Group I metals combine directly with all the halogens. The reactions
are exothermic, the greatest heats of formation being found when
the elements combine with fluorine. Except for the formation of the
fluorides, the heat of formation of a given halide increases as the
group is descended and the ionisation energies of the metallic
elements fall. The reverse is true for the fluorides, and the heat of
formation falls as the group is descended. This is due to the high
lattice energies produced from the 'combination' of the small
fluoride anion and the metal cation (p. 74). (Similar variations are
also noted with other small anions, for example nitride, carbide.)