Which Substances Are Paramagnetic

Atoms with an odd number of electrons are paramagnetic. Chemical substances are on the contrary diamagnetic in most instances. This can be illustrated with the electron shell of rock salt, NaCl, compared to the shells of the free atoms  

Free radicals are molecules that have been modified to contain an unpaired electron. They are paramagnetic. Some are stable for instance the one that was first prepared (in 1900) according to 

Most free radicals are, however, unstable, and appear as intermediate products in various chemical reactions such as during photolysis and by the influence of ionizing radiation and in certain enzymes. Under suitable conditions free radicals can have sufficient lifetime to be observed in the liquid. The appearance of free radicals in liquid hydrocarbons during radiolysis was first demonstrated in 1960. In the solid state free radicals can become stabilized almost indefinitely in many materials. 

Some molecules contain two unpaired electrons with parallel spins. This can be caused, for example, by illumination with visible and ultraviolet light, so that the energy of the system becomes excited. The total spin quantum number is then S = V2 + V2= 1. Studies of triplet state molecules by ESR are usually made in single crystal or amorphous matrices, glasses . 

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