**ELECTRON SPIN**

As we discussed in Section 32-4, whether an electron is trapped in an atom or is free. it has an intrinsic spin angular momentum S, often called simply spin. (Recall that intrinsic means that Sis a basic characteristic of an electron, like its mass and electric charge.) As we shall discuss in the next section, the magnitude of S is quantized and depends on a spin quantum numbers, which is always! for electrons0 (and for protons and neutrons). In addition, the component of S measured along any axis is quantized and depends on a spin magnetic quantum number mi, which can have only the value +

The existence of electron spin was postulated on an empirical basis by two Dutch graduate students. George Uhlenbeck and Samuel Goudsmit, from their studies of atomic spectra. The quantum physics basis for electron spin was provided a few years later. by British physicist P. A. M. Dirac. who developed (in 1929) a relativistic

quantum theory of the electron. It is tempting to account for electron spin by thinking of the electron as a tiny sphere spinning about an axis. However. that classical model. like the classical mode of orbits. does not hold up. In quantum physics. spin angular momentum is best-thought of as a measurable intrinsic property of the electron; you simply can ‘t vi- -visualize it with a classical model. . T.able 41-1. an extension of Table 40-2. shows the four quantum numbers n, I. m,. and m, that completely specify the quantum states of the electron in a hydrogen

atom. (Quantum number .s is not included because all electrons have the values =t.) The same quantum numbers also specify hallowed states of any single electron in a multielectron atom.

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