The relative energy of sublevels is decided by (n+l) value of a given subshell . Lower the n+l value , lower is the energy. Lower energy subshells are filled first .
Second shell has two subshells ( s and p ) also termed as 2s and 2p subshell
for 2s subshell, n + l value is 2+ 0 = 2 ( l=0 for s subshell)
and for 2p subshell, n +l value is 2+1 = 3 ( l=1 for p subshell)
2s subshell will be filled first and 2p subshell will be filled, once 2s subshell is filled.
Third shell has three subshells ( s ,p and d ) also termed as 3s , 3p and 3d subshell
for 3s subshell, n +l value is 3+ 0 = 3 ( l=0 for s subshell)
for 3p subshell, n +l value is 3+1 = 4 ( l=1 for p subshell)
for 3d subshell, n +l value is 3+2 = 5 ( l=2 for d subshell)
3s subshell will be filled first and 3p subshell will be filled, once 3s subshell is filled and then 3d subshell.
If two subshells have same n +l value, the one with lower n , value will be filled first.
for 2p subshell, n +l value is 2+1 = 3 , and 3s subshell , n +l , value is 3 + 0 = 3, since 2p subshell has lower n value , so it will be first filled.
