Question

# The atomic numbers of elements X, Y and Z are 19,21 and 25 respectively. The order of number of electrons present in the M shells of these elements $\begin{array}{*{20}{l}} {A{\text{ }}.{\text{ }}Z > X > Y} \\ {\;B{\text{ }}.{\text{ }}X > Y > Z} \\ {\;C{\text{ }}.{\text{ }}Z > Y > X} \\ {\;D{\text{ }}.{\text{ }}Y > Z > X} \end{array} \\ \\$

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Hint:
According to the Bohr's model in the atoms the electrons are moving around the nucleus in different shells. These shells are K, L, M, N etc.

The principle quantum numbers of the shells are, for K is 1, for L is 2, for M is 3, for N is 4 etc.
According to the principal quantum number the total electrons can be filled in a shell is 2n2.
Therefore, the total electrons in K shell is 2, in L shell is 8, in M shell is 18 and so on.
Therefore, the electron distribution of X, Y, Z with atomic number 19,21 and 25 respectively.
 K L M X (At. No=19) 2 8 9 Y (At. No.=21) 2 8 11 Z (At. No= 25) 2 8 15

From this electronic distribution the number of electrons of X, Y, Z elements in the M shell are 9, 11, 15 respectively.
The order of electrons in M shell is$\;z > y > x.$
S From this electronic distribution the number of electrons of X, Y, Z elements in the M shell are 9, 11, 15 respectively.
The order of electrons in M shell is $\;z > y > x.$

So, the correct option is, C.

Note:
Reactivity or tendency to form chemical bonds with other atoms is determined by the number of electrons in the outermost shell of a particular atom. The maximum number of electrons can be filled in the K, L, M shells are 2, 8, 18 respectively.