The increase in the atomic radii of the third $(5d)$ series of transition elements is very small,which may be accounted for by the filling of '$X$' orbitals before '$Y$' orbitals. $X$ and $Y$ are:

Cerium $(IV)$ has a noble gas configuration. Which of the following is the correct statement about it?

Given below are two statements:
Statement-$I$: The first ionization enthalpy of $Cr$ is lower than that of $Mn$.
Statement-$II$: The second and third ionization enthalpies of $Cr$ are higher than those of $Mn$.
In the light of the above statements,choose the correct answer from the options given below:

Most transition metals exhibit several characteristic properties. Which of the following statements are correct?
$I$: Form sets of compounds which display different oxidation states of the metal.
$II$: Form coloured ions in solution.
$III$: Burn vigorously in oxygen.
$IV$: Act as catalysts.

How many of the following metal ions have similar values of spin-only magnetic moment in the gaseous state?
(Given: Atomic number: $V, 23; Cr, 24; Fe, 26; Ni, 28$)
$V^{3+}, Cr^{3+}, Fe^{2+}, Ni^{3+}$

Although $Zr$ belongs to the $4d$ transition series and $Hf$ belongs to the $5d$ transition series,it is quite difficult to separate them. Why?