When a magnetic field is applied to substances, mainly two types of magnetic behaviour are observed: diamagnetism and Paramagnetism. Diamagnetic substances are repelled by the applied field while the paramagnetic substances are attracted. Substances which are attracted very strongly are said to be ferromagnetic. In fact, ferromagnetism is an extreme form of paramagnetism. Many of the transition metal ions are paramagnetic. Paramagnetism arises from the presence of unpaired electrons, each such electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum. For the compounds of the first series of transition metals, the contribution of the orbital angular momentum is effectively quenched and hence is of no significance. The magnetic moment increases with the increasing number of unpaired electrons. Thus, the observed magnetic moment gives a useful indication about the number of unpaired electrons present in the atom, molecule or ion.

How many unpaired electrons are present in 3d-orbital of central metal ion in the compound [Ni(H₂O)₆]Cl₂?

2

[Ni(H₂O)₆]Cl₂ → [Ni(H₂O)₆]²⁺ + 2Cl^-

In [Ni(H₂O)₆]²⁺ 

x + 6(0) = 2

x = 2

Ni²⁺: [Ar]3d⁸

No of unpaired electrons = 2