The first ionisation potential of `Na,Mg,Al` and `Si` are in the order

Options

A. `Na gt Mg lt A1 lt Si`
B. `Na gt Mg gt A1 Si`
C. `Na gt Mg lt A1 lt Si`
D. `Na gt Mg gt A1 lt Si`

Answer

A

Explanation

As we move from left to right across a period first ionization enthalpy `(Delta_(i) H_(1) )` generally increases.But `Delta_(i) H_(1)` of A1 is lower that of Mg because in case of Mg , an electron is to be removed from fully filled more stable 3s- orbital while in case of A1, 3p-electron has to be removed . Since removal of a s- electron requires more energy that p- electron of the same shell therefore `Delta_(i)H_(1)` of is lower than that of Mg In other words `Delta_(i)H_(1)` of these elements follows the order `: Na lt Mg gt A1 lt Si` i.e., option (a) is correct .