The first 
and the second 
ionization enthalpies (in kJ mol-1) and the 
electron gain enthalpy (in kJ mol-1) of a few elements are given below:
Elements 
I 520 7300 -60
II 419 3051 -48
III 1681 3374 -328
IV 1008 1846 -295
V 2372 5251 +48
VI 738 1451 -40
Which of the above elements is likely to be:
(a) the least reactive element.
(b) the most reactive metal.
(c) the most reactive non-metal.
(d) the least reactive non-metal.
(e) the metal which can form a stable binary halide of the formula MX2(X =halogen).
(f) the metal which can form a predominantly stable covalent halide of the formula MX(X= halogen)?
Element V is likely to be the least reactive element. This is because it has the highest first ionisation enthalpy (ΔiH1) and a positive electron gain enthalpy (ΔegH).It is a noble gas
(b) Element II is likely to be the most reactive metal as it has the lowest first ionisation enthalpy (ΔiH1) and a low negative electron gain enthalpy (ΔegH).The with lowest first ionisation enthalpy is likely to be a reactive metal
(c) Element III is likely to be the most reactive non–metal as it has a high first ionisation enthalpy (ΔiH1) but less than the noble gas elements and the highest negative electron gain enthalpy (ΔegH).
(d) Element V is likely to be the least reactive non–metal since it has a very high first ionisation enthalpy (ΔiH2) and a positive electron gain enthalpy (ΔegH).
(e) Element VI has a low negative electron gain enthalpy (ΔegH). Thus, it is a metal. Further, it has the lowest second ionisation enthalpy (ΔiH2). Hence, it can form a stable binary halide of the formula MX2 (X=halogen).
(f) The element I have low first ionisation energy and high second ionisation energy. Therefore, it can form a predominantly stable covalent halide of the formula MX (X=halogen).
