Why the configuration is like this ?!..  First s should be filled and then d, and the electrons are shifted to d only when d is half-filled, then why these elements have such configuration. In Pd, s is empty.  Kindly explain for both kinds.


Dear Student

The Aufbau's principle gives the electronic configuration correct upto 20 atomic number. After that some elements meet exception due to different reasons. 
Like , the sixth group elements have f-orbital to be filled which show poor shielding effect and thus elements like W show different configuration than it's group configuration.
Similarly, here Tc, Ru , Rh and Pd belong to group 5th accounting the d-orbital also, which show spin-spin coupling (i.e magnetic effect of spin of electrons on energy or stability) along with orbital coupling. Due to which electronic configuration differ.

Hope it helps!

Regards

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