Question

Three elements $‘X^{\prime }$, $‘Y’$ and $‘Z’$ have atomic numbers $18$, $19$ and $20$ respectively. How many electrons are present in the $M$ shells of these elements?

• A. $8$, $9$, $10$
• B. $8$, $10$, $13$
• C. $8$, $8$, $8$
• D. $8$, $9$, $12$

Answer 1

Answer: (C)

$M$ shell means it is third shell
$\Rightarrow n=3$
For element X: atomic number = 18
thus its electronic configuration:
$1s^{2}2s^{2}2p^{6}3s^{2}3p^6$
Electrons in M shell $=2+6=8$
For element Y: atomic number = 19
thus its electronic configuration:
$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$
Electrons in M shell $=2+6=8$
For element Z: atomic number = 20
thus its electronic configuration:
$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$
Electrons in M shell $=2+6=8$
thus in element $X,Y,Z$ the number of electrons in $M$ shell is $8,8,8$ respectively.