Simple Molecular Structures

A simple molecular substance is made of small, separate molecules. Each molecule contains a few atoms joined by covalent bonds (shared pairs of electrons). Examples you must know: hydrogen H₂, oxygen O₂, chlorine Cl₂, iodine I₂, water H₂O, carbon dioxide CO₂, ammonia NH₃, methane CH₄.

The single most important idea in this topic is that there are two completely different forces at work, and they have very different strengths:

• Inside each molecule — the atoms are held together by strong covalent bonds. These are hard to break.
• Between the molecules — there are only weak intermolecular forces (weak forces of attraction between whole molecules). These are easy to overcome.

Get this distinction right and the whole topic falls into place. Almost every mark in this subtopic depends on you separating "the bonds inside a molecule" from "the forces between molecules".

Why it matters. When you melt or boil the substance, you only have to break the weak forces between molecules — so it happens easily, at a low temperature. The strong covalent bonds inside the molecules survive untouched.

This is the headline exam point of the whole subtopic — and the place students lose the most marks.

When a simple molecular substance melts or boils:

• the energy supplied is used to overcome (break) the weak intermolecular forces between the molecules, so the molecules can move apart;
• it is NOT used to break the strong covalent bonds inside the molecules — those stay intact;
• because only weak forces have to be overcome, only a small amount of energy is needed, so the melting and boiling points are low.

The mark-scheme answer (memorise the shape of it):

"Simple molecular substances have low melting and boiling points because they have weak intermolecular forces between the molecules. Only a small amount of energy is needed to overcome these weak forces. The strong covalent bonds inside the molecules are NOT broken."

Three things examiners insist on:

1. The word intermolecular (or "between molecules") — not just "weak forces".
2. The forces are overcome / broken, not the covalent bonds.
3. You must not say "the covalent bonds are weak" — they are strong. It is the forces BETWEEN molecules that are weak.

Not all simple molecular substances have equally low melting points. The bigger the molecule, the stronger the intermolecular forces between molecules — so the higher the melting and boiling points.

Why? Larger molecules contain more electrons. More electrons means the intermolecular forces between molecules are stronger, so more energy is needed to pull the molecules apart.

The classic example — the halogens (Group 7):

Molecule	Size / electrons	State at room temperature
Cl₂ (chlorine)	smallest	gas
Br₂ (bromine)	medium	liquid
I₂ (iodine)	largest	solid

As you go down the group the molecules get bigger, the intermolecular forces get stronger, and the boiling point rises — so chlorine is a gas, bromine a liquid and iodine a solid at room temperature.

The same trend explains why methane CH₄ (small) is a gas while bigger molecules made of the same kind of atoms are liquids or solids.

Important: even iodine, a solid, still has a fairly low melting point compared with a giant covalent or ionic substance — because the forces being overcome are still only the (relatively) weak intermolecular forces.

Simple molecular substances do not conduct electricity in any state — solid, liquid or (for gases) gas.

To conduct electricity, a substance needs mobile charged particles — either free electrons or free ions — that can move and carry the charge. Simple molecular substances have neither:

• No free electrons. Every outer electron is either in a covalent bond or part of a lone pair — none is free to move.
• No ions. Covalent bonding means electrons are shared, not transferred, so there are no charged particles. Each molecule is neutral overall.

With no charge carriers to move, there is no electrical current.

Mark-scheme answer: "Simple molecular substances do not conduct electricity because there are no free electrons and no ions (no mobile charged particles) to carry the charge."

One thing to watch: some simple molecular substances (like HCl or ammonia) can conduct when dissolved in water, because they react with water to form ions in solution. But the pure simple molecular substance itself does not conduct.