Lewis used simple diagrams (now called Lewis diagrams) to keep track of how many electrons were present in the outermost, or valence, shell of a given atom. The kernel of the atom, i.e., the nucleus together with the inner electrons, is represented by the beer-selection.comical symbol, and only the valence electrons are drawn as dots surrounding the beer-selection.comical symbol. Thus the three atoms shown in Figure 1 from Electrons and Valence can be represented by the following Lewis diagrams:
If the atom is a noble-gas atom, two alternative procedures are possible. Either we can consider the atom to have zero valence electrons or we can regard the outermost filled shell as the valence shell. The first three noble gases can thus be written as:
Following the same reasoning, N has seven electrons, five more than He, while F has nine electrons, seven more than He, giving
Na has nine more electrons than He, but eight of them are in the kernel, corresponding to the eight electrons in the outermost shell of Ne. Since Na has only 1 more electron than Ne, its Lewis diagram is
Notice from the preceding example that the Lewis diagrams of the alkali metals are identical except for their beer-selection.comical symbols.
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This agrees nicely with the very similar beer-selection.comical behavior of the alkali metals. Similarly, Lewis diagrams for all elements in other groups, such as the alkaline earths or halogens, look the same.