Halides of Group 1 elemants

     The MX halides are prepared by direct combination of the elements ) and all the halides have large negative Δ_fH^o values. However, Table 1.1 shows that for X = F, values of Δ_fH^o (MX) become less negative down the group, while the reverse trend is true for X= Cl, Br and I. For a given metal, Δ_fH^o(MX) always becomes less negative on going from MF to MI. These generalizations can be explained in terms of a Born–Haber cycle.

For MF, the variable quantities are ΔaH^o(M), IE₁(M) and Δ_latticeH^o(MF), and similarly for each of MCl, MBr and MI.  The sum of ΔaH^o(M) and IE1(M) gives for the formation of Li‏⁺ 681, of Na‏⁺ 604, of K⁺‏ 509, of Rb⁺‏ 485 and of Cs⁺‏ 454 kJ mol^_1.

     For the fluorides, the trend in the values of Δ_fH^o(MF) depends on the relative values of {ΔaH^o(M)‏ + IE₁(M)} and Δ_latticeH^o(MF) and similarly for chlorides, bromides and iodides. Inspection of the data shows that the variation in  {Δ_aH^o(M(‏ + IE1)M(} is less than the variation in Δ_latticeH^o(MF), but greater than the variation in Δ_latticeH^o(MX) for X = Cl, Br and I. This is because lattice energy is proportional to 1 / (r‏+ ‏ +  r-)  and so variation in Δ_latticeH^o(MX) for a given halide is greatest when r^_ is smallest (for F^-) and least when r^_ is largest (for I^-). Considering the halides of a given metal the small change in the term {1/2 D(X₂)  +‏ Δ_EAH)X(}  (^_249, ^_228, ^_213, ^_188 kJ mol^_1 for F, Cl, Br, I respectively) is outweighed by the decrease in Δ_latticeH^o(MX).

   In Table 1.1, note that the difference between the values of Δ_fH^o(MF) and Δ_fH^o(MI) decreases significantly as the size of the M‏ ion increases. The solubilities of the alkali metal halides in water are determined by a delicate balance between lattice energies and free energies of hydration. LiF has the highest lattice energy of the group 1 metal halides and is only sparingly soluble, but solubility relationships among the other halides.

  The salts LiCl, LiBr, LiI and NaI are soluble in some oxygen-containing organic solvents, e.g. LiCl dissolves in THF and MeOH; complexation of the Li‏⁺ or Na₊‏ ion by the O-donor solvents is likely in all cases. Both LiI and NaI are very soluble in liquid NH₃, forming complexes; the unstable complex [Na(NH₃)₄]I has been isolated and contains a tetrahedrally coordinated Na⁺‏ ion.  In the vapour state, alkali metal halides are present mainly as ion-pairs, but measurements of M^_X bond distances and electric dipole moments suggest that covalent contributions to the bonding, particularly in the lithium halides, are important.