Magnetic Properties of Transition Elements

In studying the magnetic properties of transition elements, we must first understand that when a substance is placed in a magnetic field of strength H, the intensity of the magnetic field in the substance may be greater than or less than H. If the field in the substance is greater than H, the substance is paramagnetic. It is easier for magnetic lines of force to travel through a paramagnetic material than through a vacuum. Thus paramagnetic materials attract of force and if it is free to move a paramagnetic material will move from a weaker to a stronger part of the field. Paramagnetism arises as a result of unpaired electron spins in the atom.

If the field in the substance is less than H, the substance is diamagnetic. Diamagnetic materials tend to repel lines of force. It is harder than through a vacuum and such materials tend to move from a stronger to a weaker part of a magnetic field. In diamagnetic compounds all the electron spins are paired. The paramagnetic effect is much larger than the diamagnetic effect.

It should be noted that Iron, Fe; Cobalt, co and Nickel, Ni are ferromagnetic. Ferromagnetic materials may be regarded as a special case of paramagnetism in which the moments on individual atoms become aligned and all point in the same direction. When this happens the magnetic susceptibility is greatly enhanced compared with what it would be if all the moments behave independently. Alignment occurs when materials are magnetized and Fe, Co and Ni can form permanent magnets. Ferromagnetism is found in several of the transition metals and their compounds. It is also possible to get antiferromagnetism by pairing the moments on adjacent atoms which point in opposite directions. This gives a magnetic moment less than would be expected for an array of independent ions. It occurs in several simple salts of Fe³⁺, Mn²⁺ and Gd³⁺. Since ferromagnetism and antiferromagnetism depend on orientation, they disappear in solution.

Many compounds of the transition elements are paramagnetic, because they contain partially filled electron shells. If the magnetic moment is measured, the number of unpaired electrons can be calculated. The magnetochemistry and magnetic properties of the transition elements shows whether the d electrons are paired. This is of great importance in distinguishing between high-spin and low-spin octahedral complexes.

There are two common methods of measuring magnetic susceptibilities: the Faraday and the Gouy methods. The Faraday method is useful for measurements on a very small single crystal, but there are practical difficulties because the forces are very small. The Gouy method is more often used. Here the sample may be presented as a long rod of material, a solution or a glass tube packed with powder. One end of the material, a solution or a glass tube packed with powder. One end of the sample is placed in a uniform magnetic field and the other end in a very low or zero field. The forces observed here are much larger and can be measured using a modified laboratory balance.

The volume susceptibility K₁ of a compound is measured using a magnetic balance (Gouy balance). K₁ is dimensionless and is readily converted into the molar susceptibility X_m, which has units m₃mol^-1. Form this the magnetic moment of the compound u can be calculated if the small diamagnetic contribution is ignored:

                             u =  3kTxm

                                      N^ou_o

Where k is the Boltzman constant (1.3805 x 10^-23J K^-1)

          u_o Is the permeability of free space (4π x 10^-7 Hm^-1)

          T is the absolute temperature and N^o is the Avogadro constant.

Thus u has SI units JT^-1 and

                                      u=   (3k) / (N^ou_o)   .   xmT

it is convenient to express the magnetic moment u_o in units of Bohr magnetons u_B, where

                                      u_B=  eh   = 9.273 x 10^-24 JT^-1

                                           4πm_e

Where e is the electronic charge, h is Planck’s constant and m_e is the mass of an electron.

Thus the magnetic moment in Bohr magnetons becomes.

                                       u = constant  .   xm.T

                                      u_B

Where constant =  (3k) / (N^ou_o) / u_B = 979.5m^3/2 mol^-½ K^½

The magnetic moment u of a transition metal can give important information about the number of unpaired electrons present in the atom and the orbitals that are occupied and sometimes indicates the structure of the molecule or complex. If the magnetic moment is due entirely to the spin of unpaired electrons u_s then

                                      u_s =  4S(S + 1) . u_B