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Q.
Three elements $‘X'$, $‘ Y’$ and $‘Z ’$ have atomic numbers $18$, $19$ and $20$ respectively. How many electrons are present in the $M$ shells of these elements?
Structure of Atom
Solution:
$M$ shell means it is third shell
$\Rightarrow n =3$
For element X: atomic number = 18
thus its electronic configuration:
$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6}$
Electrons in M shell $=2+6=8$
For element Y: atomic number = 19
thus its electronic configuration:
$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{1}$
Electrons in M shell $=2+6=8$
For element Z: atomic number = 20
thus its electronic configuration:
$1 s^{2} 2 s^{2} 2 p^{6} 3 s^{2} 3 p^{6} 4 s^{2}$
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.