Question

Assertion: The $4f -$ and $5f -$ inner transition series of elements are placed separately at the bottom of the periodic table.
Reason:
(i) This prevents the undue expansion of the periodic table i.e., maintains its structure.
(ii) This preserves the principle of classification by keeping elements with similar properties in a single column.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Assertion is incorrect but Reason is correct.

Answer 1

We must need to remember that the components comprising the f-block are those where $4f$ and $5f$ orbitals are continuously filled. These components are formal individuals from group 3 however are indicated independently as f-block of the periodic table. The components of f-block are likewise known as inner transition elements.

Complete step by step answer:
We have to know that the two series of inner transition elements that are $4f$ and $5f$ series are known as Lanthanoids and actionions separately. The inner transition elements, for example, U, Th and Pa are giving acceptable sources of atomic energy.
The major characteristics of Inner Transition elements:
In the inner transition elements the third last shell is loaded up with electrons.
The inner transition elements form colored ions.
The inner transition elements exhibit variable valencies.
Actinides are radioactive ordinarily.
The inner transition elements beyond atomic number $92$ up to $103$ are manufactured just as radioactive. They are not found in nature on the world's outside.
The last electron in the configuration is added to $\left( {n - 2} \right)f$ orbitals. Lanthanides and actinides are f-block components. All components are metals. All components of the actinide arrangement are radioactive. Components after $U\left( {92} \right)$ are manufactured. Be that as it may, to keep the evenness of the table logical the components are set at the lower part of the table.
$4f$ and $5f$ inner transition elements are put independently at the lower part of the periodic table. It is on the grounds that

1. It forestalls the unjustifiable extension of the periodic table for example keeps up its structure.
2. This maintains the principle of classification by keeping elements with equal properties in a single column.
   Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.

So, the correct answer is Option A.

Note: As we know that the transition elements are in the d-block, and in the d-orbital have valence electrons. They could form several states of oxidation and contain various ions. Inner transition elements are found in the f-block, and in the f-orbital contain outermost electrons. Lanthanides and Actinides are the elements.