-Dense and soft, which we are using to drink water, or in general all other liquids. They have

Transcription

1 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) Associate Professor Andrey OLCHAK, DSc EdEx Course How Our World Is Designed? Supporting material Primary Elements of Matter We start from the scale, comparable to our own size. It is explicitly evident that we are surrounded by different kinds of media: Light and thin, which we are using to breath the air or (in general) any other gas. Gases do not have any particular volume or shape they occupy any allotted space -Dense and soft, which we are using to drink water, or in general all other liquids. They have certain volume, but take the shape of a vessel, used to store them -Dens and rigid solid bodies (stones, metals). They have both volume and certain shape, which is not so easy to change. Evidently, the world is also full of other objects which have mixed properties. For example any living being is a very complicated structure, including liquids, gazes and especially specific soft solid stuff, like our flesh and bones, which are not so rigid as stones, but still have certain shape. We should not also forget about very warm and very thin fire, which also is a quite common phenomenon in our world. So how to classify all this and how to understand how all it is organized? From the very beginning of analytical thinking, the basic trend was to reduce the variety of all of the observable objects to a limited set of primary elements forming all other objects in different combinations. The first regular attempts of such an analysis were undertaken in ancient Greece. The learning about four primary elements of matter found its final shape in Aristotelis Natural Phylosophy, which served as the basis for European education up till 18-th century.

2 According to Aristotelis the 4 basic elements are: Dry and Cold EARTH Wet and Cold WATER Wet and Hot AIR Dry and Hot FIRE Aristotelis added to this system one more FIFTH element quintessence or ether, understood as the prime cause of any motion. Very similar 5-elements structures were discussed also in India and China. Copy of a lost bronze bust of Aristotle made by Lysippos, Louvre The primary elements were understood not as real water or soil, but as ideal elements, composing real pieces of matter in different proportions. A difficulty of this system consisted in a lack of practical usage. There were no clear ideas how to extract the ideal primary elements and how to mix them in other to obtain this or that new kind of matter. But the practical usage was the last matter that worried Greek philosophers. To be fair we must mention that Greeks had also an alternative model explaining the variety of types of matter the atomic one! It dates back to a person named Democritus whose ideas can be expressed in the following assertions: 1. The atoms move in the Great Void, chaotic, and as a result of matching of the shapes, sizes, positions and orders either assemble or disassemble. 2. The resulting compounds are held together and thus produce the appearance of complex bodies. 3. A variety of bodies is due to a difference of their constituent atoms, as well as a difference in the order of assembly, in the same way as the same letters are composing different words. The problem of the model was the same: impossibility to extract and watch those atoms and finally the lack of practical usage of the model. But for ancient Greek that was the thing that really did not matter. Things started to change only in 18-th century. Two very important experiments were publicly demonstrated in England. In 1798 Benjamin Thompson (Count Rumford) posed a number of special experiments. In one

3 of them a metal ingot was placed under the water and a hole was drilled in it with a blunt drill driven by power of the two horses. After two and a half hours the water was boiling. Rumford concluded that heat is not a special primary matter, but its reason is hidden in motion. Another similar experiment was demonstrated by Hamphry Davey in Under the air pump bell, where from the air has been previously pumped out, there were placed two pieces of ice at 0 C. The two pieces could rub against each other with a special clockwork. When rubbing has caused the ice melting, the resulting water temperature was a few degrees above 0 C. In terms of the caloric theory this experiment was quite inexplicable, Davey has also concluded that the heat can only come as a result of the motion, but not as a special fluid coming from outside, which was simply impossible. It would be fair to mention that the theory considering the heat as a consequence of motion of multiple small particles (atoms) was supported in contrary to official Aristotelis theory by many outstanding researches in centuries, though previously they had no convincing evidence of their views. At the same time in 17 th - 18 th centuries Europe watched a very rapid development of the practical chemistry. It meant plenty of experiments in conversion and decomposition of different chemicals. It was experimentally established that there exists certain kinds of chemical agents that can not be further decomposed into simpler ones (chemical elements). Thus the semi-forgotten atomic theory of Democritus has received a very strong support and amendment. It was established that there are elementary particles of two types: 1. Molecules the smallest particles of chemical agents that determine its chemical properties and are capable of independent existence. 2. The atom - the smallest particle of a chemical element, which can not be further decomposed into simpler ones

4 3. Molecules are built from atoms as words are built from letters. The critical experiment was posed by English botanist and traveller Robert Brown in During his long travels in South-East Asia and Australia he gathered a collection of more than 10 thousands species of southern plants and flowers and after returning back to England studied it thoroughly. In his studies, in order to discern some very small particles of flower pollen he used quite sophisticated for his time tool the microscope. And in this study he came across a difficulty: it was very difficult to focus the microscope on a pollen particle placed on a dry surface, as the slightest puff of air immediately blows it off. He contrived to put the pollen into a dish with water. Pollen particles float on the surface and could not be puffed away. So, studying the pollen on the water surface, he noticed that those micro-particles are twitching in some strange irregular way. Being a very meticulous researcher he sketched this motion and published a special scientific article devoted to it. The only reasonable explanation for such a motion was an assumption that micro-particles of pollen are driven by hits from even much smaller and therefore invisible even through the strongest microscope particles, i.e. the molecules of water. Today we know that molecules of chemical agents, consisting of atoms as words are made of letters, are a reality. We know how to measure their size and mass. They all are mostly already measured and are listed in reference sources. As we know now, the typical size of atoms and simple molecules equals to approximately meter. The special unit is used in physics to measure interatomic distances, named Angstrem. 1 A = meter. To measure atomic masses in Physics and Chemistry we also use a special unit Dalton. 1 Dalton is equal to 1, gram approximately the mass of the simplest and smallest possible atom of hydrogen. 6, hydrogen atoms have a total mass equal to 1 gram. Therefore this number N A = 6, connecting the world of atoms and molecules with the international metric system SI has a special name Avogadro number after the known Italian chemist and Robert Brown s contemporary Amadeo Avogadro ( ). The Avogadro number is very convenient to count atoms and molecules.

5 The quantity of any substance containing exactly N A molecules is called in Physics and Chemistry 1 mole of substance. The mass of one mole of any chemical element or agent is called the atomic or molecular mass. Evidently, it always exceeds 1 gram. For example, for water (H 2 O) it equals 18.