K shell => n = 1 => number of electrons (max) = 2 n² = 2
L shell => n = 2 => number of electrons max = 2 n² = 8
M shell . n= 3. max num of electrons = 18
But given, electrons in orbit n = 3 are 2 + 8 = 10
The total num of electrons in the atom = 2 + 8 + 10 = 20 = Z
The valency shell is M shell, as it is not completely filled and the two lower orbits are completely filled.
Valency shell is M shell => num of electrons = 10.
The chemical element with Z = 20, is Calcium.