The Sequence of the filling of various sub-shells may also be determined from (n+l) rule
“The electron comes in the vacant sub-shell with lowest value of (n+l). If the value of (n+l) is same, then electron enters the sub-shell having lowest value of n.”
n=no. of shell
l= sub-shell s,p,d,f
values of l for s,p,d,f subshells is 0,1,2 & 3 respectively.
Question1. Which sub-shell is filled first 4f or 6s?
Ans. For 4f sub-shell, n=4, l=3
n+l=4+3=7
For 6s sub-shell n=6, l=0
n+l=6+0=6
The value of (n+l) is less for 6s, So 6s is filled first
Question2.Which sub-shell is filled first 4p or 5s?
Ans. for 4p sub-shell n=4, l=1
n+l= 4+1=5
for 5s sub-shell n=5, l=0
n+l= 5+0=5
Since the value of (n+l) is same for both subshells.