Most of the elements (except He, Ne and Ar) react with halogens to form halides. The order of reactivity of the halogens decreases as we descend from fluorine to iodine. The nature of bonding in halides depends on :

(a)    nature of the halogen atom, and
(b)    nature of the second element.

(a)    Nature of the halogen.

Many metals exhibit their highest oxidation state in the fluorides. They are generally ionic compounds. They are soluble in water. The corresponding chlorides are covalent in nature. The degree of covalence increases further in compounds of bromine and iodine (the same conclusions can be predicted by Fajan’s rules).

b)    Nature of the second element.

The choice of the second element plays an important role in determining the nature of bonding in halides.

(i)    Ionic lattices containing X ^- ions are formed by electropositive metals such as s-block elements. In case of ionic halides the solubility decreases in the order
                                      Iodide > bromide > chloride > fluoride

is governed by lattice energy which also increases in the same order (as the ionic radii of X ^- decreases).
 
(ii)    Covalent lattices or complex ions are readily formed by electropositive metals of p-block elements. Amongst the covalent halides, the order of solubility varies as :
                                       chloride > bromide > iodide

This may be explained in terms of increasing covalent character due to increased polarization as size of the X ^- ion increases.

 (iii) Molecular halides are formed by non-metallic elements including hydrogen. They are usually volatile. They do not volatilize if present in polymeric form. For example, Teflon (-CF₂-)n is non-volatile. Similarly, AICI₃ and SnF₄ which are molecular in the gas phase but due to the tendency of the metal atom to have a higher coordination member (4 and 6, respectively for AI and Sn in these examples), the solids acquire extended structures.

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