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By transferring one of the s electrons to a d orbital the d orbitals become completely filled. Not only is the d orbital filled all of the orbitals in the next lower shell are filled. This creates a great deal of stability.

In contrast a filled s sub shell does not provide the same level of stability. Also the energy level difference between the larger but simpler s orbital and the smaller but much more complex d orbital disappears as the number of protons in the nucleus increases drawing the d orbital electrons closer to the nucleus.

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