The first `(Delta_(i)H_(1))` and second `(Delta_(i)H_(2))` ionisation enthalpies `("in" kJ mol^(-1))` and the `(Delta_(eg)H^(ө))` electron gain enthalpy `("in" kJ mol^(-1))` of a few elements are given below:
`{:("Elements",(Delta_(i)H_(1)),(Delta_(i)H_(2)),Delta_(eg)H^(ө)),(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 `MX_2` (X=halogen).
f. the metal which can form a predominantly stable covalent halide of the formula MX(X=halogen).

Explanation

(a) Element V is likely to be the least reactive element. This is because it has the highest first ionization enthalpy `(DeltaiH1)` and a positive electron gain enthalpy `(DeltaegH)`.
(b) Element II is likely to be the most reactive metal as it has the lowest first ionization enthalpy `(DeltaiH1)` and a low negative electron gain enthalpy `(DeltaegH)`.
(c) Element III is likely to be the most reactive non–metal as it has a high first ionization enthalpy `(DeltaiH1)` and the highest negative electron gain enthalpy `(DeltaegH)`.
(d) Element V is likely to be the least reactive non–metal since it has a very high first ionization enthalpy `(DeltaiH2)` and a positive electron gain enthalpy `(DeltaegH)`.
(e) Element VI has a low negative electron gain enthalpy `(DeltaegH)`. Thus, it is a metal. Further, it has the lowest second ionization enthalpy `(DeltaiH2)`. Hence, it can form a stable binary halide of the formula `MX_(2)` (X=halogen).
(f) Element V has the highest first ionization energy and high second ionization energy. Therefore, it can form a predominantly stable covalent halide of the formula MX (X=halogen).