$Ce - 58$ is a member of

The lanthanide contraction relates to

Which element from the following lanthanoids has a half-filled $f$-orbital in its observed and expected electronic configuration?

The $Eu^{2+}$ ion is a strong reducing agent in spite of its ground state electronic configuration (outermost) : [Atomic number of $Eu=63$]

Which of the following elements shows the maximum number of different oxidation states in its compounds?

Compare the chemistry of actinoids with that of the lanthanoids with special reference to:
$(i)$ electronic configuration
$(ii)$ atomic and ionic sizes
$(iii)$ oxidation state
$(iv)$ chemical reactivity.