Justify the position of f-block elements in the periodic table.

Explanation

(1) These are present in two series, 4f and 5f series. In these elements, the differentiating electron enters in the (n-2)f sub-level.
(2) All these elements have similar outer configuration, `(n-2)f^(1-14)(n-1)d^(0-1) ns^(2)`. So all the 4f series and 5f series elements show similar properties.
For the above reasons, these elements are grouped together as lanthanides and actinides respectively and placed separately below the main body of the periodic table. These two series of elements belong to III B group only, in the `6^(th) and 7^(th)` periods respectively.