Dalton’s views ushered in a new era in chemistry. Prolonged researches were at once undertaken, in order to determine the precise atomic weights—investigations which are being carried on even today. The exact size, shape, texture, etc., of the atom was subject to endless investigation. The nature of chemical combinations (how two elements combine with one another) held the fascinated attention of chemists for a hundred years, and it is only within the past few years that a definite solution has been found, and this has only been rendered possible by the newer views of matter, entirely different from those maintained during the past century.
During the hundred years which have elapsed since Dalton’s time, a number of new elements have been discovered, and there are reasons for supposing that there are some yet to discover. It is now believed, however, that there are 92 primary elements, of which Hydrogen has the lowest atomic weight, and Uranium the highest. Typical elements are: Oxygen, Iron, Fluorine, Silver, Sodium, Sulphur, Gold, Zinc, Copper, etc. A complete list may be found in any standard Chemistry.
ATOMIC WEIGHTS
When work was undertaken, to discover the exact atomic weights of these various elements, it was soon found that these could not be expressed in exact, whole numbers. Fractions or decimal numbers were nearly always found to exist. Thus, the atomic weight of Hydrogen was not exactly 1, but 1.008; copper was 63.57, etc. For long it was thought that these variations were due to errors of experiment, and renewed attempts were made to reach more accurate conclusions, in which these apparently annoying fractions were absent. But the most painstaking experimental work only served to confirm these results, and still later researches have shown us why this is so. It would take us too far afield, however, to go into that question at present.
The various elements were given symbols for the sake of brevity; some of these represented the first letters of the name of the element; some were the first letters of the Latin word for that element. Thus, Co = Cobalt, S = Sulphur, Ne = Neon, Bi = Bismuth. On the other hand, Fe = Iron (Latin, ferrum), etc. This served greatly to abbreviate chemical language, and at the same time simplified chemical formulæ and equations.
VALENCY
We must now explain one or two terms which are extremely important for understanding what is to follow. The first of these is Valency. We know that chemical combinations take place in fixed proportions by weight; this is known as the “Constancy of Composition.” There is always an equivalence noted. This doctrine of equivalence is merely the numerical expression of the definiteness of chemical change. Calculations are made from the point-of-view of combining with a unit-weight of hydrogen (the Unit element). In chemical compounds, then, the doctrine of equivalence says that these atomic weights represent quantities of different elementary substances which are of the same chemical value as measured by their capacity for displacing the same weight of hydrogen.
A new property of the atom is thus brought out, viz., its value as measured by the number of atoms with which it can combine. This property is appropriately described as the “Valency” of the atom. If the atomic weight contains the equivalent once, i. e., if the equivalent and atomic weight are identical, that atom can combine only with one atom of hydrogen, or of chlorine, bromine, etc. The formulæ of the compounds, HCl, HBr, etc., expresses this fact. If the equivalent is contained twice in the atomic weight, then that atom can obviously combine with two atoms of hydrogen, chlorine, etc.; if it is contained three times in the atomic weight, the combining capacity or valency of the atom is three; and so forth.
THE PERIODIC LAW
The work which had been done upon the atomic weights rendered possible one of the most brilliant generalizations of modern times, in this field. This was the Periodic Law. In the year 1864, Newlands published a Table containing the various elements arranged in the order of their atomic weights. In a side column the differences between these weights were given, each being deducted from the one next higher in the scale. The next year, Newlands announced his “law of octaves,” which he deduced from his arrangement of the elements. He said in part that: “If the elements are arranged in the order of their equivalents, with a few slight transpositions ... it will be observed that elements, belonging to the same group usually appear on the same horizontal line.... It will also be seen that the number of analogous elements generally differ either by seven or by some multiple of seven; in other words, members of the same group stand to each other in the same relation as the extremities of one or more octaves in music.”