Why is 4s shell filled before 3d shell? Explain in the light of n+I rule

Why is 4s orbital filled before the filling of 3d orbital?

Na^(+) has completely filled K and L shells. Explain.

While filling up of electrons in the atomic orbitals, the 4s orbital is filled before the 3d orbital but reverse happens during the ionisation of the atom. Explain why?

How many sub-shell are there in N shell? How many orbitals are there in d sub-shell?

How many sub-shell are there in N shell ? How many orbitals are there in d sub-shell ?

The (n+l) value for 4f sub-shell is

Explain why (i) the three electrons present in 2p subshell nitrogen remain unpaired. (ii) in potassium, the 19th electron enters into 4s subshell instead of 3d subshell, (iii) chromium has configuration 3d^(5) 4s^(1) instead of 3d^(4), 4s^2, (iv) the electronic configuration of zinc can be represented as [Ar] 3d^(10) 4s^(2)?

Why 3d sub-shell has higher energy than 4s sub-shell in a muti-electron atom?

In (Fig. 3.73), if the inner shell has charge +q_0 and the outer shell is earthed, then . i. determine the charge on the outer shell, and ii. find the potential of the inner shell.

Among s, p, d and f subshells of the same shell, which has the highest ionisation energy ?