Two elements, oxygen and silicon, make up 81% of the crust, including the oceans, atmosphere, and all living matter. Hydrogen and the third-row metals raise this total to 94%, and K, Ca, and Fe in the fourth row bring it to 99.5%. The eighteen elements for which values are given in the abundance table (opposite) account for all but two atoms per 100,000. Notice that no representative elements beyond the third row are included aside from the light metals K and Ca, that only the lightest transition metals are plentiful, and that the inner transition metals do not figure among the common elements of the crust at all. A chemistry textbook should not be based on a "popularity rating" of the elements. But since the crust of the Earth is the only part about which we have extensive chemical knowledge, and is the stage on which all the chemical reactions of living organisms evolved, it is natural that we should focus attention on the elements that seem to be doing interesting things around us.