The 19th century was a time when scientists were discovering new elements and studying their chemical properties. Several chemists in the 1860s began to point out the regular recurrence of chemical properties. For example, there are obvious similarities among the alkali metals lithium, sodium, potassium, and cesium. But attempts at organization were hampered by the fact that many elements had yet to be discovered.

The Russian chemist Dmitri Mendeléev was the first to propose, in 1869, a periodic arrangement of the elements. He did so by explicitly pointing out “gaps” where, according to his hypothesis, undiscovered elements should exist. He could then predict the expected properties of the missing elements. The subsequent discovery of these elements verified Mendeléev’s organizational scheme, which came to be known as the periodic table of the elements.

FIGURE 41.19 shows a modern periodic table. (A larger and more detailed version can be found in Appendix B.) The significance of the periodic table to a physicist is the implication that there is a basic regularity or periodicity to the structure of atoms. Any successful theory of the atom needs to explain why the periodic table looks the way it does.

FIGURE 41.19 The modern periodic table of the elements, showing the atomic number $Z$ of each. A more detailed periodic table is provided in Appendix B.