The first ionization enthalpy of Na , Mg and Si , respectively, are: 496 , 737 and 786 kJ mo 1 - 1 . The first ionization enthalpy kJmol - 1 of Al is

Step-by-step solution

In the third period, Al comes after Mg and before Si . Since in Mg ( 1 s 2   2 s 2   2 p 6   3 s 2 ), the last electron is to be lost from a fully filled 3 s -orbital while in Al ( ls 2   2 s 2   2 p 6   3 s 2   3 p 1   ) it is to be lost from a 3 p -orbital, therefore, ∆ i H 1 of Al is lower than that of Mg ( 737   kJ   mol - 1 ). Further, nuclear charge increases in going from Na to Al , therefore ∆ i H 1 of Al is higher than that of Na ( 496   kJ