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84 (1) (2) DETERMINATION OF MAGNETIC MOMENT 1. Introduction Transition metals, by definition, have at least one oxidation state with an incomplete d or f subshell. Since electrons spin and generate a magnetic field, the magnetic properties of transition metals are of great interest in determining the oxidation state, electronic configuration, and so on. Most organic compounds and main group element compounds have all their electrons paired. Such molecules are diamagnetic and have very small magnetic moments. Many transition metal compounds, however, have one or more unpaired electrons, and are termed paramagnetic. The number of unpaired electrons on a given metal ion determines the magnetic moment, : , affecting it both by virtue of their spin and their orbital motion. The spin is the more important, and a close estimate of the magnetic moment can be obtained using the equation where g is the gyromagnetic ratio for an electron ( . 2) and S is the total spin of the unpaired electrons (at ½ each). For one unpaired electron (as in Ti 3+ ) The units of the magnetic moment are Bohr magnetons (BM). Actual magnetic moments are somewhat larger than the spin-only values obtained above, because of the orbital contribution. Magnetic moments are not measured directly. Instead, the magnetic moment is calculated from the magnetic susceptibility, as described in Section 2.
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85 2. Measurement of Magnetic Susceptibility There are a number of techniques that were used to determine the magnetic susceptibility of transition metals complexes. These include the Gouy method, the Faraday method, and the determination of magnetic susceptibility by nuclear magnetic resonance (NMR). Of these techniques, only the last two qualify as microtechniques, and can be carried out practically with 50 mg or less of sample. More recently, a new type of magnetic susceptibility balance, developed by D.F. Evans of Imperial College, London and Johnson Matthey/AESAR has appeared. The balance is compact, lightweight, and self-contained. It does not require a separate magnet or power supply, and is therefore easily portable. The instrument has a digital readout that provides quick and accurate readings and whose sensitivity matches that of traditional methods. this balance can handle microscale quantities of solids as well as determine the magnetic susceptibility of liquids and solutions. As such, it is an ideal instrument for microscale inorganic work. In the Gouy method, a sample is suspended from a balance between the two poles of a magnet. The balance measure the apparent change in the mass of the sample, because it is repelled or attracted by the magnetic field. The attraction is due to the magnetic field generated by the unpaired electrons in a paramagnetic sample. Diamagnetic samples are repelled by the balance. The Evans balance has the same basic equipment configuration as in the Gouy method, but instead of measuring the force that the magnet exerts on the sample, it measures the equal and opposite force the sample exerts on a suspended permanent magnet.
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This note was uploaded on 11/06/2010 for the course PHYSICS magnetic s taught by Professor Unknown during the Spring '10 term at University of Zagreb Faculty of Electrical Engineering and Computing.

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