Categories

# Why does Enthalpy of Formation of alkali metal Fluorides Decrease down the Group and for Chloride ,Bromide and Iodide Increase Down the Group?

My Question arises from the following passage

The alkali metal halides, MX, (X=F,Cl,Br,I) are
all high melting, colourless crystalline solids.
They can be prepared by the reaction of the
appropriate oxide, hydroxide or carbonate with
aqueous hydrohalic acid (HX). All of these
halides have high negative enthalpies of
formation; the $$\Delta_fH^{\circ}$$ values for fluorides
become less negative as we go down the group,
whilst the reverse is true for $$\Delta_fH^{\circ}$$ for chlorides,
bromides and iodides. For a given metal
$$\Delta_fH^{\circ}$$ always becomes less negative from
fluoride to iodide.

Why Do these Happen ?

1. The $$\Delta_fH^{\circ}$$ values for fluorides become less negative as we go down the group
2. $$\Delta_fH^{\circ}$$ for chlorides, bromides and iodides increases down the group
3. For a given metal $$\Delta_fH^{\circ}$$ always becomes less negative from fluoride to iodide