$Zr$ $(Z=40)$ and $Hf$ $(Z=72)$ have similar atomic and ionic radii because of :

Assertion $(A)$: While going from left to right in the periodic table,the atomic size decreases more rapidly for the $3d$-series compared to the $4f$-series of elements.
Reason $(R)$: $3d$-electrons experience lesser shielding than $4f$-electrons.

In the first transition series,the metallic bond strength increases from ..... to .....

Invar,an alloy of $Fe$ and $Ni$,is used in watches and meter scales. Its characteristic property is:

Which of the $3d$ series of the transition metals exhibits the largest number of oxidation states and why?

The number of valence electrons present in the metal among $Cr$,$Co$,$Fe$,and $Ni$ which has the lowest enthalpy of atomisation is