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. In periodic table for example Ag has an electron configuration of [Kr] 4d10,5s1 but when you do it yourself using Aufbau principle it should be [Kr] 5s2 4d9. Why is that?

When ask for Ag + valence electron configuration answers may now vary because of the existence of two electron configurations from the periodic table and using aufbau principle...

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  • $\begingroup$ Because the Aufbau principle is not taken as gospel by the electrons busy trying to solve Schrodinger's equation. It is not a law of nature, it is more of a general guideline for chemistry students that has problems, certainly with d electrons. $\endgroup$ – Jon Custer May 6 '16 at 13:02
  • $\begingroup$ How come Sir that it serves as a general guideline if it doesn't match in what we see in the periodic table? Thank you. $\endgroup$ – Ginee M. May 6 '16 at 13:06
  • $\begingroup$ I would not expect a general guideline to precisely match every single element in the periodic table. You have already noted that it doesn't, and basically I am saying that you should not expect it to. It is not a physical law. Adding d electrons makes for some funny things, and you have noted one of them. There are others. $\endgroup$ – Jon Custer May 6 '16 at 13:49
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As a general rule, full electron shells are more stable than unfilled shells. In silver, promoting an electron from the 5s to the 4d shell completes the d orbital, which increases the stability of the 4d shell more than it destabilizes the 5s shell. This is also explained here:

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=21&t=2807

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