in case of exceptions like cu and chromium where outer configuration is 4s1 3d 10 and 4s1 3d 5 if we want to remove a electron then it will be removed from 4s subshell  or 3d subshell?? are there anymore exceptions like these ??/

Asked by Viswanath | 15th Aug, 2014, 11:20: AM

Expert Answer:


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  • The order of energy of different orbitals in an atom is given below:

           1s < 2s < 2p <3s < 3p < 4s < 3d < 4p <5s < 4d < 5p < 6s < 4f < 5d and so on.

  • Due to anomalous behaviour of Chromium it has electronic configuration 1s2 2s2 2p6 3s2 3p6 4s1 3d5
  • The first electron that will be removed is the single 4s electron, leaving the electron configuration [Ar]3d5 because less energy will be required as compared to 3d orbital.


Aufbau process is an accurate in determining the electron configuration of most elements. There are some notable exceptions that occur within the transition metals and heavier elements. The reason these exceptions occur is because some elements are more stable with less electrons in some subshells and more electrons within others.
A list of the exceptions to the Aufbau process can be found below.

Exceptions to Electron Configuration Trends

Period 4:

Period 5:

Chromium: Z:24 [Ar] 3d54s1

Niobium: Z:41 [Kr] 5s1 4d4

Copper: Z:27 [Ar] 3d104s1

Molybdenum: Z:42 [Kr] 5s1 4d5


Ruthenium: Z:44 [Kr] 5s1 4d7


Rhodium:  Z:45 [Kr] 5s1 4d8


Palladium: Z:46 [Kr] 4d10


Silver: Z:47 [Kr] 5s1 4d10



Period 6:

Period 7:

Lanthanum: Z:57 [Xe] 6s2 5d1

Actinium: Z:89 [Rn] 7s2 6d1

Cerium: Z:58 [Xe] 6s2 4f1 5d1

Thorium: Z:90 [Rn] 7s2 6d2

Gadolinium: Z:64 [Xe] 6s2 4f7 5d1

Protactium: Z:91 [Rn] 7s2 5f2 6d1

Platinum: Z:78 [Xe] 6s1 4f14 5d9

Uranium: Z:92 [Rn] 7s2 5f3 6d1

Gold: Z:79 [Xe] 6s1 4f14 5d10

Neptunium: Z:93 [Rn] 7s2 5f4 6d1


Curium: Z:96 [Rn] 7s2 5f2 6d1


Lawrencium: Z:103 [Rn] 7s2 5f14 7p1


Topperlearning Team.

Answered by Hanisha Vyas | 18th Aug, 2014, 12:30: PM