The Hunds rule develop. When scientists hunds working on electron distribution in orbitals. He finds that each orbital has a different energy level occupied by different no. of electrons on the different shells. Where he observed that each subgroup of orbital 1st occupied a single electron on every subgroup of the orbital. Then it’s occupying double. ↑↓↑↓↑↓↑   1s2s2px2py2pzX(Wrong) ↑↓↑↓↑↑↑  1s2s2px2py2pz√(Right)