Electronic configuration
How to fill electrons in
orbitals and subshells?
Three rules to be considered
for writing electronic configuration.
Aufbau rule:
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.
1^{st} shell has only one subshell and only
one orbital.
Subshell name of 1^{st} shell is s subshell and it has only one
orbital. It is 1s
2^{nd} shell has two subshells which are s and p.
s subshell has only one orbital while p subshell has
three orbitals.2s and 2p
3^{rd} shell has three subshells which are s,p
and d.d subshell has five orbitals. 3s,3p and 3d
4^{th} shell has four subshells which are s,p,d
and f. f subshell has seven orbitals.4s,4p,4d and 4f
5^{th} shell also has four subshells which are
s,p,d and f.f subshell has seven orbitals.5s,5p,5d and 5f
Shell |
Subshell |
Number |
Max Number |
1^{st} |
1s |
1 |
2 |
2^{nd} |
2s |
1+3 |
2+6=8 |
3^{rd} |
3s,3p |
1+3+5 |
2+6+10=18 |
4^{th} |
4s,4p,4d |
1+3+5+7 |
2+6+10+14=32 |
5^{th} |
5s,5p,5d |
1+3+5+7 |
2+6+10+14=32 |
6^{th} |
6s,6p |
1+3+5 |
2+6+10=18 |
7^{th} |
7s |
1+3 |
2+6=8 |
8^{th} |
8s |
1 |
2 |
To learn in a easy way the sequence of orbitals is as
follows:
Electrons will start to fill from 1s and go as the below sequence.ssp sps dps dps fdps fdps
1s^{2} 2s^{2}
2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10} 4p^{6}
5s^{2} 4d^{10} 5p^{6} 6s^{2} 4f^{14} 5d^{10}
6p^{6} 7s^{2} 5f^{14} 6d^{10}7p^{6}8s^{2}
For example
H = 1= 1s^{1 }
He =2= 1s^{2}
Li =3= 1s^{2} 2s^{1}
Be =4= 1s^{2} 2s^{2}
B=5= 1s^{2} 2s^{2}
2p^{1}
Na =11= 1s^{2} 2s^{2}
2p^{6} 3s^{1}
K =19= 1s^{2} 2s^{2}
2p^{6} 3s^{2}3p^{6} 4s^{1}
Cr=24= 1s^{2} 2s^{2}
2p^{6} 3s^{2} 3p^{6} 4s^{1} 3d^{5}
Cu =29=1s^{2} 2s^{2}
2p^{6} 3s^{2} 3p^{6} 4s^{1} 3d^{10}
Zn= 30=1s^{2} 2s^{2}
2p^{6} 3s^{2} 3p^{6} 4s^{2} 3d^{10}
Why is 3d not
filled before 4s?
Because
3d has lower energy than 4s so it is filled later than 4s.
Why is 4s^{2} changed to 4s^{1 }and 3d^{4}
to 3d^{5}?
Because 3d^{5} is more stable than 3d^{4}
due to half filled orbitals.