Transition elements show magnetic moment due to spin and orbital motion of electrons. Which of the following metallic ions have almost same spin only magnetic moment?
Co2+ has electronic configuration of 3d7 and Cr3+ has electronic configuration of 3d3. Both have the same number of unpaired electrons, i.e 3.
Magnetic moment = 
Where n is the number of unpaired electrons. So for Cr3+ and Co2+ , the number of unpaired electrons equals 3.
Magnetic moment =
= 3.87 B.M
1