Properties of Simple Covalent Compounds

Explaining Low Melting and Boiling Points

Why does water boil at just $100^\circ\text{C}$ while other substances require much higher temperatures? The answer lies in distinguishing between the bonds inside the molecules and the forces between them.

Within a single molecule, atoms are tightly held together by very strong covalent bonds. However, the individual molecules are only attracted to each other by weak intermolecular forces. These covalent bonds are roughly 20 to 30 times stronger than the weak forces pulling the separate molecules together.

When you heat a simple molecular substance to melt or boil it, you are only supplying enough thermal energy to separate the molecules from one another. You completely overcome the weak intermolecular forces, but the strong covalent bonds remain completely intact. Because these intermolecular forces are so weak, very little energy is needed, resulting in low melting and boiling points.

The Effect of Molecular Size

A heavy box takes more effort to push across the floor than a light one, and molecules follow a remarkably similar rule. As the size of a simple molecule increases, the strength of its intermolecular forces also increases.

Larger molecules have stronger forces of attraction between them. Consequently, they require much more heat energy to overcome these forces and separate the molecules. This creates a clear trend in groups like the halogens:

Substance	Formula	Size / Mass	State at Room Temperature	Boiling Point
Fluorine	$F_2$	Smallest	Gas	Lowest
Bromine	$Br_2$	Medium	Liquid	Medium
Iodine	$I_2$	Largest	Solid	Highest

Buckminsterfullerene ($C_{60}$) is an important exception that examiners love to test. Despite having 60 carbon atoms, it is still just a large simple molecular substance, not a giant covalent lattice. Therefore, it has a much lower melting point than diamond or graphite.

Worked Example: Explaining Boiling Points

Explain why pentane ($C_5H_{12}$) has a higher boiling point than methane ($CH_4$).

Step 1: Identify the structural difference.

• Pentane is a larger molecule than methane, meaning it has a higher relative molecular mass.

Step 2: Relate the size to the intermolecular forces.

• Larger molecules have stronger intermolecular forces of attraction between them.

Step 3: State the energy consequence.

• Therefore, more thermal energy is required to overcome these stronger intermolecular forces in pentane, giving it a higher boiling point.

Explaining Poor Electrical Conductivity

If you drop a plugged-in appliance into a bathtub, it is the dissolved salts in the tap water that conduct the deadly current, not the pure water molecules themselves. In fact, pure simple molecular substances are excellent insulators in all states (solid, liquid, and gas).

This lack of conductivity occurs because simple molecular compounds consist entirely of neutral molecules. They do not have an overall electrical charge. Furthermore, all of their outer-shell electrons are tightly shared in localized covalent bonds.

For any substance to conduct electricity, it must contain charge carriers that are free to move. Simple molecular substances have no delocalised electrons and no mobile ions available. Without these mobile charged particles, an electric current simply cannot pass through the substance. Even $C_{60}$, which has spare electrons within its structure, cannot conduct electricity because those electrons cannot move between the molecules.