9. The lanthanides and the subsequently discovered actinides do not fit into the Mendel eef table and can only be fitted into the modern
table by expanding it sideways to an inconvenient degree. They are.
therefore, placed separately at the bottom of the table. These two
series of elements are now recognised as being inner transition elements,
when electrons enter a quantum level two units below that of
the outer. Many properties depend upon the outer electronic configurations
and hence we can correctly predict that the lanthanides
and actinides are two series of closely similar elements.