States and colours
The halogens are non-metals and occur as diatomic molecules, such as and . Their colours become progressively darker down Group 7.
| Halogen | Formula | State and appearance |
|---|---|---|
| Fluorine | Very pale yellow gas | |
| Chlorine | Green-yellow gas | |
| Bromine | Red-brown liquid that gives orange-brown fumes | |
| Iodine | Grey-black solid that sublimes to form purple vapour |
Boiling point and volatility
Volatility describes how readily a substance evaporates. A substance with a lower boiling point is more volatile.
The melting and boiling points of the halogens increase down the group, so volatility decreases. Among the four halogens shown above, volatility is greatest for fluorine and lowest for iodine.
Halogen molecules have simple molecular structures. Separate molecules attract one another through instantaneous dipole–induced dipole forces.
Down the group, each diatomic molecule contains more electrons. The intermolecular attractions therefore become stronger, so more energy is required to separate the molecules. This causes the boiling point to increase.