Boiling Point Trend

The boiling and melting points of the halogens increase down the group. This is due to the increasing strength of the London dispersion forces as the number of electrons increases when the size and relative mass of the atoms increases. As substance is said to be volatile if it has a low boiling point so volatility decreases down the group.

Reactivity Trend

Halogen atoms react by gaining an electron in their outer shell. This means they're reduced and therefore oxidising agents. When halogens are reduced, halide ions are formed. As you go down the group, the atomic radius increases so the outer electrons are further from the nucleus. The outer electrons are also shielded more from the attraction of the positive nucleus, because there are more inner electrons. This makes it harder for larger atoms to attract the electron needed to form an ion, despite the increased charge on the nucleus. So, larger atoms are less reactive and so less oxidising.

Displacement Reactions

The halogens relative oxidising strengths can be seen in their displacement reactions with halide ions. More reactive halogens will oxidise and displace the halide ions of less reactive halogens. Reactivity decreases down the group so a halogen will displace a halide from solution if the halide is below it in the periodic table.

Chlorine will displace bromide and iodide
Bromine will displace iodide
Iodine has no reaction

With these displacement reactions, there are colour changes

Chlorine water is colourless and will displace potassium bromide solution and potassium iodide solution
Bromine water is orange and will displace potassium iodide solution
Iodine solution is brown and does not displace any halides

The brown and orange of bromine water and iodine solution can often look similar so reacting the mixture with an organic solvent like hexane will dissolve with the halogen and the different halogens form different colours.

A violet/pink colour shows the presence of iodine
An orange red colour shows bromine
A very pale yellow/green shows chlorine.

	Created by Sarah . over 7 years ago

A level Chemistry Note on OCR A - A level Chemistry Module 3, created by Sarah . on 03/02/2017.