Multiple choiceQuestion 1 · Recall
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By the end you can describe the colour and physical state of chlorine, bromine and iodine, and explain the trend in their volatility using instantaneous dipole–induced dipole forces.
Chlorine, bromine and iodine sit together in Group 17. They are the three halogens you meet at AS Level.
At room temperature they look nothing alike:
Same group, same type of molecule (), yet one is a gas, one is a liquid and one is a solid.
Volatility
How easily a substance turns from a liquid or solid into a gas at a given temperature.
A more volatile substance evaporates more easily and has a lower boiling point. A less volatile substance needs a higher temperature to become a gas.
Volatility decreases down Group 17.
| halogen | colour | state at room temperature | boiling point |
|---|---|---|---|
| pale yellow-green | gas | −34 °C | |
| red-brown | liquid | 59 °C | |
| grey-black | solid | 184 °C |
Chlorine is already a gas at room temperature, so it is the most volatile of the three. Iodine is a solid and only becomes a gas at 184 °C, so it is the least volatile.
Halogen molecules are non-polar: the two atoms share their electrons equally. So there are no permanent dipoles to attract one molecule to another.
But electrons are not still. At any instant, the electron cloud can be lopsided, giving the molecule a temporary dipole. That temporary dipole pushes the electrons in a neighbouring molecule aside and induces a matching dipole. The two molecules then attract each other.
These attractions are instantaneous dipole–induced dipole forces (also called London forces or dispersion forces).
Down the group, the molecules get bigger: , then , then . So the forces get stronger, and volatility falls.
Where the picture stops being useful: these forces act between molecules. They are not the covalent bonds inside each molecule. Boiling a halogen does not break the – bond.