For centuries, philosophers have asked, "Are there a small set of fundamental constituents out of which everything is made? Or might it be that we will always find structure within structure, layers within layers like an onion?" Thales of Miletus, who suggested that water was the single fundamental entity from which matter is built, may be the first person recorded to suggest the appealing notion of an ultimate form of matter. The following material briefly sketches how this thinking has progressed to the present time.
Thales' student Anaximander added earth, fire, and air to water to the list of fundamental building blocks. The important notion that rational principles could explain what was observed was contained in this philosophy.
What are earth, air, fire, and water made of? Addressing this question led, by the nineteenth century, to recognition of the familiar chemical elements. The definition adopted then for an "element" was a substance that cannot be decomposed further into simpler substances by ordinary chemical means. Thus, the world consisted of a number of distinct substances (at the time, only about 30 elements were identified; today, there are more than 100). It was discovered that elements combine with other elements according to very simple rules: Two hydrogens plus one oxygen make one water "molecule." The essentially limitless number of chemical "compounds" that are found in nature are then the result of combinations of elements. At the beginning of the nineteenth century, John Dalton proposed an atomic theory of matter: The elements themselves are collections of tiny indestructible atoms characterized by their atomic weights (oxygen atoms weigh 16 times as much as hydrogen atoms). This can be considered the first theory of "elementary particles" having a sound scientific basis.
Dmitri Mendeleev arranged a table of the elements in order of increasing atomic weight and thereby made one of the most important discoveries in the history of science: The properties of the elements are "periodic" functions of their atomic weights. For example, in Mendeleev's "periodic table," the metals copper, silver, and gold, which have vastly different atomic weights, all appear in the same column. There were also gaps in this table: Elements not yet discovered that should exist if this atomic theory were correct. Confirmation of Mendeleev's "predictions" then made this scheme the basis of chemical thinking in the late nineteenth century; by the early twentieth century, further experiments established atoms as real physical entities.