How to fill electrons in
orbitals and subshells?
Three rules to be considered
for writing electronic configuration.
Aufbau means building up and it is not a name of any scientist.
Electrons to be filled from lower energy
shell(subshell-orbital) to higher energy shell(subshell-orbital)
Before moving further you must know the following:
Shell or energy level has subshell/s.
Subshell has orbitals.
1st shell has only one subshell and only
Subshell name of 1st shell is s subshell and it has only one
orbital. It is 1s
2nd shell has two subshells which are s and p.
s subshell has only one orbital while p subshell has
three orbitals.2s and 2p
3rd shell has three subshells which are s,p
and d.d subshell has five orbitals. 3s,3p and 3d
4th shell has four subshells which are s,p,d
and f. f subshell has seven orbitals.4s,4p,4d and 4f
5th shell also has four subshells which are
s,p,d and f.f subshell has seven orbitals.5s,5p,5d and 5f
To learn in a easy way the sequence of orbitals is as
Electrons will start to fill from 1s and go as the below sequence.ssp sps dps dps fdps fdps
2p6 3s2 3p6 4s2 3d10 4p6
5s2 4d10 5p6 6s2 4f14 5d10
6p6 7s2 5f14 6d107p68s2
H = 1= 1s1
He =2= 1s2
Li =3= 1s2 2s1
Be =4= 1s2 2s2
B=5= 1s2 2s2
Na =11= 1s2 2s2
K =19= 1s2 2s2
2p6 3s23p6 4s1
Cr=24= 1s2 2s2
2p6 3s2 3p6 4s1 3d5
Cu =29=1s2 2s2
2p6 3s2 3p6 4s1 3d10
Zn= 30=1s2 2s2
2p6 3s2 3p6 4s2 3d10
Why is 3d not
filled before 4s?
3d has lower energy than 4s so it is filled later than 4s.
Why is 4s2 changed to 4s1 and 3d4
Because 3d5 is more stable than 3d4
due to half filled orbitals.