Stories About Chemistry
33. One, Two, Three, Many…
That was about what the counting abilities of primeval man amounted to. His mathematical apparatus included only two quantitative magnitudes, namely, “much,” and “little.”
People used much the same criterion about a hundred years ago when they tried to estimate the amounts of the separate elements our planet had stored away in its “granaries.” For example, lead, zinc, and silver had found wide usage in practice; there was much of them. Hence, these elements were considered abundant. But the rare earths (lanthanides) were rare because they were hardly ever encountered on Earth.
There was little of them. See how easy it was to reason only a century ago.
The first inspectors of the chemical element storehouses had an easy job to do. Our contemporaries laugh to think of their “activities.”
And how can they help laughing since they can now state exactly how much there is of everything! If they can even tell how many atoms of each element there are in the Earth’s crust. They know for certain that the notorious rare earths are just slightly less abundant in the minerals of our planet than lead, zinc, and silver all taken together.
Scrupulous “accounting” of the reserves of chemical elements started with a scientific feat accomplished by the American scientist Clark. He performed more than 5500 chemical analyses of a great variety of minerals from the tropics and from the tundra, of all kinds of water from lakes in the depths of the wilderness and from the Pacific Ocean. He studied samples of various soils from all parts of the world. This titanic work took him twenty years. Thanks to Clark and other scientists mankind got a quite distinct idea of the abundance of different elements on Earth.
So was born the science of geochemistry. It told wonderful stories such as had never before been known to man. It appeared that the first 26 representatives of the Mendeleyev Table, from hydrogen to iron, form practically the entire crust of the Earth. They constitute 99.7 per cent of its weight, leaving only a “miserly” three tenths of per cent for all the other 67 elements occurring in nature.
Now what is there the most of on Earth? Neither iron, nor copper, nor tin, though man has been using them for thousands of years and the supply of these metals seemed immense, even inexhaustible.
The most abundant element is oxygen. If we place all the Earth’s resources of oxygen on one pan of an imaginary pair of scales and all the rest of the elements on the other, the scales will strike an almost perfect balance. Almost half of the Earth’s crust is oxygen. It is everywhere: in water, in the atmosphere, in an enormous number of rocks, in any animal and plant, and everywhere it plays a very important part.
One quarter of the Earth’s “firmament” is silicon. It is the ultimate foundation of inorganic nature. Further, the elements of the Earth arrange themselves in the following order of abundance: aluminium, 7.4 per cent; iron, 4.2 per cent; calcium, 3.3 per cent; sodium, 2.4 per cent; potassium and magnesium, 2.35 per cent each; hydrogen, 1.0 per cent; and titanium, 0.6 per cent. Such are the ten most abundant chemical elements on our planet.
But what is there the least of on Earth? There is very little gold, platinum and platinum metals. That is why they are valued so highly. But it is a curious paradox that gold was the first of the metals to become known to man. Platinum was discovered before oxygen, silicon or aluminium had ever been heard of.
The noble metals possess a unique feature. They do not occur in nature as compounds but in the native state. No effort is required to smelt them from their ores, that is why they were found on Earth, precisely found, so very long ago.
However, these metals do not “take the cake” for rarity. This lamentable prize goes rather to the secondary radioactive elements. We could rightly call them ghost elements. The geochemists tell us that the amount of polonium on Earth totals only 9600 tons; the amount of radon is still smaller, 260 tons; there is 26 thousand tons of actinium.
Radium and protactinium are veritable giants among the ghosts: they total about 100 million tons, but compared to gold and platinum this is a very small quantity.
As to astatine and francium, they can hardly be classed even as ghosts, because they are still less material. The terrestrial reserves of astatine and francium are measured, ridiculous though it sounds, in milligrams. The name of the rarest element on Earth is astatine (69 milligrams in all of the Earth’s crust). No further comments are necessary.
The first transuranium elements, neptunium and plutonium, have also been found to exist on Earth. They are born in nature as a result of very rare nuclear reactions between uranium and free neutrons. These ghosts can “boast” of hundreds and thousands of tons. But as to promethium and technetium, which are also due to uranium (the latter is capable of spontaneous fission, with its nuclei breaking up into two approximately equal fragments), there is nothing that can be said of them.
Scientists have found hardly perceptible traces of technetium, and are still looking for promethium in uranium minerals. The balance has yet to be invented on which the Earth’s “reserves” of promethium and technetium could be weighed.