Aufbau principle and its deviations

An easy guide to one of the most important principles taught in secondary chemistry, The Aufbau Principle. Helpful for students who consider such an amazing topic to be a nightmare. Happy Chemistry!

1. AUFBAU PRINCIPLE AND
ITS DEVIATIONS

2. WHAT IS AUFBAU PRINCIPLE?
Aufbauis a Germanword that means
“Building Up.”
Aufbau Principle states that when electrons
organize themselves in different energy shells
or orbits, they occupy the lower energy shells
at first and gradually make their way to the
higher energy levels.

3. SEQUENCE OF ENERGY SHELLS
GRADUALLY OCCUPIED BY ELECTRONS
FOLLOWING AUFBAU PRINCIPLE
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d
6s 6p
7s

4. 1s
2s
2p 3s
3p
4s 3d
4p
5s 4d
5p
6s 4f
5d
6p 7s

5. SOME EXAMPLES
•H(1)=1s
1
•He(2)=1s
2
•Li(3)=1s
2
2s
1
•Na(11)=1s
2
2s
2
2p
6
3s
1
•Ar(18)=1s
2
2s
2
2p
6
3s
2
3p
6

6. DEVIATIONS OF AUFBAU PRINCIPLE
Some elements do not follow the general rule of
Aufbau Principle because for such elements
the stability is determined by the presence of
either a half filled or a fully filled orbital.
Hence such elements gain more stability in the
presence of np
3
,np
6
,(n-1)d
5
,(n-1)d
10
,(n-1)f
7
,(n-
1)f
14
in their electronic configuration.

7. ELEMENTS THAT DEVIATE FROM
AUFBAU PRINCIPLE
Cr(24)=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
4
(WRONG)
=1s
2
2s
2
2p
6
3s
2
3p
6
4s
1
3d
5
(CORRECT)
Cu(29)=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
9
(WRONG)
=1s
2
2s
2
2p
6
3s
2
3p
6
4s
1
3d
10
(CORRECT)
Mo(42)=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
10
4p
6
5s
2
4d
4
(WRONG)
=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
10
4p
6
5s
1
4d
5
(CORRECT)
Ag(47)=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
10
4p
6
5s
2
4d
9
(WRONG)
=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
10
4p
6
5s
1
4d
10
(CORRECT)
Au(79)= 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
9
(WRONG)
=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
10
4p
6
5s
2
4d
10
5p
6
6s
1
4f
14
5d
10
(CORRECT)

8. IMPORTANT THINGS TO REMEMBER
WHILE WRITING AUFBAU PRINCIPLE
•1.NEVER PUT SPACES BETWEEN THE DENOTIONS
OF THE ORBITS AND ORBITALS. IT SHOULD NEVER
BE WRITTEN LIKE 1s 2s 2p or 1s,2s,2p. IT SHOULD
ALWAYSBE WRITTEN LIKE 1s2s2p.
•2.AFTER WRITING THE SEQUENCE OF ENERGY
LEVELS ACCORDING TO AUFBAU PRINCIPLE, ONE
HAS TO ARRANGE THE ENERGY LEVELS AGAIN
ACCORDING TO THE ASCENDING ORDER OF
SHELLS IRRESPECTIVE OF THE ENERGY. EXAMPLE:
Va(23)=1s
2
2s
2
2p
6
3s
2
3p
6
4s
2
3d
3
=1s
2
2s
2
2p
6
3s
2
3p
6
3d
3
4s
2