Explain why :
(i) The three electrons present in 2p sub-shell of nitrogen remain unpaired.
(ii) In potassium , the `19^(th)` electron enters 4s sub-shell instead of 3d sub-shell.
(iii) Chromium has configuration `3d^(5)4s^(1)` and not `3d^(4)4s^(2)`.