Chemistry - f-block Question with Solution | TestHub

The stability of an ion's oxidation state is often related to achieving a completely empty, half-filled, or completely filled f-subshell. Europium (Eu) can attain a stable half-filled 4f configuration by exhibiting a +2 oxidation state. Terbium (Tb) can attain a stable half-filled 4f configuration by exhibiting a +4 oxidation state.

Eu ($Z=63$) has the electronic configuration

$[Xe]4f^{7}6s^2$. Losing two electrons gives

$E{u}^{2+}$: $[Xe]4f^7$, which is a stable half-filled configuration. Tb ($Z=65$) has the electronic configuration

$[Xe]4f^{9}6s^2$. Losing four electrons gives

$T{b}^{4+}$: $[Xe]4f^7$, which is also a stable half-filled configuration.