Group 17 VIIA The Halogens

All neutral halogens are diatomic and readily reduced to halide ions. All combine with hydrogen to form gases which, except for HF, are strong acids in aqueous solution. Some physical properties of the halogens are summarized in Table 8-14. 

Element Ionization Energy (kJ mol ) Electron Affinity (kJ mol ) Electro- negativity Melting Point rc) Boiling Point rc) x—x Distance (pm) XHof Dissociation (kJ mol ) 

Source See Table 8-3. Ionization energy for At is from J. Emsley, The Elements, Oxford University Press, New York, 1989, p. 23. 

One of the most striking properties of F2 is its remarkably low bond dissociation enthalpy, an extremely important factor in the high reactivity of this molecule. Extrapolation from the bond dissociation enthalpies of the other halogens would yield a value of approximately 290 kJ mol , nearly double the actual value. Several suggestions 

Of the hydrohalic acids, HF is by far the weakest in aqueous solution (pKa = 3.2 at 25° C) HCl, HBr, and HI are all strong acids. Although HF reacts with water, strong hydrogen bonding occurs between F and the hydronium ion (F — H — OH2) to form the ion pair H30 F, reducing the activity coefficient of H30. As the concentration of HF increases, however, its tendency to form H3O increases as a result of further reaction of this ion pair with HF  

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