1) The electron configuration of oxygen is 1s2 2s2 2p4 because the maximum number of electrons that can be filled in s orbital is 2 and so the extra electron should be entered in 2p.
2) The correct electron configuration of nitrogen is
The reason is that the pairing of electrons does not take place until each degenerate orbital is filled with one electron each (Hund’s principle).
3) The correct electronic configuration of scandium is 1s2 2s2 2p6 3s2 3p6 4s2 because after completion of 3p orbital electron enters in 4s because the energy of 4s orbital is less than 3d (Aufbau principle).
4) The correct electronic configuration of chromium is 1s2 2s2 2p63s2 3p6 4s2 3d5 . Because atoms having half filled or completely filled orbitals are more stable. So by transferring one electron from 4s to 3d the atom gets extra stability.
