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Periodic Table (Chemistry) 673

of four primordial substances—earth, air, fire, and water. and silica—may in fact be complex substances has been
The two greatest philosophers of classical antiquity, Plato substantiated. We now know them to be oxides of calcium,
(428–347 BC) and Aristotle (384–322 BC), adopted magnesium,barium,aluminum,andsilicon.Twoofhisen-
Empedocles’s four elements, though Aristotle added a tries, light and heat, are not materials at all. Nevertheless,
fifth, the quita essentia, which made up the crystalline Lavoisier’s concept of simple substances and his prelim-
spheres of the heavens. While some scholars accepted inary list proved to be of inestimable importance for the
these elements as late as the 17th century, it is important development of chemistry and the periodic classification
to realize that these four elements were more philo- of the elements.
sophical constructs than specific entities with chemical For example, a clearly defined working concept of an
consequences. element was essential for the elaboration of the atomic
Similarly, atoms were proposed by the ancient Greeks theory by John Dalton (1766–1844). In 1808, this English
not so much to explain specific natural phenomena as schoolmaster published A New System of Chemical Phi-
to account for permanence amid change. Atomists like losophy in which he put forth his postulates about the
Democritus of Abdera (ca. 460–370 BC) argued that a structure of matter. Each element, Dalton argued, is consti-
body could not be infinitely subdivided. The process ul- tuted of identical, immutable, and uniquely characteristic
timately had to stop at the level of an “uncuttable” par- atoms. When elements combine to form compounds, their
ticle, literally, ατoµos. The Roman poet, Lucretius (ca. atoms unite in a fixed ratio that is characteristic of the com-
100–55 BC), claimed that atoms and the void constitute pound. Because these numbers are fixed, the elementary
all things and elaborated that idea to explain meteorology composition of the compound, by mass, is also constant.
and geology, sensation and sex, cosmology and sociol- This latter property could be quite accurately determined,
ogy, and even life and the mind. In spite of such ambitious even in Dalton’s time. However, Dalton had no direct way
comprehensiveness, atoms were not as widely accepted of ascertaining the correct atomic ratio of the elements of
as were the four earthly elements, largely because Aris- any compound. As a consequence, he could not calculate,
totle rejected them. Nevertheless, the concept of ultimate with confidence, the relative masses of the atoms of the
particles of matter was occasionally invoked as a working various elements—their atomic masses or atomic weights.
hypothesis by scientists such as Gassendi, Galileo, Boyle, The atomic masses of the elements and the atomic ra-
and Newton. Indeed, Newton went so far as to express this tios characteristic of specific compounds are manifested in
opinion: “It seems probable to me, that God in the Begin- the elementary mass composition of the compounds. Once
ning form’d Matter in solid, massy, hard, impenetrable, the correct atomic ratio is known, atomic masses can be
movable Particles.” readily calculated from mass composition. Alternatively,
Thus, by 1700, the progress made in chemistry was call- knowledge of atomic masses permits conversion of com-
ing the idea of four elements into serious question, while position by mass to composition by atomic ratio. How-
the idea of atoms was gaining scientific currency. The ever, Dalton faced the dilemma of an equation with two
century which followed did much to further establish unknowns.
the latter and to overthrow the former. Antoine Laurant That dilemma continued to plague chemistry for 50
Lavoisier (1743–1794), the Frenchman often being cred- years and, hence, limited the utility of the atomic theory.
ited with being the “Father of Modern Chemistry,” enun- Because of the importance of mass relationships in chem-
ciated a working definition of “simple substance” in his ical reactions, there was a consensus that the characteris-
´
1789 text, Trait´ e Elementaire de Chimie. The first edi- tic atomic mass of an element was a significant property.
tion included a table of 33 elementary substances that However, there was little agreement on what the correct
could not be decomposed by chemical operations. Within values were. Hydrogen was generally recognized to be the
this table, the elements were classified into groups such as “lightest” element and, hence, was typically assigned an
m´ etalliques and non m´ etalliques. Most of the metals listed atomicmassof1.Relativetothisstandard,theatomicmass
by Lavoisier (for example, antimony, silver, copper, and of oxygen was approximately 8, according to some scien-
iron) had been known and used for centuries. This was also tists, or 16, according to others. The situation had reached
the case with certain of the nonmetals such as carbon and such a sorry impass that the progress of chemistry was
sulfur. However, Lavoisier’s list also included hydrogen, being impeded. Therefore, a special international confer-
oxygen, and nitrogen, three gases which had recently been ence was convened in Karlsruhe, Germany, in 1860, with
isolated and, thanks to his theoretical system, correctly the expressed aim of resolving the confusion over atomic
identified as specific, elementary substances. Essentially masses. Significantly, the two scientists most clearly
all of Lavoisier’s original 33 substances still appear in associated with the development of the periodic table,
a modern periodic table. Lavoisier’s guess that the five the Russian Dmitri Ivanovich Mendeleev and the German
“earths” he includes—lime, magnesia, baryta, alumnia, Julias Lothar Meyer, were among those in attendance.