Chemistry · Ch 8 — Transition and Inner Transition Elements
Magnetic Properties
Magnetic Properties
The compounds of transition metals exhibit magnetic properties due to the unpaired electrons present in their atoms or ions. When a magnetic field is applied, substances which are attracted towards the applied magnetic field are called paramagnetic, while the ones which are repelled are called diamagnetic. Some substances are attracted very strongly, and these are called ferromagnetic substances.
Paramagnetism and ferromagnetism arises due to presence of unpaired electrons in a species. When all electron spins are paired, the compound becomes diamagnetic.
Among transition metals, Fe, Co, Ni are ferromagnetic. When a magnetic field is applied externally, all the unpaired electrons in these metals and their compounds align in the direction of the applied magnetic field. Due to this, the magnetic susceptibility is enhanced. These metals can be magnetized — that is, they acquire a permanent magnetic moment.
Each unpaired electron gives rise to a small magnetic field (magnetic moment) due to its spin angular momentum and orbital angular momentum. In the case of the first row transition elements, the contribution from the orbital angular momentum is quenched, and hence can be neglected. The spin-only formula for magnetic moment is :
where n is the number of unpaired electrons and μ is the magnetic moment expressed in Bohr Magneton (BM). A single unpaired electron has magnetic moment μ = 1.73 BM.
From the magnetic moment (μ) measurements of the metal complexes of the first row transition elements, the number of unpaired electrons can be calculated with the use of the spin-only formula. As the magnetic moment is directly related to the number of unpaired electrons, the value of μ will vary directly with the number of unpaired electrons.
Magnetic moments are determined experimentally, in solution or in the solid state, where the central metal is hydrated or bound to ligands. A slight difference in the calculated and observed values of magnetic moments thus can be noticed. Table 8.7 gives the calculated and observed magnetic moments of cations of the 3d series.
Table 8.7 Magnetic moments of ions of first transition series elements (values in BM)
| Ion | Outer electronic configuration | Number of unpaired electrons | Calculated value of magnetic moment | Experimental value |
|---|---|---|---|---|
| 0 | 0 | 0 | ||
| 1 | 1.73 | 1.75 | ||
| 2 | 2.84 | 2.76 | ||
| 3 | 3.87 | 3.86 | ||
| 4 | 4.90 | 4.80 | ||
| 5 | 5.92 | 5.96 | ||
| 4 | 4.90 | 5.3-5.5 |