Dichlorine, dibromine and diiodine

   Dichlorine is a pale green-yellow gas with a characteristic odour. Inhalation causes irritation of the respiratory system and liquid Cl₂ burns the skin. Reaction 1.1 can be used for small-scale synthesis, but, like F₂, Cl₂ may be purchased in cylinders for laboratory use.

       (1.1)

   Dibromine is a dark orange, volatile liquid (the only liquid non-metal at 298 K) but is often used as the aqueous solution ‘bromine water’. Skin contact with liquid Br₂ results in burns, and Br₂ vapour has an unpleasant smell and causes eye and respiratory irritation. At 298 K, I₂ forms dark purple crystals which sublime readily at 1 bar pressure into a purple vapour.   In the crystalline state, Cl₂, Br₂ or I₂ molecules are arranged in layers as represented in Figure 1.1. The molecules Cl₂ and Br₂ have intramolecular distances which are the same as in the vapour (compare these distances with r_cov, Table 16.1). Intermolecular distances for Cl₂ and Br₂ are also listed in Figure 1.1; the distances within a layer are shorter than 2r_v (Table 16.1), suggesting some degree of interaction between the X₂ molecules. The shortest intermolecular X…..X distance between layers is significantly longer. In solid I₂, the intramolecular I^_I bond distance is longer than in a gaseous molecule, and the lowering of the bond order (i.e. decrease in intramolecular bonding) is offset by a degree of intermolecular bonding within each layer (Figure 1.1).

Fig. 1.1 Part of the solid state structures of Cl₂, Br₂ and I₂ in which molecules are arranged in stacked layers, and relevant intramolecular and intermolecular distance data.

   The molecules Cl₂ and Br₂ have intramolecular distances which are the same as in the vapour (compare these distances with r_cov, Table 16.1). Intermolecular distances for Cl₂ and Br₂ are also listed in Figure 1.1; the distances within a layer are shorter than 2r_v  suggesting some degree of interaction between the X₂ molecules. The shortest intermolecular X…..X distance between layers is significantly longer. In solid I₂, the intramolecular I_I bond distance is longer than in a gaseous molecule, and the lowering of the bond order (i.e. decrease in intramolecular bonding) is offset by a degree of intermolecular bonding within each layer (Figure 1.1). It is significant that solid I₂ possesses a metallic lustre and exhibits appreciable electrical conductivity at higher temperatures; under very high pressure I₂ becomes a metallic conductor. 

     Chemical reactivity decreases steadily fromCl₂ to I₂, notably in reactions of the halogens with H₂, P₄, S₈ and most metals. The values of Eo inTable 16.1 indicate the decrease in oxidizing power along the series Cl₂ > Br₂ > I₂, and this trend is the basis of the methods of extraction of Br₂ and I₂ described in Section 16.2. Notable features of the chemistry of iodine which single it outamong the halogens are that it ismore easily:

• oxidized to high oxidation states;
• converted to stable salts containing I in the 1 oxidation state (e.g. Figure 16.4).