How the atoms are constructed and relate to one another bespeaks economy and awesome organization in chartlike order. About 300 years ago, only 12 elements were known—antimony, arsenic, bismuth, carbon, copper, gold, iron, lead, mercury, silver, sulfur, and tin. As more were discovered, scientists noticed that the elements reflected a distinct order. Because there were gaps in the order, scientists such as Mendeleyev, Ramsay, Moseley, and Bohr theorized the existence of unknown elements and their characteristics. Those elements were subsequently discovered just as predicted. Why could those scientists predict that there were forms of matter that were unknown at the time?
