Question

Three elements ‘X’, ‘Y’ and ‘Z’ have atomic numbers 18, 19 and 20 respectively. Find the number of electrons present in the M shell of these elements.
A. 8, 9, 10
B. 8, 10, 13
C. 8, 8, 8
D. 8, 9, 12

Answer 1

There are four shells in the electronic configuration which are K, L, M and N. So, M is the third shell. Make the electronic configuration of the elements ‘X’, ‘Y’ and ‘Z’ and then, get the number of electrons which came in the third shell.

Complete Step by Step Solution:
The electronic configuration can be defined as the distribution of electrons of an atom or molecule. There are four orbitals s, p, d and f which contains electrons. s – orbital can hold maximum of 2 electrons, p – orbital can hold maximum 6 electrons, d – orbital can hold maximum of 10 electrons and f- orbital can hold maximum 14 electrons. For example, the atomic number of neon is 10 so its electronic configuration will be $1{s^2}2{s^2}2{p^6}$ where, 1,2… are the shells.
Now, in the question it is asked for the number of electrons present in the M shell. There are four shells in the electronic configuration which are K, L, M and N. So, the M shell means the third shell. Therefore, from the question, it is given that the atomic numbers are –
X = 18
Y = 19
Z = 20
So, the electronic configuration of atom X is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}$
From the above we already know that M is the third shell, so, we have to count the number of electrons present in the $3s$ and $3p$ orbitals. So, the electrons present in the M shell for atom X are - $6 + 2 = 8$ electrons.
Similarly, the electronic configuration of atom Y is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^1}$ .
Total electrons $= 6 + 2 = 8$ electrons
the electronic configuration of atom Z is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}$
Total electrons $= 6 + 2 = 8$ electrons
So, the electrons present in the M shell of the X, Y and Z atoms are 8, 8, 8 respectively.

Hence, the correct option is (C).

Note: We can also find the number of electrons present in the M shell by directly making the electronic configuration for K, L, M and N shells. For example, the electronic configuration for atom X can be –
2, 8, 8 where 2 electrons are present in K shell, 8 electrons are present in L shell and other 8 electrons are present in M shell and N shell has 0 electrons present. So, from this we directly got our answer for the M shell has 8 electrons.