Halides and oxidation numbers

The standard enthalpy change is (8X5 g), and it is equal to the sum of the clockwise steps in the cycle  

Now look at the terms on the right-hand side of Equation 6.26. AH (S, g) is unaffected by changes in X, so the formation of 8X5 will be most favoured when AH (X, g) is small and the bond energy term, fi(8—X), is large, because then AH (8X5, g) will be at its most negative. 

A thermodynamic breakdown of the standard enthalpy change for the formation of a gaseous, covalent halide, SX lg) from its elements. 

Fluorine (AgX2, s) is most negative when lAHf (X , g) is most negative, and when r(X ) is at its smallest. Both these conditions are fulfilled by fluorine. 

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