Question

Explain why transition elements with electronic configuration 3d⁴4s² and 3d⁹4s² do not exist.

Answer 1

1. d orbitals are degenerate orbitals and they acquire extra stability when half-filled (3d⁵) or completely filled (3d¹⁰). Hence 3d⁴ and 3d⁹ electronic configurations are less stable.
2. The energy difference between 3d and 4s subshells is very low, hence there arises a transfer of one electron from 4s orbital to 3d orbital.
   The electronic configuration changes as,
   3d⁴4s² → 3d⁵4s¹
   3d⁹4s² → 3d¹⁰4s¹
   Therefore transition elements, with electronic configurations 3d⁴4s² and 3d⁹4s² do not exist.