Assertion: The $5^{th}$ period of the periodic table contains 18 elements and not 32.

Reason: When n = 5, the value of l = 0, 1, 2, 3. The order in which the energy of available orbits 4d, 5s and 5p increases is 5s < 4d < 5p, and the total number of orbitals available is 9, thus 18 electrons can be accommodated.

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 preserve the principle of classification by keeping elements with similar properties in a single column.

Assertion : Manganese (atomic number 25 ) has a less favourable electron affinity than its neighbours on either side because.

Reason : The Manganese has stable, ${\left[Ar\right]}^{18}3d^{5}4s^2$ electrons configuration.