Statement-1: The $4f-$and $5f-$ inner transition series of elements are placed separately at the bottom of the periodic table.

Statement-2: (i) This prevents the undue expansion of the periodic table, i.e., maintains its structure. (ii) This preserve the principle of classification by keeping elements with similar properties in a single column.

The inner transition series elements, designated $4f$ and $5f$,are positioned independently at the bottom of the periodic table. It does so for two reasons: 

1) It preserves the periodic table's structure by preventing unwarranted expansion.
2) By grouping objects with comparable attributes into a single column, this maintains the classification concept.