# The electrons in a poly-electronic atom are filled one by one in order of increasing energy level.The energy of sub shells and orientation of orbitals depends upon the values of three quantum numbers(i.e n,l and m respectively ) derived from Schrodinger wave equation. The different orbitals of subshells however possess same energy level and are called degenerate orbitals but their energy level changes in presence of magnetic field and the orbitals are non-degenerate. A spectral line is noticed if an electron jumps from one level to other. The paramagnetic nature of element is due to the presence of unpaired electron. The total Magnetic momentum of $Ni^{2+}$ ion is....

$(a)\;\sqrt6 BM\qquad(b)\;\sqrt8 BM\qquad(c)\;\sqrt{15} BM\qquad(d)\;\sqrt{12}BM$

$Ni^{2+} : 3d^8$
i.e n = 2 unpaired electrons
$\therefore$ Magnetin Moment = $\sqrt{n(n+2)} BM$
$=\sqrt{2(2+2)}$
$=\sqrt8$