Name: iodine	Group number: 17
Symbol: I	Group name: Halogen
Atomic number: 53	Period number: 5
Atomic weight: 126.90447 (3)	Block: p-block
CAS Registry ID: 7553-56-2	Voice:
Standard state: solid at 298 K	Colour: violet-dark grey, lustrous
Classification: Non-metallic	Availability:

Iodine is a bluish-black, lustrous solid. It volatilises at ambient temperatures into a pretty blue-violet gas with an irritating odour.

It forms compounds with most elements, but is less reactive than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulphide to form beautiful purple solutions. It is only slightly soluble in water. Iodine compounds are important in organic chemistry and very useful in medicine and photography. Lack of iodine is the cause of goitre (Derbyshire neck). The deep blue colour with starch solution is characteristic of the free element. It is assimilated by seaweeds from which it may be recovered, and is found in Chilean saltpetre, caliche, old salt brines, and salt wells.

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The picture above shows the result of touching nitrogen triiodide (NI3)! Nitrogen triiodide is percussion sensitive. Do not attempt this reaction unless are a professionally qualified chemist and you have carried out a legally satisfactory hazard assessment. Nitrogen triiodide is dangerous! Select a movie icon to see the result of touching nitrogen triiodide.

Isolation

Iodine is available commercially so it is not normally necessary to make it in the laboratory. Iodine occurs in seawater but in much smaller quantities than chloride or bromide. As for bromine, with suitable sources of brine, it is recovered commercially through the treatment of brine with chlorine gas and flushing through with air. In this treatment, iodide is oxidized to iodine by the chlorine gas.

2I^- + Cl₂ 2Cl^- + I₂

Small amounts of iodine can be made through the reaction of solid sodium iodide, NaI, with concentrated sulphuric acid, H₂SO₄. The first stage is formation of HI, which is a gas, but under the reaction conditions some of the HI is oxidized by further H₂SO₄ to form iodine and sulphur dioxide.

NaI (s) + H₂SO₄ (l) HI (g) + NaHSO₄ (s)

2HI (g) + H₂SO₄ (l) I₂ (g) + SO₂ (g) + 2H₂O (l)