Use electron configurations to account for the stability of the lanthanide ions Ce⁴⁺ and Eu²⁺.

The Ce metal has electronic configuration as follows

[Xe] 4f¹5d¹6s²

After losing 4 electrons, it gains noble gas configuration,. So Ce ⁺⁴ is stable.

Eu has electronic configuration as follows

[ Xe ] 4 f ⁷6s²

[ Xe ] 4 f ⁷

Its outermost orbit contains 2 electrons so Eu²⁺ is stable. Its + 3 oxidation state is also stable.

Ce⁺²