The electron configuration for chromium is NOT #1s^2 2s^2 2p^6 3s^2 3p^6 3d^4 4s^2#, but #color(blue)(1s^2 2s^2 2p^6 3s^2 3p^6 3d^5 4s^1)#.

Interestingly enough, Tungsten is much more stable v an electron setup of #4f^14 5d^4 6s^2#.

Unfortunately, over there is no easy way to describe these deviations in the ideal order because that each element.

To describe Chromium"s electron configuration, we might introduce:

The exchange energy #Pi_e# (a stability quantum mechanical element that is directly proportional to the variety of pairs of electron in the exact same subshell or an extremely close-energy subshells through parallel spins)The coulombic repulsion energy #Pi_c# (a destabilizing variable that is inversely proportional come the number of electron pairs)These incorporate to create an in its entirety pairing energy #Pi = Pi_c + Pi_e#.

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The previous is stabilizing and the latter is destabilizing, together shown listed below (suppose configuration 2 is at pairing power #Pi = 0#):

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One explanation for Chromium, then, is that:

The maximized exchange energy #Pi_e# stabilizes this configuration (#3d^5 4s^1#). The maximization comes from how there space #5# unpaired electrons, rather of just #4# (#3d^4 4s^2#).The minimized coulombic repulsion energy #Pi_c# more stabilizes this configuration. The minimization comes from having every unpaired electron in the #3d# and also #4s# (#3d^5 4s^1#), rather than one electron pair in the #4s# (#3d^4 4s^2#).The small-enough orbital size way that the electron thickness is not as spread out as it could be, which makes it favorable enough for a maximum total spin to give the many stable configuration.

However, Tungsten"s #5d# and #6s# orbitals being bigger than the #3d# and #4s# orbitals (respectively) spreads the end the electron density enough the the pairing power (#Pi = Pi_c + Pi_e#) is small enough.

The more the electron circulation is spread out out, the less electron-pair repulsion over there is, and thus the lower #Pi_c# is. Therefore, the lower #Pi# is.

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Thus, electron pairing is favorable enough for Tungsten.

There is no hard and also fast preeminence for this, however that is an explanation the correlates with speculative data.