3. Lithium Through Neon   Previous PageNext Page
     The Buildup of the Elements

Why should atoms contain outermost electron shells that have room for a maximum of eight electrons? G. N. Lewis first proposed this "rule of eight" at the turn of the century, and initially pictured the electrons as occupying the corners of a cube around the atom. Some of his sketches from a notebook page of 1902 are shown opposite. Lewis soon realized that the eight electrons were arranged in four electron pairs, like the electron pairs involved in chemical bonding. No good explanation for these four electron pairs per shell was forthcoming until physicists began applying quantum mechanics to chemistry in the 1920's. We will use their results in Chapters 7 and 8, but will avoid the mathematics that led to the results. For the moment we will say only that in the second electron shell there are four different "orbitals" or ways of locating electrons in space around the nucleus, with each of these orbitals capable of holding two electrons. In Chapter 9 we shall see that the calculated arrangement of these orbitals in space leads to a correct prediction of the observed shapes of molecules, a powerful indication that the quantum- mechanical treatment is correct.

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