Mendeleyev set out the details of each element on a card and began pinning his cards to the wall. He arranged and rearranged them, endeavoring to get them in order by weight, at the same time taking their various properties and characteristics into consideration. Soon a pattern began to emerge. He found that if he put the first seven elements (with the exception of hydrogen, which is really in a class by itself) in order in a column and then listed the next seven beside them, there was a remarkable similarity between each pair of elements. Sodium fell beside lithium, these being two of the elements called alkali metals because they react with water to form strong alkalis. Chlorine was paired with fluorine, two gases called halogens because of their notable tendency to form salts. These were the first two “periods” in what became his periodic table.
Continuing in a third column, Mendeleyev found that potassium fell beside sodium, then calcium beside magnesium. So far, so good. But from there on, things became more complicated. After trying various arrangements of his cards, he found that he could place all the following elements up as far as iodine in two long periods, of 17 elements each. By dividing the short periods, as shown on the chart, he found two rows at the top of each period and three rows at the bottom that corresponded exactly to well-known families of chemical elements. In the middle of the long periods, he found the metallic elements, including those metals most familiar to us in daily experience.
However, in order to achieve this beautiful arrangement, Mendeleyev had to leave several gaps, three in the first long period and one in the second. These blank spaces did not deter him from publishing his table. So strong now was his confidence that the elements had been created in an orderly pattern that he boldly announced that elements as yet unknown would be discovered to fill the gaps. With presumptuous audacity, he went so far as to describe the properties of these missing elements. He predicted their atomic weights, densities and the types of chemical combinations that they would form. He gave them tentative names, “eka-boron,” “eka-aluminum” and “eka-silicon,” in line with their expected family characteristics.
His confidence in the orderliness of “nature” was not misplaced. Much sooner than anyone might have expected, his missing elements started turning up. Gallium (eka-aluminum) was discovered in France in 1876, scandium (eka-boron) in Sweden in 1879, and, in 1886, germanium (eka-silicon) was discovered in Germany. Astonishingly—to everyone but Mendeleyev—the physical properties and atomic weights of each element were almost precisely what he had predicted. Incidentally, germanium has found an indispensable place today in the production of transistors.
After these discoveries, scientists who at first had paid little attention to Mendeleyev’s table came to acclaim him internationally as a scientific genius. His periodic chart came to be an indispensable aid in chemical research and teaching, and is to this day found everywhere on the walls of chemistry classrooms and laboratories. No one doubted that the other elements needed to fill in the blanks in the table would ultimately come to light.
