Classification of Elements

During the 19th century, chemists isolated many more elements and the known list grew steadily -- today 118 elements are known, of which 92 (atomic numbers 1 to 92) occur naturally. As the list grew, chemists noticed genuine similarities in properties among certain elements, and this observation is what drove the search for a systematic classification: grouping elements by their properties promises real benefits for using them effectively. Several classification schemes were proposed before ordering elements by atomic weight finally produced a workable periodic table.

Doebereiner's Triads (1817). J. W. Doebereiner noticed that some sets of three elements with similar chemical properties -- chlorine, bromine and iodine is the classic example -- could be arranged so that the atomic weight of the middle element is close to the arithmetic mean of the atomic weights of the outer two. He called such a set of three a triad. Only a limited number of elements could be grouped this way, and the idea broke down for triads such as [Fe, Co, Ni], [Ru, Rh, Pd] and [Os, Ir, Pt], whose atomic masses are too close together for the averaging relationship to mean anything.

de Chancourtois's telluric helix (1862). A. E. B. de Chancourtois proposed that "the properties of bodies are the properties of numbers," meaning the atomic weight. He arranged the elements in order of increasing atomic weight along a helix traced at 45 degrees to the vertical axis of a cylinder whose circumference was 16 units. One complete turn of the helix corresponded to an atomic-weight increase of 16, so elements lying on the same one of the 16 equally spaced vertical lines on the cylinder's surface showed similar properties. This was the first genuinely systematic attempt at a periodic arrangement, though it attracted little attention at the time. …