The periodic table is one of the most notable achievements in chemistry because it helps to organize what would otherwise be a bewildering array of properties of the elements. However, the fact that its structure corresponds to the electronic structure of atoms was unknown to its discoverers. The periodic table was developed solely from a consideration of physical and chemical properties of the elements.

In 1860, the Congress of Karlsruhe brought together many prominent chemists in an attempt to resolve issues such as the existence of atoms and the correct atomic masses. One of the new ideas presented was Avogadro’s principle—that the numbers of molecules in samples of different gases of equal volume, pressure, and temperature are the same. This principle allowed the relative atomic masses of the gases to be determined. Two scientists attending the congress were the German Lothar Meyer and the Russian Dmitrii Mendeleev, both of whom left with copies of Avogadro’s paper. In 1869, Meyer and Mendeleev discovered independently that the elements fell into families with similar properties when they were arranged in order of increasing atomic mass. Mendeleev called this observation the periodic law.

Mendeleev’s chemical insight led him to leave gaps for elements that would be needed to complete the pattern but were unknown at the time. When they were discovered later, he turned out (in most cases) to be strikingly correct. For example, his pattern required an element that he named “eka-silicon” below silicon and between gallium and arsenic. He predicted that the element would have a relative atomic mass of 72 (taking the mass of hydrogen as 1) and properties similar to those of silicon. This prediction spurred the German chemist Clemens Winkler in 1886 to search for eka-silicon, which he eventually discovered and named germanium. It has a relative atomic mass of 72,59 and properties similar to those of silicon, as shown in the accompanying table.

One problem with Mendeleev’s table was that some elements seemed to be out of place. For example, when argon was isolated, it did not seem to have the correct mass for its location. Its relative atomic mass of 40 is the same as that of calcium, but argon is an inert gas and calcium a reactive metal. Such anomalies led scientists to question the use of relative atomic mass as the basis for organizing the elements. When Henry Moseley examined X-ray spectra of the elements in the early twentieth century, he realized that he could infer the atomic number itself. It was soon discovered that elements fall into the uniformly repeating pattern of the periodic table if they are organized according to atomic number, rather than atomic mass.