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Question
Although \[\ce{Cr^3+}\] and \[\ce{Co^2+}\] ions have same number of unpaired electrons but the magnetic moment of \[\ce{Cr^3+}\] is 3.87 B.M. and that of \[\ce{Co^2+}\] is 4.87 B.M. Why?
Solution
Magnetic moment of an electron/dipole moment is caused by its intrinsic properties of spin and electric charge. It depends upon the number of unpaired electrons in its valence shell. The more the number of unpaired electrons, the greater will be the value of magnetic moment.
\[\ce{Cr^{3+} = [Ar] 3d^3 = 3 unpaired electrons}\]
\[\ce{Co^{2+} = [Ar] 3d^7 = 3 unpaired electrons}\]
But as both the ions have the same no. of unpaired electrons, there’s one other factor called orbital contribution which also influences the magnetic moment. As, the three unpaired electronic configurations is symmetrical in \[\ce{Cr^3+}\] this contributes zero orbital contribution but this is quite appreciable in case of \[\ce{Co^2+}\] and hence it has larger value of Magnetic moment than \[\ce{Cr^3+}\].
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