(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

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Y, as the first shell can never hold up 3 elections. To fill their duplets they need jst 2 electrons and as soon the atom will get 2 electrons in its first or (K) shell the another electron will jump into next shell or (L) shell. 😊
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(a) 2.
(b) flourine- 2,7 -1
aluminium- 2,8,3 - 3
argon - 2,8,8 - 0
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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.
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