Electronic configuration of $Cr$ is $[Ar] 3d^5 4s^1$. For $Cr^{2+}$, it is $[Ar] 3d^4$. Number of unpaired electrons $n = 4$. Magnetic moment $\mu = \sqrt{n(n+2)} = \sqrt{4(4+2)} = \sqrt{24} \approx 4.90$ BM.