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 MX2 (X=halogen).
f. the metal which can form a predominantly stable covalent halide of the formula MX(X=halogen).

Explanation

( a) the element V has highest first ionization enthalpy `(Delta_(i) H_(1))` and positive electron gain enthalpy `(Delta_(eg) H)` and hence it is the least reactive element . Since inert gases have positive `Delta_(eg)H,` therefore the element - V must be an inert gas. The values of `Delta_(i) H_(1) , Delta_(i) H_(2)` and `Delta_(eg) H` match that of He.
( b) The element II which has the least first ionization `(Delta_(i) H_(1))` and a low negative electron gain enthalpy `(Delta_(eg) H)` is the most reactive metal . The values of `Delta_(i) H_(1) , Delta_(i) H_(2)` and `Delta_(eg)H` match that of K ( potassium )
( c) the element III which has high first ionization enthalpy `(Delta_(i) H)` but not so high first ionization enthalpy `(Delta_(eg) H)` Therefore it is the least reactive non- metal . the values of `Delta_(i) H_(1) Delta_(i) H_(2)` and `Delta_(eg) H ` match that of I (Iodine )
( e) the element I has low first ionization `Delta_(i) H_(1)` but a very high second ionization enthalpy `(Delta_(i) H_(2))` therefore it must be an alkali metal .Since the metal forms a predominarly stable covalent halide of the formula MX ( X= halogen ) therefore the alkali metal must b e least reactive i.e., Li The values of `Delta_(i)H_(1), Delta_(i) H_(2)` and `Delta_(eg)` H match that of Li (lithium)