delocalised electrons to carry charge.

Both explanations begin with the same particle model—separate covalently bonded molecules—but then follow a different chain of reasoning for the property being explained.