In the "full deck" metaphor of the chapter opening, we now have inspected the deck and marked the most valuable cards for use in future games. There now are 106 elements in all, and more probably will be made artificially at the high atomic number end of the periodic table. These latter elements are only laboratory curiosities, however, and the elements of real importance to the planet and to life are known already. Only 18 of the 106 elements make up 99.98% of the Earth's crust, all from the first four rows of the periodic table. Only 11 of the 106 elements make up 99.99% of living organisms, again all from the first four rows. Another 13 elements are needed by living organisms in trace amounts, and the others, as far as we know, are not involved in life processes. The selection of these elements based on chemical properties has been superimposed on earlier selections based on chemical and physical properties, as the planet stratified, as the solar system formed earlier, and originally as the elements were synthesized in the stars.

We have tended to focus on the elements that are important to us and to our environment and to avoid the chemistry of the unusual elements. This is why we neglected many heavier elements, and all the inner transition metals. These first ten chapters are a study of matter and an introduction to the chemical elements. The next seven chapters are a study of energy and reactivity. These chapters will add quantitative and time dimensions to what so far has been only a descriptive science. They also will provide the necessary background for the last part of the book, the carbon compounds and the chemistry of living organisms.