What is the number of unpaired electrons present in $d$ orbitals of $Cr (Z=24)$ to have extra stability?

Identify the correct statements among the following:
$i. E_{2s}(H) > E_{2s}(Li) < E_{2s}(Na) > E_{2s}(K)$
$ii.$ The maximum number of electrons in the shell with principal quantum number $n$ is equal to $2n^2$.
$iii.$ Extra stability of half-filled subshell is due to smaller exchange energy.
$iv.$ Only two electrons,irrespective of their spin,may exist in the same orbital.

The spin angular momentum for an electron is given by .......

$A$ completely filled $d$ orbital $(d^{10})$ configuration is:

As per the modern periodic law,the physical and chemical properties of elements are periodic functions of their

The number of angular and radial nodes of $4d$ orbital respectively are