Calcium has 20 electrons. These occupy K, L, M, and N shells. Which shell or shells are incomplete

Atomic number of Calcium is Z = 20.  Its ground state electronic configuration is shown below:

$\frac{1{\text{s}}^2}{\text{K}}\frac{2{\text{s}}^{2}2{\text{p}}^6}{\text{L}}\frac{3{\text{s}}^{2}3{\text{p}}^6}{\text{M}}\frac{4{\text{s}}^2}{\text{N}}$

Therefore, M and N shell are incompletely filled orbitals.

Note: M shell (n = 3) has 3 – subshells they are 3s, 3p and 3d.

N shell (n = 4) has total 4 – subshells they are 4s, 4p, 4d and 4f.