Question
Inner-transition elements have been kept separately below the main periodic table. Why?
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Answer
Inner-transition elements are the elements in which lanthanoids and actinoids are present. Lanthanoid series starts after $La _{51}$ and actinoid series starts after ${ }_{89} Ac$. La and Ac are the elements of third group. The element after Lanthanoids is ${ }_{72} Hf$ and element after actinoid is $104 U _{ nq }$. Hence in reality all lanthanoids and actinoids are $d$-block elements but if we put in periodic table that the structure of periodic table get disturbed. Therefore, to maintain its structure and to follow the principle of classification by placing elements with similar properties in the same class these elements have been kept separately at the bottom of the periodic table.
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