See explanation below.
Work Step by Step
Europium, a lanthanide element of atomic number 63.Its electronic configuration is:[Xe]6s2 4f7. The outermost 2s electrons are removed to form Eu2+ ion. So electronic configuration of Eu2+ is [Xe]4f7 Electronic configuration of Eu3+ is [Xe]4f6 Electronic configuration of Eu4+ is [Xe]4f5 Half filled f orbital in Eu2+ makes it more stable. b.Terbium, a lanthanide element of atomic number 65.Its electronic configuration is[Xe]6s2 4f9. The outermost 2s electrons are removed to form Tb2+ ion.Electronic configuration of Tb2+ is [Xe]4f9 Electronic configuration of Tb3+ is [Xe]4f8 Electronic configuration of Tb4+ is [Xe]4f7