I have always been confused as to why elements of atomic numbers 20-28, i.e., Calcium to Nickel have 2 valence electrons in their outer shell. Starting from Potassium, why is one electron present in the valence shell, making the configuration 2,8,8,1 and why doesn't it become 2,8,9 ? Same for Calcium, why does another electron join the Valence shell making the configuration 2,8,8,2 ? For scandium, 2,8,9,2 ? And in Copper, 2,8,18,1 ? why does one electron go from being in the valence shell to going in the 3rd shell?

What makes Chromium's and Copper's electronic configuration so weird?

Why does this happen?

So, in general how is this weird behavior defined?



2 Answers 2


Chromium has atomic number 24 and an electronic configuration of 2 8 13 1 but according to the rule i mentioned it must be 2 8 12 2. The electron fills the 3rd shell's 3rd subshell to give the electronic configuration 2 8 12 2 however this electronic configuration is not so stable so, an electron jumps from the 4th shell's first subshell to the 3rd shells 3rd subshell. Because half filled subshells are more stable than those which are one less than half filled.

This is why it happens in simple terms: The 3rd shell's 3rd subshell is a large one. It has 5 orbitals or boxes (remember, each can accomodate maximum of two electrons only ).You fill all the boxes first with one electron and then only after that you pair them (this is a complicated topic again). For example: After Calcium 2 8 8 2 the 3rd orbital's 3rd subshell starts filling up. It has 5 boxes. The first box gets filled with an electron. You get scandium. The second electron is filled in the second box and you get titanium (remember pairing will not be done first).The third electron goes in the third box giving vanadium. And finally its time for the 4th electron to go in the 4th box for chromium leading to a problem. As now the 3rd subshell of the 3rd shell is one less than half filled (all boxes filled with one electron) the electron jumps from the 4th shells first subshell ( The fourth shell's first subshell has only one box and right now it has 2 electrons) to the third shell's 3rd subshell as its better to keep both the subshells half filled than to keep one hungry.The real reason lies in a concept called exchange energy and I suggest you read about it yourself.

Similarly in copper (Z=29) the expected atomic configuration would be 2 8 17 2 but again one electron jumps from the 4th shell's first subshell to the 3rd shell's 3rd subshell giving 2 8 18 1. In this case the 3rd shell's 3rd subshell gets fully filled with electrons and is the most happy (even more than the half filled one).


In accordance to Aufbau's principle, the subshells with the lower energy will be first filled with electrons. Assuming that you are a student and are not aware of subshells and orbitals yet( By looking at the way you wrote down the electronic configuration), I believe that you will learn about it in a higher grade. I will still try to explain in simple terms. The atom has shells. These shells are divided into subshells which are further divided into orbitals. The orbitals can be considered to be boxes in which only two electrons can fit. The subshell is a collection of these boxes and the shell is a collection of subshells. The first shell has only one subshell and two electrons can fit in it. The second shell has two subshells of different sizes and a total of 8 electrons can fit in it. The third shell has three subshells and 18 electrons (maximum) can fit in it and so on.

The electrons are filled in the lower energy subshells first. The ones closer to nucleus have less energy and electrons are filled in the first shell first then the second and then the third. However while filling the third (which has 3 subshells) we fill the first and the second just fine and in sequence (this sums up to 8 electrons in that shell). Now the third subshell in the third shell has a problem. It can accomodate 10 electrons in total but the problem is that it has much higher energy than the next shell's first subshell. So the electron rather prefers to enter the higher shell as in potassium leaving the third subshell unfilled and with only 8 electrons.Two electrons can be filled in the first subshell of the fourth shell. The second subshell of the fourth shell now has higher energy than the third subshell of the third shell.So the electron now prefers to enter the third shell once again as in scandium. The same pattern continues. '


  • $\begingroup$ Thanks a lot, my confusions have been solved to a great extent, and I completely understood what you wrote. Why do Chromium and Copper posses exceptional electronic confs? Also, why is this post getting so many downvotes? $\endgroup$
    – S.A
    Mar 6 at 13:44
  • $\begingroup$ I answered your question and i suggest you edit your question and add this question as well. As for the downvotes, the question (as well as the answer) may sound somewhat amateur. $\endgroup$ Mar 6 at 14:59

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