Relativistic stabilization of the ns orbitals, the destabilization of the (n-1)d orbitals and their SO splitting for the group 5 elements.
see above
2006
<cite class="book" style="font-style:normal">"Transactinides and the future elements" in   (2006) <i> The Chemistry of the Actinide and Transactinide Elements</i> (3rd ed.), Dordrecht, The Netherlands:  <a href="//commons.wikimedia.org/w/index.php?title=Springer_Science%2BBusiness_Media&amp;action=edit&amp;redlink=1" class="new" title="Springer Science+Business Media (page does not exist)">Springer Science+Business Media</a>  <small><a href="https://en.wikipedia.org/wiki/International_Standard_Book_Number" class="extiw" title="en:International Standard Book Number">ISBN</a>:  <a href="//commons.wikimedia.org/wiki/Special:BookSources/1-4020-3555-1" title="Special:BookSources/1-4020-3555-1">1-4020-3555-1</a>. </small></cite>