Question

Number of electrons in the M shell of an element with atomic number 28 is ______.
A. 18
B. 16
C. 8
D. 2

Answer 1

The electrons of an atom are arranged in shells (K, L, M, N), which surround the nucleus, and with each successive shell, the electrons being farther from the nucleus. The electron shells consist of one or more subshells (s, p, d, f).

Complete step by step answer:
We know that the shells are further divided into subshells. According to the following theoretical constraints electron shells of an atom are filled as:
-Each s subshell can hold a maximum 2 electrons.
-Each p subshell can hold a maximum 6 electrons.
-Each d subshell can hold a maximum 10 electrons.
-Each f subshell can hold a maximum 14 electrons.
-Each g subshell can hold a maximum 18 electrons.
In the K shell, we have only 1 subshell that is s, so the maximum number of electrons we can accommodate in the K shell is 2.
In L shell, we have 2 subshells that includes s and p, so the maximum number of electrons in L shell will be 8 i.e. 2 electrons in s and 6 electrons in p; $2 + 6 = 8$
Now, in M shell we have 3 subshells that includes s, p and d, so the maximum number of electrons in M shell will be 18 i.e. 2 electrons in s, 6 electrons in p and 10 electron in d; $2 + 6 + 10 = 18$.
In N shell, we have s, p, d and f subshells, so the maximum number of electrons in N shell will be 32 i.e. 2 electrons in s, 6 electrons in p, 10 electron in d and 14 electrons in f; $2 + 6 + 10 + 14 = 32$.
Atomic number 28 represents the electronic configuration K, L, M as 2, 8, 18. Hence, M shell will contain 18 electrons.
So, the correct answer is “Option A”.

Note:
During the filling of electrons, some rules need to be considered as the orbitals should be filled in the order of increasing $n + 1$. If two orbitals have the same value of $n + 1$, then they will be filled in order of their increasing n. Therefore, we follow this order for the filling of the orbitals:
${\rm{1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, and 7p}}$