(i) Occurrence : It mainly occurs in sea weeds or alkali metal iodides. Caliche (crude chile salt petre) which is mainly sodium nitrate contains iodine as sodium iodate (NaIO₃).

(ii) Preparation of iodine :On a commercial scale iodine is prepared from sea weeds and caliche.

(a) From sea weeds : Sea weeds (Laminaria variety) are dried, burnt and ash (called kelp contains about 1% as iodides of alkali metals besides chlorides and sulfates) is extracted with hot water. sulfates and chlorides are separated by fractional crystallization, the mother liquor is treated with Cl₂ gas or heated with MnO₂ and conc. H₂SO₄ to liberate I₂ which is cooled and condensed to give violet crystals.

2NaI + Cl₂ —> 2NaCl + I₂

2NaI + MnO₂ + 3H₂SO₄ —> 2NaHSO₄ + MnSO₄ + 2H₂O + I₂

(b) From Caliche : The mother liquor left after crystallization of NaNO₃ is treated with NaHSO₃ to liberate I₂ from NaIO₃.

2NaIO₃ + 5NaHSO₃ —> 3NaHSO₄ + 2Na₂SO₄ + H₂O + I₂

In the laboratory, I₂ is prepared by heating a mixture of potassium iodide and MnO₂ with conc. H₂SO₄.

2KI + MnO₂ + 3H₂SO₄ —> 2KHSO₄ + MnSO₄ + H₂O + I₂

(iii) Properties: It is a dark violet shining solid which sublimes on heating. It is least soluble in water. However, its solubility can be increased by adding 100%KI solution due to the formation of I^–₃ complex ion in which I^– ion acts as a lewis base (ligand) and I₂ molecule behaves as a lewis acid (central atom) which accommodates lone pair of electrons donated by I^– ion in the antibonding sigma P_s molecular orbital.

I₂ + I^– ——> I^–₃ (complex ion)

The aqueous solution containing I^–₃ complex ion has a brown color. It is soluble in many organic solvents. Its solution in CS₂ .CHCl₃ and CCl₄ is violet while in strong donor solvents like alcohols, ethers and amines is brown.

With cold, dilute NaOH, iodine gives hypoiodous acid

However, with hot, conc. solution of NaOH, the reaction is similar to that of Cl₂ or Br₂.

Iodine does not displace chlorine and bromine from chlorides and bromides respectively, but it displaces them from their oxy salts

2KClO₃ + I₂ ——> 2 KIO₃ + Cl₂

2KBrO₃ + I₂ ——> 2 KIO₃ + Br₂

With Na₂S₂O₃, iodine solution is decolorized due to the formation of colorless iodide and tetrathionate ions.

2Na₂S₂O₃ + I₂ —> 2NI + Na₂S₄O₆

With ammonia it reacts as follows

2NH₃ + 3I₂ ——> NI₃.NH₃ + 3HI

With strong oxidizing agents such as HNO₃,O₃ and Cl₂ , iodine gives iodic acid (HIO₃)

I₂ + 10 HNO₃ ——> 2 HIO₃ + 10 NO₂ + 4 H₂O

I₂ + H₃O + O₃ ——> 2 HIO₃ + 5 O₂

I₂ + 5 Cl₂ + 6H₂O ——> 2 HIO₃ + 10 HCl