Now, in order to write electronic configuration, we need to apply (n+l) rule Energy order :1s<1p<2s<1d<2p<3s<2d..... Option1) 13:1s21p62s21d3 is not half filled Option 2 ) 9:1s21p62s1 is the first alkali metal because after losing one electron, it will achieve first noble gas configuration Option 3 ) 8:1s21p6 is the first noble gas because after 1p6e− willenter 2 s hence new period Option 4) 6:1s21p4 has 1p valencesubshell.