The origin and development of the atomic theory

The two principal links in the chain which binds the world of the ancient Greek (and one Roman) Atomists with those of our own times are Descartes (1596-1650) and Gassendi (1592-1655). Gassendi revived and ably supported the atomic theory of Epicurus; while Descartes resuscitated the mechanistic conceptions of the Greek Atomists on the basis of a corpuscular theory of matter, corresponding in many respects to the doctrine of Democritus.

The work of Leucippus connects through Descartes and Gassendi, Huygens, Boyle and Newton, directly with that of Higgins and Dalton, founders of the modern chemical atomic theory.

Dalton had already adopted a physical atomic theory, based no doubt on the work of his illustrious predecessors, and upon his own thorough study of the gases of the atmosphere, before he formulated his chemical atomic hypothesis. He opposed the prevailing belief that the various gases exist in the atmosphere in a state of chemical combination, asserting that the atmosphere is a physical mixture.

In 1805, he extended his atomic hypothesis to the explanation of chemical phenomena. His first postulate was, of course, that all matter is made up of small particles; these he found possessed the power of attracting (and holding) other particles. He therefore concluded that these invisible particles never subdivide in taking part in chemical changes, and that all atoms of any one element must be alike. But the atoms of the different elements vary in weight, form and combining power. He established the rule, already assumed by William Higgins (1789), that different atoms tend to combine in the proportion of atom to atom.

When a compound was composed of two elements only, it was presumed to be binary; that is, since water, for example, was known to be composed of hydrogen and oxygen, it was supposed by Dalton that it must consist of one atom of each of these elements; as was also assumed in the case of ammonia, which he knew to be a combination of hydrogen and nitrogen. Now we know that one atom of hydrogen combined with one atom of oxygen forms, not water, but oxide of hydrogen; while the combining of two atoms of hydrogen with one atom of oxygen forms water.

Dalton established the fact that elements combine only in definite proportions—e.g., that oxygen and hydrogen will combine only in the proportion of 8 to 1. Had Dalton known that a molecule of water contains two atoms of hydrogen, and one of oxygen, he would have known that one atom of oxygen must weigh sixteen times—instead of eight times—as much as one atom of hydrogen. One part of hydrogen by weight does combine with eight parts by weight of oxygen, but this does not prove that the portion of hydrogen in water contains only one atom. Since it contains two atoms, instead of one, then, in order to preserve the relative combining (or “equivalent”) weights of the two substances, we must assume that the atomic weight of oxygen is 16, for the combining weights of elements represent the relative (not actual) weights of the atoms. And each element has its own fixed combining weight, ascertained by experiment.

Water, then, is not a mere mixture or combination of hydrogen and oxygen, in the sense that a pound of hydrogen and eight and a half pounds of oxygen will, when exploded, produce 9½ pounds of water. Combination would indeed be effected, but there would be a residue in the container of just one-half pound of oxygen. The mass of oxygen that has combined will weigh exactly eight times as much as the hydrogen; and now we know that the water formed will contain exactly twice as many atoms of hydrogen as there are oxygen atoms present in the combination, uniting not only “one by one,” but, in this case, two to one. What the law of definite proportions proves is that chemical combinations always take place between atoms.

But there is also a law of multiple proportions. It is found that when one element forms more than one compound with a second element, the quantities of the first element which combine with the combining weight of the second element are always simple multiples—never fractions—of the combining weight of the first element.

We have seen that the atomic weight of oxygen is 16—i.e., 16 times heavier than the lightest element, hydrogen, which is therefore (on the relative scale of atomic weights) 1. But the combining weight of oxygen is 8—i.e., eight parts of oxygen will combine with so many parts (combining weight) of some other element. Suppose we take for example nitrogen. Now the combining weight of nitrogen is 14, and its atomic weight is also 14. Fourteen parts of nitrogen will combine therefore with eight parts of oxygen. But fourteen parts of nitrogen will also combine with four simple multiples of eight parts of oxygen, forming in all five different compound substances. For example:

John Dalton advanced a theory satisfactorily to explain why in successive compounds the amount of oxygen goes up in jumps, and why this jump in each case is equivalent to the combining weight of oxygen. The regular jumps in the case of the five nitrogen compounds simply mean that one more atom has entered, in each successive case, into combination. These combining weights, as we have seen, simply represent the relative weights of the various elements. “Atomic weight” is merely the name given by Dalton to the relative combining weights of the atoms. He had no information as to how many atoms actually combine in making a compound, much less of the actual weight of the single atoms of the elements.

Nor did he know why atoms combine at all, in any proportions, but, being a thorough-going Newtonian, both his physical theory and his chemical theory have a common basis in Newton’s doctrine of mutually repulsive atoms. If an element A unites with an element B, the repulsion of the atoms of B from one another must tend, he thought, to the formation of a binary compound. “Binary compounds must first be formed, then ternary, and so on, till the repulsion of the atoms of B refuses to admit any more.”[28]

While we have traveled quite a distance in atomic and chemical theory since the days of John Dalton (1766-1844), it is to the everlasting credit of this (at the time) obscure and poverty-stricken teacher of mathematics and physics that he gave us, in 1808, “A new System of Chemical Philosophy” which unites the atomic theory of modern science, first advanced by his admirers and contemporaries, Baron J. J. Berzelius (1779-1848) and Sir Humphrey Davy (1778-1829).

Little did Dalton dream that the “atomic weight” of the elements is equivalent to the number of electrons in each atom of a given substance![29] Or that the atomic number of an element, arranged (successively) in the order of their atomic weights, from 1 (hydrogen) to 92 (uranium), is an index to the number of positive electrical charges on the atomic nucleus, around which revolve in “planetary orbits” an equivalent number of negatively charged electrons—the real building-stones of the Universe.