Why in auf bau Principle 4s filled first before 3d ?

1.	Why in auf bau Principle 4s filled first before 3d ?
Answer» Because of the n+l rule in this we have to check that which orbit have a greater n+l value where n is the principal quantum no..and l is the azimuthal quantum no....and it value is ...s=0,p=1,d=2,f=3.so 4+0=4 and 3+2=5 ....hence 3d>4s<br>Because of the n+l rule where n=the orbit and l=azimuthal quantum no....So. In 4s, n=4 and l=s suborbit =0.:. Using n+l rule, 4+0=4Now in 3d, n=3 and l=d suborbit =2.:. Using n+l rule, 3+2 =5So, point to be noted is 1)s=0, p=1,d=2,and f=32)The the subshell with less energy will be filled first. Since, 4s has less energy i.e. 4,than 3d,i.e. 5, 4s is filled before 3d.....

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