1.14 Group 7: the halogens

The halogens

The halogens are non-metals. They exist as diatomic molecules: F₂, Cl₂, Br₂ and I₂. They are toxic, so they are handled in a fume cupboard.

Trends in physical properties

Going down the group:

• melting and boiling points increase, because the molecules get larger (higher relative molecular mass), so the forces between molecules are stronger
• the colour gets darker
• the state changes from gas to liquid to solid

So you can predict that astatine, below iodine, is a dark solid with an even higher melting point.

Test for chlorine

Hold damp blue litmus paper in the gas. Chlorine turns it red and then bleaches it white.

Reactions

• With metals, halogens form metal halides (salts). These are ionic, with a 1− halide ion such as \mathrm{Cl^-}. 2Na + Cl₂ → 2NaCl and 2Fe + 3Cl₂ → 2FeCl₃
• With hydrogen, halogens form hydrogen halides: H₂ + Cl₂ → 2HCl
• Hydrogen halides are gases that dissolve in water to form acidic solutions. Hydrogen chloride forms hydrochloric acid.
• With other non-metals, halogens form covalent compounds made of molecules.

Why reactivity decreases down the group

A halogen atom reacts by gaining one electron to complete its outer shell. Going down the group:

1. The atoms have more shells, so the outer shell is further from the nucleus.
2. There is more shielding by inner electrons.
3. So there is a weaker attraction for an extra electron.
4. An electron is gained less easily, so the halogen is less reactive.

Fluorine is the most reactive halogen.