Halides

The alkali metal halides $MX$ ($X = F, Cl, Br, I$) are colourless, high-melting crystalline solids, typically made by reacting the corresponding oxide, hydroxide or carbonate with the appropriate hydrohalic acid $HX$. All have large negative enthalpies of formation, though the trend down the group differs by halide: for the fluorides, $\Delta_fH^{\ominus}$ becomes less negative on descending the group, whereas for chlorides, bromides and iodides the opposite happens — $\Delta_fH^{\ominus}$ becomes more negative down the group. Looking instead across a single metal, $\Delta_fH^{\ominus}$ always becomes less negative in passing from the fluoride to the iodide. …