Question

In the ground state electronic configuration of $Co$ , how many electrons are present in $'M'$ shell?
$a$ .$13$
$b$ .$2$
$c$ .$15$
$d$ .$3$

Answer 1

Cobalt is $d$ - block element its atomic number is $27$ and its symbol is $Co$ . The electronic configuration on cobalt in ground state is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^8}4{s^2}$ . In electronic configuration numbers $1,2,3$ and so on represent the shell and $s,p$ and $d$ represent the name of the sub shell.

Complete step by step answer:
Before solving the question we have to get some idea what shell is. So, the shell is the pathway followed by electrons around an atom’s nucleus. It can hold a maximum of 32 electrons. The electron shell is labeled $K,L,M,N,O,P$ and $Q$ or it can also labeled as $1,2,3,4,5,6,$ and $7$ .
Here is a table which tells us about the shell that how many number of electrons, number of sub-shell and name of sub-shell

Principle quantum shell	Maximum number of electrons	Number of sub-shell	Name of sub-shell
$K,n = 1$	$2$	$1$	$1s$
$L,n = 2$	$8$	$2$	$2s,2p$
$M,n = 3$	$18$	$3$	$3s,3p,3d$
$N,n = 4$	$32$	$4$	$4s,4p,4d,4f$

Now, cobalt has electronic configuration in ground state is $1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}3{d^8}4{s^2}$ .For $M$ shell,$n = 3$ it can hold maximum 18 electrons in its shell. From electronic configuration of $Co$ in ground state there are $15$ electrons present in $'M'$ shell or $n = 3$ .
Thus, the correct answer is 15.
Therefore, the correct option is (C).

Additional information: Cobalt is a hard, lustrous, brittle and hard ferromagnetic element. It is found in earth crust in combined form .It is used to make alloy and it is also used to make ceramic and glassware. Cobalt shows any properties like variable oxidation state due to their empty $d$ -orbital, magnetic property due to their unpaired electron is valence $d$ -orbital. Cobalt generally shows $+ 3$ oxidation state and $+ 2$ oxidation state.

Note: Electronic configuration should be done using Aufbau’s principle; it states that the electron first fills in the atomic orbital of the lowest energy than higher. The maximum number of electrons present in a shell can be calculated by using formula. Maximum number of electrons in a shell$=$ $2{n^2}$.