A Magnetic Personality - Paramagnetism and Diamagnetism

We are dealing in our model with electrons in orbitals, which are defined to have both orbital motion and spin motion both contribute to the (para)magnetic moment. Quantum theory associates quantum numbers with both these motions. The spin and orbital motion of an electron in an orbital involve quantum numbers for both spin momentum (.S ), which is actually related to the number of unpaired electrons (n) as S = nil, and the orbital angular momentum (L). The magnetic moment (pi) (which is expressed in units of Bohr magnetons, pis) is a measure of the magnetism, and is defined by an expression (7.1) involving both quantum numbers. 

The introduction of an equation involving quantum numbers may be daunting, but we are fortunately able to simplify this readily. Firstly, for first-row transition metal ions, the effect of L on pi is small, so a fairly valid approximation can be reached by neglecting the L component, and then our expression reduces to the so-called spin-only  

Thus we have reduced our expression to one involving simply the number of unpaired electrons. Using this spin-only Equation (7.3), the value of pi can be readily calculated and predictions compared with actual experimental values (Table 7.8). 

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