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.
