Question

The noble gas core electron configuration of Sc is [Ar]4s²3d¹ and that of Y, which lies immediately below it on the periodic table, is [Kr]5s²4d¹. Explain why a similar pattern is not observed for Fe (noble gas core electron configuration of [Ar]4s²3d⁶) and Ru (noble gas core electron configuration of [Kr] 5s¹ 4d⁷).

Answer 1

Filling of electrons in an orbital occurs via their position in energy diagram. So a lower energy orbital will be filled first before a higher energy orbital.

For Fe with initial 3d5 being a stable half filled configuration, 4s orbital gets filled. Once the 4s is completely filled its energy becomes greater than the 3d orbital and thus, 3d gets filled.

On the other hand, in case of Kr, as the electrons are filled in the 5s orbital its e-e repulsion raises the energy of the orbital and thus the further electrons are filled in the now lower energy 4d orbital.