Group VII properties

Cambridge·Chemistry·8.3·Extended

Physical properties of the halogens

The halogens are non-metals in Group VII. Their elements exist as diatomic molecules: each molecule contains two atoms of the same element. For example, chlorine, bromine and iodine exist as $$\ce{Cl2}$$, $$\ce{Br2}$$ and $$\ce{I2}$$ respectively.

Physical properties at room temperature and pressure

  • Chlorine is a pale yellow-green gas.
  • Bromine is a red-brown liquid.
  • Iodine is a grey-black solid.

From chlorine to bromine to iodine, the state changes from gas to liquid to solid. Density also increases down the group: bromine is denser than chlorine, and iodine is denser than bromine.

Predicting halogen properties from position

These patterns can be used to predict properties of other halogens. An element below iodine in Group VII would be expected to have a higher density than iodine and to be solid at room temperature and pressure. Such predictions use the observed trend; they are not a substitute for measured properties.

Halogen reactivity and displacement

Halogens become less reactive down Group VII. Of the three halogens considered here, the order is:

chlorine > bromine > iodine

A more reactive halogen can displace a less reactive halogen from a compound containing its halide ions. During the reaction, the more reactive halogen gains electrons and forms halide ions. The less reactive halide ions lose electrons and form the displaced halogen.

Predicting halogen displacement

Compare the halogen added with the halogen represented by the halide ions:

  • Chlorine displaces bromide ions and iodide ions.
  • Bromine displaces iodide ions, but not chloride ions.
  • Iodine displaces neither chloride ions nor bromide ions.

For example, chlorine displaces bromine from bromide ions:

ClX2+2 BrX−→2 ClX−+BrX2\ce{Cl2 + 2Br^- -> 2Cl^- + Br2}

In this reaction, chlorine forms chloride ions, while bromide ions form bromine. The reverse displacement does not occur: bromine cannot displace chloride ions because bromine is less reactive than chlorine.

The same rule applies to another Group VII element when its position is known. A halogen can displace the halide ions of an element below it in the group, but not those of an element above it.