Quantum number refers to the set of four special numbers which provide the entire information about electrons of a particular atom.

Thus, the letter ‘n’ represents the principle quantum number and the Azimuthal quantum number ‘l’ can have any value between to n-1.

For n = 1 [ K shell] has L = 0 [ one subshell]

For n = 2 [L shell] has L = 0, 1 [2 subshell]

For n = 3 [M shell] has L = 0, 1, 2 [3 subshell]

For n = 4 [N shell] has L = 0, 1, 2, 3, [4 subshell]

For n = 5 [O shell] has L = 0, 1, 2, 3, 4, [5 subshell]

∴ For l = 4, minimum value of n = 5