Aufbau principle

Aufbau principle , subshellsource : slide

Aufbau (German word) means ‘building up’ according to Aufbau principle.

“The vacant subshell having lowest energy is filled first. When this subshell is filled  completely, then the filling of next sub-shell with higher energy starts.”

The energy level diagram is



Order of Filling of Subshell

According to Aufbau principle energy of various subshell  increases in the order.


1H        -1s1

2He       – 1s2

3Li        – 1s2, 2s1

4Be       -1s2, 2s2

5B        – 1s2, 2s2,2p1

6C        – 1s2,2s2,2p2

7N        – 1s2,2s2,2p3

8O        -1s2,2s2, 2p4

9F         -1s2,2s2,2p5

10Ne     -1s2, 2s2,2p6

11Na     -1s2,2s2,2p6,3s1

12Mg     -1s2, 2s2,2p6,3s2

13Al      -1s2,2s2,2p6,3s2,3p1

14Si      – 1s2,2s2,2p6,3s2,3p2

15P       – 1s2,2s2,2p6,3s2,3p3


16S       1s2,2s2,2p6,3s2,3p4

17Cl      1s2,2s2,2p6,3s2,3p5

18Ar      1s2,2s2,2p6,3s2,3p6

19K      1s2,2s2,2p6,3s2,3p6,4s1

20Ca     1s2,2s2,2p6,3s2,3p6,4s2

21Sc      1s2,2s2,2p6,3s2,3p6,4s2,3d1

22Ti      1s2,2s2,2p6,3s2,3p6,4s2, 3d2

23V      1s2,2s2,2p6,3s2,3p6,4s2, 3d3

24Cr      1s2,2s2,2p6,3s2,3p6,4s1, 3d5

25Mn     1s2,2s2,2p6,3s2,3p6,4s2, 3d5

26Fe      1s2,2s2,2p6,3s2,3p6,4s2, 3p6

27Co     1s2,2s2,2p6,3s2,3p6,4s2, 3d7

28Ni      1s2,2s2,2p6,3s2,3p6,4s2, 3d8

29Cu     1s2,2s2,2p6,3s2,3p6,4s1, 3d10

30Zn     1s2,2s2,2p6,3s2,3p6,4s2, 3d10

There are few  exceptions to Aufbau principle ,such as

24Cr      1s2,2s2,2p6,3s2,3p6,4s1,3d5

29Cu     1s2,2s2,2p6,3s2,3p6,4s1,3d10

41Nb     1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s1,4d4

42Mo     1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s1,4d5

44Ru     1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s1,4d7

45Rh     1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s1,4d8

46Pd     1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s0,4d10

47Ag     1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s1,4d10

78pt      1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s2,4d10,5p6,6s1,4f14,5d9

79Au     1s2,2s2,2p6,3s2,3p6,4s2,3d10,4p6,5s2,4d10,5p6,6s1,4f14,5d10

Give electronic configuration of  given ions according to Aufbau principle

11Na+, 12Mg++, 9F, 8O––, 29Cu++, 29Cu+, 24Cr+, 24Cr3+, 25Mn++, 22Ti++, 30Zn++

No of electrons in Na+=11-1=10


no.of electrons in Mg++=12-2=10


no. of electrons in F=9+1=10


no. of electrons in O– –=8+2=10

O– –=1s2,2s2,2p6

no. of electrons in Cu+=29-1=28

Note : When d-subshell is present then electrons are removed from outer most shell (n), then penultimate shell (n–1)d.

Cu       1s2,2s2,sp6,3s2,3p6,4s1,3d10

Cu+      1s2,2s2,2p6,3s2,3p6,4s0,3d10

Cu++    1s2,2s2,2p6,3s2,3p6,4s0,3d9

24Cr      1s2,2s2,2p6,3s2,3p6,4s1,3d5

Cr+       1s2,2s2,2p6,3s2,3p6,4s0,3d5

Cr3+     1s2,2s2,2p6,3s2,3p6,4s0,3d3

25Mn     1s2,2s2,2p6,3s2,3p6,4s2,3d5

Mn++1s2, 2s2,2p6,3s2,3p6,4s0,3d5

22Ti      1s2,2s2,2p6,3s2,3p6,4s2,3d2

Ti++      1s2,2s2,2p6,3s2,3p6,4s0,3d2

30Zn     1s2,2s2,2p6,3s2,3p6,4s2,3d10

Zn++     1s2,2s2,2p6,3s2,3p6,4s0,3d10