(a) Which of the following electronic configurations are not possible? Give reason.  (i) X - 2,8,4  (ii) Y - 3,8,2  (iii) Z - 2,8,9 (b) Write electronic configurations of the following elements and predict their valencies :  Fluorine - 9, Aluminium – 13, Argon – 18
 

Here both (ii) and (iii), electronic configurations are not correct.
Explaination -  As per the Bohr - Bury's scheme of filling electrons in shells, the first shell should have only 2 electrons in the innermost shell. The first shell can not accomadate more than 2 electrons. In the option (ii), the the innermost shell has more than 2 electrons. Thus, its incorrect.
                         Now, coming to the next option (iii), the outermost shell can accomodate 8 electrons, not more than 8. In option (iii), the outermost shell has 9 electrons which is not possible. The outermost shell, as per the Bohr - Bury's rule, should be 8 only, not more than that. Therefore, the configuration in Option (iii) is incorrect.