Mendeleyev’s periodic table

The existence of qualitative changes in matter was known long before human beings began to think about science, but it was not really understood until the advent of atomic theory. Earlier, physics took the changes of state from solid to liquid to gas as something that occurred, without knowing exactly why. Only now are these phenomena being properly understood.

The science of chemistry made great strides forward in the 19th century. A large number of elements were discovered. But, rather like the confused situation that exists in particle physics today, chaos reigned. Order was established by the great Russian scientist Dimitri Ivanovich Mendeleyev (1834-1907) who, in 1869, in collaboration with the German chemist Julius Lothar Meyer (1830-95), worked out the periodic table of the elements, so called because it showed the periodic recurrence of similar chemical properties.

The existence of atomic weight was discovered in 1862 by Stanislao Cannizzaro (1826-1910). But Mendeleyev’s genius consisted in the fact that he did not approach the elements from a purely quantitative standpoint, that is, he did not see the relation between the different atoms just in terms of weight. Had he done so, he would never have made the breakthrough he did. From the purely quantitative standpoint, for instance, the element tellurium (atomic weight = 127.61) ought to have come after iodine (atomic weight = 126.91) in the periodic table, yet Mendeleyev placed it before iodine, under selenium, to which it is more similar, and placed iodine under the related element, bromine. Mendeleyev’s method was vindicated in the 20th century, when the investigation of X-rays proved that his arrangement was the correct one. The new atomic number for tellurium was put at 52, while that of iodine is 53.

The whole of Mendeleyev’s periodic table is based on the law of quantity and quality, deducing qualitative differences in the elements from quantitative differences in atomic weights. This was recognised by Engels at the time:

Finally, the Hegelian law is valid not only for compound substances but also for the chemical elements themselves. We now know that ‘the chemical properties of the elements are a periodic function of their atomic weights,’ … and that, therefore, their quality is determined by the quantity of their atomic weight. And the test of this has been brilliantly carried out. Mendeleyev proved that various gaps occur in the series of related elements arranged according to atomic weights indicating that here new elements remain to be discovered. He described in advance the general chemical properties of one of these unknown elements, which he termed eka-aluminium, because it follows after aluminium in the series beginning with the latter, and he predicted its approximate specific and atomic weight as well as its atomic volume. A few years later, Lecoq de Boisbaudran actually discovered this element, and Mendeleyev’s predictions fitted with only very slight discrepancies. Eka-aluminium was realised in gallium… By means of the – unconscious – application of Hegel’s law of the transformation of quantity into quality, Mendeleyev achieved a scientific feat which it is not too bold to put on a par with that of Leverrier in calculating the orbit of the until then unknown planet Neptune.^[26]

Chemistry involves changes of both a quantitative and qualitative character, both changes of degree and of state. This can clearly be seen in the change of state from gas to liquid or solid, which is usually related to variations of temperature and pressure. In Anti-Dühring, Engels gives a series of examples of how, in chemistry, the simple quantitative addition of elements creates qualitatively different bodies. Since Engels’ time the naming system used in chemistry has been changed. However, the change of quantity into quality is accurately expressed in the following example:

and so on to C₃₀H₆₀O₂, melissic acid, which melts only at 80° and has no boiling point at all, because it does not evaporate without disintegrating.”^[27]

The study of gases and vapours constitutes a special branch of chemistry. The great British pioneer of chemistry, Michael Faraday (1791-1867), thought that it was impossible to liquefy six gases, which he called permanent gases – hydrogen, oxygen, nitrogen, carbon monoxide, nitric oxide and methane. But in 1877, the Swiss chemist Raoul Pierre Pictet (1846-1929) managed to liquefy oxygen at a temperature of –140°C under a pressure of 500 atmospheres. Later, nitrogen, oxygen and carbon monoxide were all liquefied at still lower temperatures. In 1900, hydrogen was liquefied at –240° and, at a lower temperature, it even solidified. Finally, the most difficult challenge of all, the liquefaction of helium, was achieved at –255°. These discoveries had important practical applications. Liquid hydrogen and oxygen are now used in large amounts in rockets. The transformation of quantity into quality is shown by the fact that changes of temperature bring about important changes of properties. This is the key to the phenomenon of superconductivity. Through super-cooling, certain substances, beginning with mercury, were shown to offer no resistance to electric currents.

The study of extremely low temperatures was developed in the mid-19th century by the mathematician and physicist William Thomson (later Lord Kelvin) (1824-1907), who established the concept of absolute zero (the lowest possible temperature), which he calculated to be –273°C. At this temperature, he thought, the energy of molecules would sink to zero. This temperature is sometimes referred to as zero Kelvin, and used as the basis for a scale to measure very low temperatures. However, even at absolute zero, motion is not done away with altogether. There is still some energy, which cannot be removed. For practical purposes, energy is said to be zero, but that is not actually the case. Matter and motion, as Engels pointed out, are absolutely inseparable – even at “absolute zero”.

Nowadays, incredibly low temperatures are routinely achieved, and play an important role in the production of superconductors. Mercury becomes superconductive at exactly 4.12° Kelvin (K); lead at 7.22°K; tin at 3.73°K; aluminium at 1.20°K; uranium at 0.8°K, titanium at 0.53°K. Some 1,400 elements and alloys display this quality. Liquid hydrogen boils at 20.4°K. Helium is the only known substance which cannot be frozen even at absolute zero. It is the only substance that possesses the phenomenon known as superfluidity. Here too changes of temperature produce qualitative leaps. At 2.2°K, the behaviour of helium undergoes so fundamental a change that it is known as helium-2 to distinguish it from liquid helium above this temperature (helium-1). Using new techniques, temperatures as low as 0.000001°K have been reached, though it is thought that absolute zero is unattainable.

So far, we have concentrated on chemical changes in the laboratory and in industry. But it should not be forgotten that these changes take place on a much vaster scale in nature. The chemical composition of coal and diamonds, barring impurities, is the same – carbon. The difference is the result of colossal pressure, which, at a certain point transforms the contents of the coal-sack into a duchess’ necklace. To convert common graphite into diamonds would require the pressure of at least 10,000 atmospheres over a very long period of time. This process occurs naturally beneath the earth’s surface. In 1955, the big monopoly GEC succeeded in changing graphite into diamonds with a temperature of 2,500°C, and a pressure of 100,000 atmospheres. The same result was obtained in 1962, with a temperature of 5,000°C, and a pressure of 200,000 atmospheres, which turned graphite into diamond directly, without the aid of a catalyst. These are synthetic diamonds, which are not used to adorn the necks of duchesses, but for far more productive purposes – as cutting tools in industry.

1. Engels, F. Dialectics of Nature, pp. 90-1.

2. Engels, F. Anti-Dühring, p. 162.