Even if we are not allowed to think of the atoms in the molecules as held together by hooks, it is well to have some sort of concrete picture of molecular structure. It is possible to represent the tetravalent carbon atom in the form of a tetrahedron, and to consider the united atoms or atomic groups as placed at the four vertices. With such a spatial representation we can get an idea about many chemical questions which otherwise would be difficult to explain. We know, for example, that two compound molecules having the same kind and number of atoms and the same bonds (and hence the same structural formulæ), may yet be different in that they are images of each other like a pair of gloves. Substances whose molecules are symmetrical in this way can be distinguished from each other by their different action on the passage of light. This molecular chemistry of space, or stereo-chemistry as it is called, has proved of great importance in explaining difficult problems in organic chemistry, i.e. the chemistry of carbon. Although there have never been many chemists who really have believed the carbon atom to be a rigid tetrahedron, we must admit that in this way it has been possible to get on the track of the secrets of atomic structure.
In comparing the properties of the elements with their atomic weights, there has been discovered a peculiar relation which remained for a long time without explanation, but which later suggested a certain connection between the inner structure of an atom and its chemical properties. We refer to the natural or periodic system of the elements which was enunciated in 1869 by the Russian chemist, Mendelejeff, and about the same time and independently by the German, Lothar Meyer. This system will be understood most clearly by examining the [table on p. 23], where the elements with their respective atomic weights and chemical symbols are arranged in numbered columns so that the atomic weights increase upon reading the table from left to right or from top to bottom. It will be seen that in each of the nine columns there are collected elements with related properties, forming what may be called chemical families. The table as here given is of a recent date and differs from the old table of Mendelejeff, both in the greater number of elements and in the particulars of the arrangement. With each element there is associated a number which indicates its position in the series with respect to increasing atomic weight. Thus hydrogen has the number 1, helium 2, etc., up to uranium, the atom of which is the heaviest of any known element, and to which the number 92 is given. In each of the columns the elements fall naturally into two sub-groups, and this division is indicated in the table by placing the chemical symbols to the right or left in the column.
On close examination it becomes evident that the regularity in the system is not entirely simple. First of all some cases will be found where the atomic weight of one element is greater than that of the following element. (The cases of argon and potassium on the one hand and cobalt and nickel on the other are examples.) Such an interchange is absolutely necessary if the elements which belong to the same chemical family are to be placed in the same column. As a second instance of irregularity, attention must be called to Column VIII. in the table. While in the first score or so of elements it is always found that two successive elements have different properties and clearly belong to distinct chemical families, in the so-called iron group (iron, cobalt and nickel) we meet with a case where successive elements resemble each other in many respects (for instance, in their magnetic properties). Since there are two more such “triads” in the periodic system, however, we cannot properly call this an irregularity. But in addition to these difficulties there is what we may even call a kind of inelegance presented by the so-called “rare earths” group. In this group there follow after lanthanum thirteen elements whose properties are rather similar, so that it is very difficult to separate them from each other in the mixtures in which they occur in the minerals of nature. (In the table these elements are enclosed in a frame.)
On the other hand, the apparent absence of an element in certain places in the table (indicated by a dash) cannot by any means be looked upon as irregular. In Mendelejeff’s first system there were many vacant spaces. With the help of his table Mendelejeff was, to some extent, able to predict the properties of the missing elements. An example of this is the case of the element between gallium and arsenic. This is called germanium, and was discovered to have precisely the properties which had been predicted for it—a discovery which was one of the greatest triumphs in favour of the reality of the periodic system. On the whole, the elements discovered since the time of Mendelejeff have found their natural positions in the table. This is seen, for example, in the case of the so-called “inactive gases” of the atmosphere, helium, argon, neon, xenon, krypton and niton, which have the common property of being able to form no chemical combinations whatever. Their valence is therefore zero, and in the table they are placed by themselves in a separate column headed with zero.
To explain the mystery of the periodic system, it was necessary to make clear not only the regularity of it, but also the apparent irregularities which seemed to be arbitrary individual peculiarities of certain elements or groups. In the periodic system, chemistry laid down some rather searching tests for future theories of atomic structure.
THE PERIODIC OR NATURAL SYSTEM
OF THE ELEMENTS
| 0 | I. | II. | III. | IV. | V. | VI. | VII. | VIII. |
|---|---|---|---|---|---|---|---|---|
| 1 Hydrogen H 1·008 | ||||||||
| 2 Helium He 4·00 | 3 Lithium Li 6·94 | 4 Beryllium Be 9·1 | 5 Boron B 11·0 | 6 Carbon C 12·0 | 7 Nitrogen N 14·0 | 8 Oxygen O 16 | 9 Fluorine F 19·0 | |
| 10 Neon Ne 20·2 | 11 Sodium Na 23·0 | 12 Magnesium Mg 24·3 | 13 Aluminium Al 27·1 | 14 Silicon Si 28·3 | 5 Phosphorus P 31·0 | 16 Sulphur S 32·1 | 17 Chlorine Cl 35·5 | |
| 18 Argon A 39·9 | 19 Potassium K 39·1 | 20 Calcium Ca 40·1 | 21 Scandium Sc 44·1 | 22 Titanium Ti 48·1 | 23 Vanadium V 51·0 | 24 Chromium Cr 52·0 | 25 Manganese Mn 54·9 | 26 Iron 27 Cobalt Fe 55·8 Co 59·0 28 Nickel Ni 58.7 |
| 29 Copper Cu 63·6 | 30 Zinc Zn 65·4 | 31 Gallium Ga 69·9 | 32 Germanium Ge 72·5 | 33 Arsenic As 75·0 | 34 Selenium Se 79·2 | 35 Bromine Br 79·9 | ||
| 36 Krypton Kr 82·9 | 37 Rubidium Rb 85·4 | 38 Strontium Sr 87·6 | 39 Yttrium Y 88·7 | 40 Zirconium Zr 90·6 | 41 Niobium Nb 93·5 | 42 Molybdenum Mo 96·0 | 43 — | 44 Ruthenium 45 Rhodium Ru 101·7 Rh 102·9 46 Palladium Pd 106·7 |
| 47 Silver Ag 107·9 | 48 Cadmium Cd 112·4 | 49 Indium In 114·8 | 50 Tin Sn 118·7 | 51 Antimony Sb 120·2 | 52 Tellurium Te 127·5 | 53 Jodine J 126·9 | ||
| 54 Xenon X 130·2 | 55 Caesium Cs 132·8 | 56 Barium Ba 137·3 | 57 Lanthanum La 139·0 | 58 Cerium 59 Praseodymium 60 Neodymium 61 —62 Samarium Ce 140·2 Pr 140·6 Nd 144·3 Sm 150·4 | ||||
| 63 Europium 64 Gadolinium 65 Terbium 66 Dysprosium Eu 152·0 Gd 157·3 Tb 159·2 Dy 162·5 | ||||||||
| 67 Holmium 68 Erbium 69 Thulium 70 Ytterbium71 Cassiopeium Ho 163·5Er 167·7Tm 168·5 Yb 173·0Cp 175·0 | ||||||||
| 72 Hafnium Hf 179 | 73 Tantalum Ta 181·5 | 74 Tungsten W 184·0 | 75 — | 76 Osmium 77 Iridium Os 190·9 Ir 192·1 78 Platinum Pt 195·2 | ||||
| 79 Gold Au 197·2 | 80 Mercury Hg 200·6 | 81 Thallium Tl 204·0 | 82 Lead Pb 207·2 | 83 Bismuth Bi 209·0 | 84 Polonium Po 210·0 | 85 — | ||
| 86 Niton Ni 222·0 | 87 — | 88 Radium Ra 226·0 | 89 Actinium Ac? | 90 Thorium Th 232 | 91 Protactinium Pa? | 92 Uranium U 238 | ||
The Molecular Theory of Physics.
From a consideration of the chemical properties of the elements we shall now turn to an examination of the physical characteristics, although in a certain sense chemistry itself is but one special phase of physics.
If matter is really constructed of independently existing particles—atoms and molecules—the interplay of the individual parts must determine not only the chemical activities, but also the other properties of matter. Since most of these properties are different for different substances, or in other words are “molecular properties,” it is reasonable to suppose that in many cases explanations can be more readily given by considering the molecules as the fundamental parts. It is natural that the first attempts to develop a molecular theory concerned gases, for their physical properties are much simpler than those of liquids or solids. This simplicity is indeed easily understood on the molecular theory. When a liquid by evaporation is transformed into a gas, the same weight of the element has a volume several hundred times greater than before. The molecules, packed together tightly in the liquid, in the gas are separated from each other and can move freely without influencing each other appreciably. When two of them come very close to each other, mutually repulsive forces will arise to prevent collision. Since it must be assumed that in such a “collision” the individual molecules do not change, they can then to a certain extent be considered as elastic bodies, spheres for instance.