Group VII - the halogens

The electronic configuration of each halogen is one electron less than that of a noble gas, and it is not surprising therefore, that all the halogens can accept electrons to form X ions. Indeed, the reactions X(g) + e - X (g), are all exothermic and the values (see Table 11.1), though small relative to the ionisation energies, are all larger than the electron affinity of any other atom. 

Numerous ionic compounds with halogens are known but a noble gas configuration can also be achieved by the formation of a covalent bond, for example in halogen molecules, X2, and hydrogen halides, HX. When the fluorine atom acquires one additional electron the second quantum level is completed, and further gain of electrons is not energetically possible under normal circumstances, i.e 

One surprising physical property of fluorine is its electron affinity which, at — 333 kJmoPS is lower than that of chlorine, -364 kJmol , indicating that the reaction X(g) + - X (g) is more 

The very low bond dissociation enthalpy of fluorine is an important factor contributing to the greater reactivity of fluorine. (This low energy may be due to repulsion between non-bonding electrons on the two adjacent fluorine atoms.) The higher hydration and lattice enthalpies of the fluoride ion are due to the smaller size of this ion. 

ENTHALPY DATA FOR HALIDE ION FORMATION IN AOLEOL S SOLI DON 

FlfWIlt, 4l( ( number Oilier electrons Atomic ratlins Inml Radius ofioiiX (nm  

All the remaining halogens have unfilled d orbitals available and the covalency of the element can be expanded. Compounds and complex ions are formed both with other halogens and with oxygen in which the halogen can achieve a formal oxidation state as high as + 7. for example chlorine has formal oxidation states of +1 in the chlorate(I) anion CIO + 5 in the chlorate(V) anion CIO 3, and + 7 in the chlorate(VII) anion CIO4.  

One surprising physical property of fluorine is its electron affinity which, at — 333 kJmol is lower than that of chlorine, —364 kJmol-1, indicating that the reaction X(g) + e - X (g) is more exothermic for chlorine atoms. In view of the greater reactivity of fluorine a much higher electron affinity might reasonably have been expected. The explanation of this anomaly is found when the steps involved in a complete reaction are considered. For example, with a Group I metal ion M+(g) the steps to form a crystalline solid are, 

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GROUP VII THE HALOGENS 325 OTHER DISPLACEMENT AND OXIDATION REACTIONS... 

Eleetronegativities inerease aeross a period to a maximum with Group VII, the halogens, for whieh reaetivity deereases from top to bottom of the table. Elements in Group 0 are unreaetive... 

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