Problems are listed in approximate order of difficulty. A single dot (•) indicates straightforward problems involving just one main concept and sometimes requiring no more than substitution of numbers in the appropriate formula. Two dots (••) identify problems that are slightly more challenging and usually involve more than one concept. Three dots (•••) indicate problems that are distinctly more challenging, either because they are intrinsically difficult or involve lengthy calculations. Needless to say, these distinctions are hard to draw and are only approximate.
• One of the triumphs of nineteenth-century science was the discovery by Mendeleev of the periodic table of the elements, which brought order to the confusing multitude of known elements, as described in Chapter 10. A crucial step in this discovery was to list the elements in the correct order — that is, as we now know, in order of increasing atomic number (which we define in Section 3.4 as the number of electrons in the neutral atom). At the time, atomic number was an unknown concept, and chemists listed the elements in order of atomic mass. Fortunately, these two orderings are very nearly the same. Use the periodic table inside the back cover of the book (where both parameters are listed) to find out how many times the two orders are different. The exception involving argon and potassium was a stumbling block in the development of the periodic table since it was found necessary to reverse the order of these two (when ordered by mass) to get the periodic table to make sense. We will see in Chapter 16 why it is that atomic number and atomic mass increase almost perfectly in step.
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