📝 Halogens
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The halogens chlorine, bromine and iodine exist as covalent diatomic molecules. They become increasingly less volatile and more deeply coloured on descending Group 17. The volatility decreases as van der Waals’ forces between molecules increase.
The melting and boiling points of the halogens increase steadily down the group. This is due to the increase in strength of the van der Waal's forces between the molecules, which results from the increasing number of electrons in the molecule and the increasing surface area of the molecule.
As the number of shells increases, the shielding increases and the electrons in the covalent bond are further from (and more shielded from) the nucleus. Therefore, they are less strongly attracted to the nucleus and the electronegativity decreases.
All the halogens are oxidizing agents. Fluorine is the strongest oxidising agent and iodine is the weakest. The reactivity of the halogens decreases descending the group. It gets easier to oxidise the hydrogen halides going down Group 17 as the strength of the hydrogen–halogen bond decreases.
The more reactive halogens (i.e. the strongest oxidising agents) will displace the more reactive halides (i.e. the strongest reducing agents) from solutions of their ions:
Chlorine will displace bromide and iodide ions from solution. Bromine will displace iodide ions from solution, but not chloride ions. Iodine cannot displace either bromide or chloride ions from solutions. As size increases down a group, the ability of the atoms to accept electrons decreases and hence their oxidising power decreases.
Chlorine reacts with cold hydroxide ions in a disproportionation reaction. This reaction produces commercial bleach. A different disproportionation reaction takes place between chlorine and hot alkali, producing NaCl(aq), NaClO3(aq) and water.
The halogens all have important industrial uses, especially chlorine, which is used in the manufacture of many other useful products. Possibly the most important use of chlorine is in the prevention of disease by chlorination of water supplies.
If silver nitrate solution and a little nitric acid, is added to an aqueous solution containing a halide ion, the following reactions take place:
If dilute or concentrated ammonia is added to AgCl(s), the precipitate dissolves.
If dilute ammonia is added to AgBr(s), there is no reaction but if concentrated ammonia is added then the precipitate dissolves.
Silver iodide does not dissolve either in dilute or in concentrated ammonia.