The first ionisation potential of Na ,Mg and Si are respectively 496,737 and 786 `Kjmol^-1` the ionisation potential of Al will be closer to

Options

A. 760 kJ `mol^(-1)`
B. `575 kJ mol^(-1)`
C. `801 kJ mol^(-1)`
D. `419 kJ mol^(-1)`

Answer

B

Explanation

In the third period., A1 comes after Mg and before Si . Since in Mg `(1s^(2) 2s^(2) 2p^(6) 3s^(2))` . The last electron is to be lost from a filled 3s- orbital while in `A1 (1s^(2) 2s^(2) 2p^(6) 3s^(2) 3p^(1))` it is to be lost from a 3p- orbital therefore `Delta_(i)H_(1)` of A1 is lower than that of Mg (737 kJ `mol^(-1)`) .Further since nuclear charge increases in going from Na (496 kJ `mol^(-1)`) but lower than that of Mg `(737 kJ mol^(-1))` . Thus option ( b) is correct.