I wrote facts about the elements on individual paper cards. On these cards, I wrote information such as the elements' melting points, densities, colors, atomic masses, and bonding powers. Once my cards were made, I tried arranging them in various ways. Finally, I noticed that patterns appeared when the elements were arranged in order of increasing atomic mass. One of the trends that I noticed showed that the bonding power of the elements from lithium to fluorine change in an orderly way. For example, after fluorine, the next heaviest element I knew was sodium, which has the same bonding power as lithium. Using this knowledge, I placed the card for sodium below the card for lithium. This worked well -- as I laid out cards, each element had properties similar to the elements above and below it. Arranging the elements by increasing atomic mass left three blank spaces in the table. Despite this development, however, I boldly proposed that these blank spaces would be filled by elements that had not yet been discovered.
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