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At the end of electronic configuration, we were taught that, electron orbitals are most stable when they are either fully filled or half filled. E.g., the final valence configuration of chromium is $\ce{(4s)^1 (3d)^5}$ and not $\ce{(4s)^2 (3d)^4}$. But the final electronic configuration of chlorine is $\ce{(3s)^2 (3p)^5}$ and not $\ce{(3s)^1 (3p)^6}$.

Why doesn't chlorine follow the half filled or full filled theory?

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    $\begingroup$ +1. I have no idea why you got -2, but a new user like you should be made to feel welcome here. $\endgroup$ – user1271772 Feb 22 at 15:03
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Having a half-filled or filled subshell is stabilising, as you say. But for Cl, the difference in energy between 3s and 3p is greater than the additional stabilisation from filling the 3p subshell. 3d and 4s are closer in energy, so for Cr it is favourable to promote a 4s electron and have a 4s1 configuration.

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  • $\begingroup$ Great answer @atbm! I would like to invite you to participate as one of the first group of users in the Materials Modeling Stack Exchange: area51.stackexchange.com/proposals/122958/… We will soon be in the private beta phase, where only a select group of users can participate. These users help to determine the style of questions for the future. Can you help us by committing? $\endgroup$ – user1271772 Feb 22 at 15:16
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In the case of 3$d$ and 4$s$ energies, there is an "anomaly" in the ordering, being the 3$d$ orbital energy higher than that of 4$s$. However, you are comparing the configuration of chromium (with 3$d$ and 4$s$ orbitals) with chlorine. The valence shell of the latter contains 3$s$ and 3$p$ orbitals, and the energy of 3$s$ orbitals are lower than that of 3$p$.

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  • $\begingroup$ I have no idea why this got a -1. I've give you a +1 to even that out. I would like to invite you to participate as one of the first group of users in the Materials Modeling Stack Exchange: area51.stackexchange.com/proposals/122958/…. We will soon be in the private beta phase, where only a select group of users can participate. These users help to determine the style of questions for the future. Can you help us by committing? $\endgroup$ – user1271772 Feb 22 at 15:04

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