Why small molecules melt low and do not conduct

When solid iodine turns into a purple vapour, does anything inside its molecules break? A French chemist, Bernard Courtois, was the first to see that vapour, in eighteen eleven. He added too much acid to some seaweed ash, and a violet vapour rose and settled again as dark, shiny crystals. Something came apart to make that gas, yet the iodine came out the other side still iodine. To see what came apart in that vapour, start with one iodine molecule up close.

A substance made of small molecules has two different things holding it together, and they could hardly be less alike. Inside each molecule, each atom is joined to its neighbour by covalent bonds. A covalent bond is a shared pair of electrons, and it is strong. In iodine, I two, one covalent bond holds one iodine atom to the other. Between one molecule and the next, there are intermolecular forces. Inter means between, and these forces are weak, far weaker than the bond inside the molecule. Foundation and Higher papers both test this split. Now warm some solid iodine gently, and watch the purple vapour form. Which gives way as iodine turns to vapour: the covalent bonds, or the forces between molecules? The forces between the molecules. The weak attractions are what give way, and every iodine molecule leaves as a whole I two, with its bond intact.

That is the whole story of melting and boiling for any substance made of small molecules. The molecules move apart, and the intermolecular forces between them are overcome. The covalent bonds inside each molecule are not broken. The molecule that leaves as a gas is the same molecule that sat in the solid, the same size and the same shape. Only the gaps between the molecules change. The forces being overcome are weak, so little energy is needed. That is why the melting and boiling points are low, and why so many of these substances are liquids or gases at room temperature. Boil a kettle dry and the steam that left it was water, not hydrogen and oxygen. Every water molecule went up the spout whole. Now water itself, which boils at one hundred degrees C. Pause and explain why water boils so low: say what is overcome, and what is not broken. Water is made of small molecules. When it boils, the weak intermolecular forces between those molecules are overcome, which needs little energy. The covalent bonds inside each molecule are not broken. Listen to the verbs in that answer. Forces are overcome. Bonds are broken. Keep each verb with its own noun, and no examiner can read your answer as breaking the bonds. One examiner's report lists the mix-ups it keeps meeting: "Candidates mix up simple covalent molecules and intermolecular forces. The misconceptions are that intermolecular forces are bonds rather than forces, that covalent bonds are broken for melting to occur and that these bonds are weak." Three errors, one cause: treating the bond inside and the force between as one thing. The fix is to name them apart every time. Strong covalent bonds inside, weak forces between, and only the forces are overcome. Here is a student's sentence: fluorine has a low boiling point because it has weak covalent bonds between the molecules. That one sentence hides two separate errors. Can you find both errors in the student's sentence about fluorine? First, covalent bonds are inside a molecule, never between molecules. Second, covalent bonds are strong, not weak. The sentence should say weak intermolecular forces between the molecules.

Molecule size changes things too. The bigger the molecule, the stronger the intermolecular forces between molecules, so the higher the melting and boiling points. Three substances from one family have molecules of increasing size. Their boiling points, smallest molecule first, are minus forty-two, nought, and thirty-six degrees C. Then a fourth member of the family arrives, with a bigger molecule still. Does that fourth member boil above or below thirty-six degrees C, and why? Above. Its molecules are bigger again, so the forces between them are stronger, and more energy is needed to overcome them. Push that trend a very long way, to molecules that are enormously long chains, and you reach a polymer. The video on polymers picks up exactly there.

The second property is conduction. To conduct electricity, a substance needs charged particles that are free to move. A substance made of small molecules is built from molecules and nothing else. Why does a small-molecule substance not conduct? One sentence, without the word bond. Its molecules have no overall charge, so there are no charged particles free to move and carry the current. A full answer about a substance like this has three parts, in order. It is made of small molecules. The weak intermolecular forces between them are overcome. Only a little energy is needed. On a question about propane, one examiner's report says: "Few students recognised that it is the structure of propane as small molecules that is important rather than it being a hydrocarbon with single bonds." It adds: "Whilst more than a half of students scored at least one mark, only one in seven scored all three." More than half got started, so the idea is within reach. The fix is the opening: begin every answer with made of small molecules, and the other two parts follow from it.

Before the purple crystals come back, four quick checks. Inside a molecule, what holds each atom to the next, and what acts between molecules? Inside: strong covalent bonds. Between: weak intermolecular forces. Now one you have not seen: chlorine boils at minus thirty-four degrees C, bromine at fifty-nine. Which has stronger forces? Bromine. Its molecules are bigger, so the intermolecular forces are stronger and need more energy to overcome. A different one: why can a substance made of small molecules not carry a current? Its molecules have no overall charge, so there is nothing charged that is free to move. One more, back to Courtois: when iodine crystals turn to purple vapour, what is overcome, and what stays whole? The weak forces between the iodine molecules are overcome. The covalent bond inside each I two stays whole, so the purple gas is made of the same molecules as the crystals.

Use the thumbs-up as a record for yourself. If this one is in the bag, press it, and you know not to spend revision time on it again. Leave the thumb off if it is not there yet, and save the video so you can find it. One small trick helps here: say the forces-and-bonds rule out loud once, and it sticks.

Next in the chapter: polymers, recognising one, and why polymers are solids. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh five, oh four.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.