Question
Explain the meaning of inner-transition series.

Answer

A series of $f$-block elements having electronic configuration $(n-2) f^{1-14}(n-I) d^{0-1} n s^2$ placed separately in the periodic table represents inner transition series. The $f$-orbitals lie much inside the e/ orbitals.
Since the last electron enters pre-penultimate shell, these elements are inner transition elements.
There are two inner transition series as follows :
$4 f$-series ${ }_{58} Ce \rightarrow{ }_{71} Lu$
$5 f$-series $90 Th \rightarrow{ }_{103} Lr$

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