The main provisions of the ICT. Proof of the existence of molecules
The main provisions of the molecular kinetic theory.
one). Any substance has a discrete (discontinuous) structure. It consists of the smallest particles - molecules and atoms, separated by intervals. Molecules are the smallest particles with chemical properties of this substance. Atoms are the smallest particles with the properties of chemical elements that make up a given substance.
2). Molecules are in a state of continuous chaotic motion, called thermal. When a substance is heated, the speed of thermal motion and the kinetic energy of its particles increase, and when cooled, they decrease. The degree of heating of a body is characterized by its temperature, which is a measure of the average kinetic energy of the translational motion of the molecules of this body.
3). In the process of their interaction, the forces of attraction and repulsion arise between the molecules.
^ Experimental substantiation of the molecular kinetic theory
The presence of permeability, compressibility and solubility in substances indicates that they are not continuous, but consist of separate, spaced particles. Through modern methods studies (electron and ion microscopes) managed to obtain images of the largest molecules.
Observations of Brownian motion and particle diffusion have shown that the molecules are in continuous motion.
The presence of strength and elasticity of bodies, wettability, adhesion, surface tension in liquids, etc. - all this proves the existence of forces of interaction between molecules.
^ Brownian motion.
In 1827, the English botanist Brown, observing a suspension of pollen in water through a microscope, discovered that the grains of pollen were constantly moving chaotically. The disorderly motion of very small particles of a solid suspended in a liquid is called Brownian motion. It was found that Brownian motion takes an unlimited time. The intensity of movement of particles suspended in a liquid does not depend on the substance of these particles, but depends on their size. Large particles remain stationary. The intensity of Brownian motion increases with increasing temperature of the liquid and decreases with decreasing temperature. Particles suspended in a liquid move under the influence of liquid molecules that collide with them. Molecules move chaotically, so the forces with which they act on suspended particles are continuously changing in magnitude and direction. This leads to the disordered movement of suspended particles. Thus, Brownian motion clearly confirms the existence of molecules and the chaotic nature of their thermal motion. (The quantitative theory of Brownian motion was developed in 1905 by Einstein.)
By diffusion the phenomenon of spontaneous mutual penetration of molecules of substances adjoining each other into the intermolecular gaps of each other is called. (Diffusion through semi-permeable partitions is called osmosis.) An example of diffusion in gases is the diffusion of odors. In liquids, a clear manifestation of diffusion is mixing against the action of gravity of liquids of different densities (in this case, the molecules of a heavier liquid rise up, and a lighter liquid - go down). Diffusion also occurs in solids... This is proved by this experience: two polished flat plates of gold and lead, laid on top of each other, were kept at room temperature for 5 years. During this time, the plates have grown together, forming a single whole, with gold molecules penetrating into lead, and lead molecules into gold to a depth of 1 cm. 1 The rate of diffusion depends on the state of aggregation of the substance and temperature. With an increase in temperature, the diffusion rate increases, and with a decrease, it decreases.
^ Sizes and mass of molecules
The size of the molecule is a conditional value. It is evaluated as follows. Along with the attraction forces, the repulsive forces also act between the molecules, therefore the molecules can approach only to a certain distance. The distance of the maximum approach of the centers of two molecules is called the effective diameter of the molecule and is denoted by o (in this case, it is conventionally considered that the molecules have a spherical shape). With the exception of molecules of organic substances containing a very large number of atoms, most molecules in order of magnitude have a diameter of 10 -10 m and a mass of 10 -26 kg.
^ Relative molecular mass
Since the masses of atoms and molecules are extremely small, calculations usually use not absolute, but relative masses obtained by comparing the masses of atoms and molecules with atomic unit mass, which is chosen as 1/12 of the mass of a carbon atom (i.e., use the carbon scale of atomic masses). Relative molecular(or atomic) mass M r(or BUT r) of a substance is called a value equal to the ratio of the mass of a molecule (or atom) of this substance to 1/12 of the mass of a carbon atom 12 C. Relative molecular (atomic) mass is a quantity that has no dimension. The relative atomic mass of each chemical element is indicated in the periodic table. If a substance consists of molecules formed from atoms of various chemical elements, the relative molecular weight of this substance is equal to the sum of the relative atomic masses of the elements that make up this substance.
^ Amount of substance
The amount of substance contained in a body is determined by the number of molecules in this body (or the number of atoms). Since the number of molecules in macroscopic bodies is very large, to determine the amount of matter in a body, the number of molecules in this body is compared with the number of atoms in 0.012 kg of carbon. In other words, the amount of substance v is called a value equal to the ratio of the number of molecules (or atoms) N in a given body to the number of atoms N A in 12 g of carbon, i.e.
v = N / N A . The amount of the substance is expressed in moles. A mole is equal to the amount of matter in a system containing the same number of structural elements (atoms, molecules, ions) as there are atoms in carbon-12 weighing 0.012 kg.
^ Avogadro's constant. Molar mass
According to the definition of a mole, 1 mole of any substance contains the same number of molecules or atoms. This is the number N A, equal to the number atoms in 0.012 kg (i.e. in 1 mol) of carbon, called Avogadro's constant. The molar mass M of any substance is called the mass of 1 mol of this substance. Molar mass substances are expressed in kilograms per mole.
The amount of a substance can be found as 
The mass of one molecule can be found as 
or given that the relative molecular weight is numerically equal to the mass of one molecule expressed in amu. (1 amu = 1.6610 -27 kg).
^ 2. The structure of gaseous, liquid and solid bodies
There are four aggregate states substances - solid, liquid, gaseous and plasma.
If the minimum potential energy W P of the molecules of a substance is much less than the average kinetic energy of their thermal motion W K (i.e., W P> W K, then the substance is in a solid state.
In gases at low pressures and low temperatures, the molecules are located from each other at distances that are many times greater than their size. Under such conditions, gas molecules are not bound by intermolecular forces of attraction. They move chaotically along the entire volume occupied by the gas. The interaction of gas molecules occurs only when they collide with each other and with the walls of the vessel in which the gas is located. The momentum transfer in these collisions determines the pressure produced by the gas. The distance that a molecule travels between two successive collisions is called the molecular mean free path. If gas molecules consist of two or more atoms, then upon collision they acquire rotary motion... Thus, in gases, molecules perform mainly translational and rotational motion.
In liquids, the distance between molecules is comparable to their effective diameter. The forces of interaction of molecules with each other are quite large. Liquid molecules oscillate around temporary equilibrium positions. However, in liquids, W P ~ W K, therefore, having received an excess of kinetic energy as a result of chaotic collisions, individual molecules overcome the attraction of neighboring molecules and move to new equilibrium positions around which they again perform oscillatory motion. The time of vibration of liquid molecules near equilibrium positions is very short (about 10 -10 - 10 -12 s), after which the molecules make a transition to new positions. Consequently, the molecules of the liquid make an oscillatory motion around the temporary centers of equilibrium and abruptly move from one position of equilibrium to another (due to such movements, the liquid has fluidity and takes the form of the vessel in which it is located). A liquid consists of many microscopic regions in which there is a certain order in the arrangement of nearby molecules, not repeating throughout the entire volume of the liquid and changing over time. This type of particle ordering is called short-range order.
In solids, the distance between molecules is even less than in liquids. The forces of interaction of molecules of solids with each other are so great that the molecules are held relative to each other in certain positions and vibrate around constant centers of equilibrium. Solids are divided into crystalline and amorphous. Crystalline bodies are characterized by the so-called crystal lattices - an ordered and periodically repeating arrangement of molecules, atoms or ions in space. If through an arbitrary node crystal lattice draw a straight line in any direction, then along this straight line at an equal distance there will be other nodes of this lattice, that is, this structure is repeated throughout the entire volume of the crystalline body. This kind of particle ordering is called long-range order. In amorphous bodies (glass, resin and a number of other substances) there is no long-range order and crystal lattice, which makes amorphous bodies similar in properties to liquids. However, in amorphous bodies, molecules vibrate around temporary equilibrium positions much longer than in liquids. In solids, molecules perform predominantly oscillatory motion (although there are also individual molecules moving translationally, as evidenced by the phenomenon of diffusion).
^ 3. Stern's experience. Velocity distribution of molecules
Gas molecules move with high speeds straight ahead of the collision. At room temperature, the speed of air molecules reaches several hundred meters per second. The distance that molecules travel on average from one collision to another is called the average free path of molecules. At room temperature, air molecules have an average free path of the order of 10 -7 m. Due to the randomness of motion, molecules have very different velocities. But at a given temperature, it is possible to determine the speed close to which greatest number molecules.
The speed in, close to which the largest number of molecules has, is called the most probable speed.
Only a very small number of molecules have a speed close to zero, or close to infinite great value many times the most probable speed. And, of course, there are no molecules whose speed is zero or infinitely high. But most of the molecules have a speed close to the most probable.
With an increase in temperature, the velocities of the molecules increase. But the number of molecules with a speed close to the most probable decreases, since the spread in velocities increases, and the number of molecules whose velocities differ significantly from the most probable one increases. The number of molecules moving at high speeds increases, and at lower speeds - decreases. AND 
due to the huge number of molecules in any volume of gas, their directions of motion along any coordinate axis are equally probable if the gas is in a state of equilibrium, that is, there are no flows in it. This means that any directed motion of one molecule corresponds to the antidirectional motion of another molecule with the same speed, that is, if one molecule moves, for example, forward, then there will certainly be another molecule that moves back with the same speed. Therefore, the speed of movement of molecules, taking into account their direction, cannot be characterized by the average speed of all molecules, it will always be zero, because a positive speed co-directional with one of the coordinate axes will add up with a negative speed that is anti-directional to this axis. If the values of the velocities of all molecules are squared, then all the minuses will disappear. If, then add up the squares of the velocities of all molecules, and then divide by the number of molecules N, that is, determine the average value of the squares of the velocities of all molecules, and then extract Square root from this value, then it will no longer be equal to zero and it will be possible to characterize the speed of movement of molecules. The square root of the mean of the squares of the velocities of all molecules is called their root mean square velocity 
... It follows from the equations of molecular physics that 
.
^ Stern's Experience.
The first experimental determination of the velocity of molecules was made in 1920 by the German physicist O. Stern. It determined the average speed of the atoms. The schematic of the experiment is shown in Fig.
On a flat horizontal base, two coaxial cylindrical surfaces 1 and 2 are fixed, which, together with the base, can rotate around the vertical axis of the OO 1. Surface 1 is solid, and n 
Surface 2 has a narrow slit 4 parallel to the axis OO 1. This axis is a platinum silver-plated wire 3 through which an electric current is passed. The entire system is located in a chamber from which air is evacuated (i.e. in a vacuum). The wire is heated to a high temperature. Silver atoms, evaporating from its surface, fill the inner cylinder 2. A narrow beam of these atoms, passing through slit 4 in the wall of cylinder 2, reaches the inner surface of cylinder 1. If the cylinders are stationary, silver atoms are deposited on this surface in the form of a narrow strip parallel to slots (point B), (section of the cylinders with a horizontal plane).
When the cylinders are rotated with a constant angular velocity around the axis OO 1 during the time t, during which the atoms fly from the slit to the surface of the outer cylinder (i.e., they cover the distance AB equal to the difference 
the radii of these cylinders), the cylinders rotate through an angle , and the atoms are deposited in the form of a strip in another place (point C, Fig. b). The distance between the deposition sites of atoms in the first and second cases is equal to s.
We denote 
is the average velocity of atomic motion, and v = R is the linear velocity of the outer cylinder. Then 
... Knowing the parameters of the installation and measuring experimentally s, it is possible to determine the average velocity of motion of atoms. In Stern's experiment, it was found that the average speed of silver atoms is 650 m / s.
Mountains, stars, people - everything that we see around is made of tiny atoms. Atoms are small. Very, very much. We know from childhood that all matter is made up of clusters of these tiny things. We also know that they cannot be seen with the naked eye. We are forced to blindly believe these statements, without being able to verify. Atoms interact with each other and make up our world brick by brick. How do we know this? Many people do not like to take the claims of scientists at face value. Let us, along with science, go from understanding atoms to directly proving their existence.
It might seem like there is an easy way to prove the existence of atoms: to put them under a microscope. But this approach won't work. Even the most powerful light focusing microscopes cannot visualize a single atom. The object becomes visible because it reflects light waves. Atoms are so much smaller than the wavelength of visible light that they do not interact at all. In other words, atoms are invisible even to light. However, atoms do have observable effects on some of the things we can see.
Hundreds of years ago, in 1785, the Dutch scientist Jan Ingenhauzh studied a strange phenomenon that he could not understand. Tiny particles of coal dust were poking around the surface of some alcohol in his laboratory.
50 years later, in 1827, Scottish botanist Robert Brown described something remarkably similar. By examining the pollen granules under a microscope, Brown found that some of the granules emitted tiny particles - which were then removed from the pollen in a random, nervous dance.
At first, Brown thought the particles were some kind of unknown organism. He repeated the experiment with other substances, such as stone dust, which were clearly inanimate, and again saw the strange movement.
It took almost a hundred years for science to find an explanation. Einstein came in and developed a mathematical formula that predicted that very particular type of motion - then called Brownian motion, after Robert Brown. Einstein's theory was that the particles of the pollen granules were constantly moving as millions of tiny water molecules - molecules made up of atoms - slammed into them.
“He explained that this nervous movement that you observe was actually caused by the action of individual water molecules on the dust particles or whatever you have,” explains Harry Cliff of Cambridge University, also curator of the Science Museum in London.
By 1908, observations, backed by calculations, showed that atoms were real. Physicists have made significant progress in ten years. By stretching individual atoms, they began to understand their internal structure.
It was a surprise that atoms can be separated - especially in light of the fact that the very name "atom" came from the Greek "atomos", meaning "indivisible." But physicists now know that atoms are far from basic bricks. They are made up of three main parts: protons, neutrons, and electrons. Imagine that protons and neutrons together form a "sun", or nucleus, at the center of the system. Electrons orbit this nucleus, like planets.
If atoms are unimaginably small, then these are subatomic particles at all. It's funny, but the smallest particle of the three was discovered first - an electron. To understand the difference in size, keep in mind that protons in a nucleus are 1,830 times larger than an electron. Imagine a lollipop in orbit hot air balloon- the discrepancy will be something like this.
But how did we know these particles are there? The answer is that, although small, they have a big impact. The British physicist Thomson, who discovered electrons, used an excellent method to prove their existence in 1897.
He had a Crookes pipe - a ridiculous piece of glass from which most of the air was sucked by the machine. A negative electrical charge was applied to one end of the tube. This charge was enough to knock out some of the electrons from the molecules of the gas remaining in the tube. The electrons are negatively charged, so they flew to the other end of the tube. Thanks to the partial vacuum, the electrons flew through the tube without encountering large atoms in their path.
The electric charge caused the electrons to move very quickly - on the order of 59,500 kilometers per second - until they crashed into the glass at the far end, knocking out even more electrons that were hiding in its atoms. Surprisingly, the collision between these mind-bogglingly tiny particles produced so much energy that it produced a fantastic green-yellow glow.
“It was, in a sense, one of the first particle accelerators,” says Cliff. "It accelerates electrons at one end of the tube to the other, and they slam into the screen at the other end, producing a phosphorescent glow."
Because Thomson discovered that he could manipulate beams of electrons with magnets and electric fields, he knew they weren't just strange beams of light - they were charged particles.
And if you're wondering how these electrons can fly independently of their atoms, it's due to an ionization process in which - in this case - an electrical charge changes the structure of an atom, knocking electrons out into space nearby.
In particular, because electrons are so easy to manipulate and move, electrical circuits have become possible. The electrons in a copper wire travel like a train from one copper atom to another - that's why the wire is transmitted through the wire. Atoms, as we have already said, are not whole pieces of matter, but systems that can be modified or disassembled into structural elements.
The discovery of the electron showed that it was necessary to learn more about atoms. Thomson's work showed that electrons are negatively charged - but he knew that atoms by themselves have no common charge. He suggested that they must contain mysterious positively charged particles in order to offset the negatively charged electrons.
The early 20th century revealed these positively charged particles and at the same time revealed the inner structure of the atom - similar to the solar system.
Ernest Rutherford and his colleagues took a very thin metal foil and placed it under a beam of positively charged radiation - a stream of tiny particles. Most of the powerful radiation went straight through, as Rutherford thought, given the thickness of the foil. But, to the surprise of scientists, part of it bounced off.
Rutherford suggested that the atoms in the metal foil must contain small, dense regions with a positive charge - nothing else would have the potential to reflect such powerful radiation. He discovered positive charges in an atom - and simultaneously proved that they are all bound in a dense mass, unlike electrons. In other words, he demonstrated the existence of a dense nucleus in an atom.
The problem remained. By that time, they could already calculate the mass of the atom. But given the data on how heavy the particles in the nucleus must have been, the idea that they were all positively charged didn't make sense.
“Carbon has six electrons and six protons in its nucleus - six positive charges and six negative charges,” explains Cliff. "But the carbon nucleus doesn't weigh six protons, it weighs the equivalent of 12 protons."
At first, it was suggested that there are six other nuclear particles with the mass of a proton, but negatively charged: neutrons. But nobody could prove it. In fact, neutrons could not be found until the 1930s.
Cambridge physicist James Chadwick was desperate to discover the neutron. He has worked on this theory for many years. In 1932, he managed to make a breakthrough.
Several years earlier, other physicists had experimented with radiation. They launched positively charged radiation - of the type that Rutherford used in his search for a nucleus - into beryllium atoms. Beryllium gave off its own radiation: radiation that was not positively or negatively charged and could penetrate deep into the material.
By this time, others had figured out that gamma radiation was neutral and penetrated deeply, so physicists believed that it was precisely it that was emitted by beryllium atoms. But Chadwick didn't think so.
He independently produced new radiation and directed it at a substance that he knew was rich in protons. Unexpectedly, it turned out that the protons were knocked out of the material as if by particles with an identical mass - like billiard balls by other balls.
Gamma radiation cannot reflect protons in this way, so Chadwick decided that the desired particles must have the mass of a proton, but a different electric charge: and these are neutrons.
All the main particles of the atom have been found, but the story doesn't end there.
Although we learned a lot more about atoms than we knew before, they were difficult to visualize. In the 1930s, no one had any pictures of them - and many people wanted to see them in order to accept their existence.
It's important to note, however, that the methods used by scientists like Thomson, Rutherford, and Chadwick paved the way for new equipment that ultimately helped us produce these images. The electron beams that Thomson generated in his Crookes tube experiment were particularly useful.
Today, such beams are generated by electron microscopes, and the most powerful of these microscopes can actually take pictures of individual atoms. This is because an electron beam has a wavelength thousands of times shorter than a beam of light - so short, in fact, that waves of electrons can bounce off tiny atoms and produce a picture that light beams cannot.
Neil Skipper of University College London says such images are useful for people who want to study the atomic structure of specialty substances, such as those used to make batteries for electric vehicles, for example. The more we know about their atomic structure, the better we are at designing batteries, making them efficient and reliable.
You can also figure out what atoms look like by simply poking at them. This is essentially how atomic force microscopy works.
The idea is to bring the tip of an extremely small probe to the surface of a molecule or substance. With enough proximity, the probe will be sensitive to the chemical structure of what it is pointing to, and the change in resistance as the probe moves will allow scientists to take pictures of, for example, a single molecule.
Skipper adds that many atomic scientists are investigating how the structure of things changes when exposed to high pressure or temperature. Most people know that when a substance heats up, it often expands. It is now possible to detect the atomic changes that occur in this process, which often turns out to be useful.
“When you heat a liquid, you can see how its atoms take on a disordered configuration,” says Skipper. "You can see it directly from the structure map."
Skipper and other physicists can also work with atoms using neutron beams, first discovered by Chadwick in the 1930s.
“We are launching a lot of neutron beams into samples of materials, and from the resulting scattering pattern, you can understand that you are scattering neutrons in nuclei,” he says. "You can roughly estimate the mass and size of the object that was visible through."
But atoms aren't always just there, in a stable state, waiting to be studied. Sometimes they decay - that is, they are radioactive.
There are many naturally occurring radioactive elements. This process generates the energy that formed the basis of nuclear power - and nuclear bombs... Nuclear physicists are generally trying to better understand the reactions that go through fundamental changes like these in the nucleus.
Laura Harkness-Brennan of the University of Liverpool specializes in the study of gamma rays, a type of radiation emitted by decaying atoms. A radioactive atom of a certain type emits special form gamma ray. This means that you can identify atoms only by registering the energy of gamma rays - this, in fact, is what Harkness-Brennan is doing in his laboratory.
“The types of detectors you should use are detectors that will allow you to measure both the presence of radiation and the energy of radiation that has been deposited,” she says. "This is because all cores have a special imprint."
Since all types of atoms can be present in the area where radiation has been detected, especially after a major nuclear reaction, it is important to know exactly which radioactive isotopes are present. Such detection is usually carried out at nuclear plants or in areas where a nuclear disaster has occurred.
Harkness-Brennan and her colleagues are now working on detection systems that can be placed in such locations to show in three dimensions where radiation may be present in a particular room. “You need techniques and tools that will allow you to make a three-dimensional map of space and tell you where in this room, in this pipe,” - she says.
You can also visualize the radiation in the "Wilson chamber". As part of this special experiment, alcohol vapor, cooled to -40 degrees Celsius, is sprayed into a cloud over a radioactive source. Charged radiation particles flying from the radiation source knock electrons out of the alcohol molecules. The alcohol condenses into a liquid near the path of the emitted particles. The results of this type of detection are impressive.
We did not work much directly with atoms - except that we realized that these are beautiful complex structures that can undergo amazing changes, many of which occur in nature. By studying atoms in this way, we improve our own technologies, extract energy from nuclear reactions and we understand better natural world around us. We also got the opportunity to protect ourselves from radiation and study how substances change under extreme conditions.
“Given how small an atom is, it's incredible how much physics we can get out of it,” notes Harkness-Brennan aptly. Everything that we see around us consists of these tiny particles. And it is good to know that they are there, because it is thanks to them that everything around became possible.
Based on materials from the BBC
J. Dalton's theory
The first truly scientific substantiation of the atomistic theory, convincingly demonstrating the rationality and simplicity of the hypothesis that everyone chemical element consists of the smallest particles, was the work of the English school teacher of mathematics J. Dalton (1766-1844), whose article on this problem appeared in 1803. Dalton's atomic postulates had the advantage over the abstract reasoning of ancient Greek atomists that his laws made it possible to explain and link among themselves the results of real experiments, as well as predict the results of new experiments. He postulated that: 1) all atoms of the same element are identical in all respects, in particular, their masses are the same; 2) atoms of different elements have different properties, in particular, their masses are not the same; 3) a compound, unlike an element, includes a certain integer number of atoms of each of its constituent elements; 4) in chemical reactions, a redistribution of atoms can occur, but not a single atom is destroyed or created again. (In fact, as it turned out at the beginning of the 20th century, these postulates are not quite strictly fulfilled, since atoms of the same element can have different masses, for example, hydrogen has three such varieties, called isotopes; in addition, atoms can undergo radioactive transformations and even completely collapse, but not in the chemical reactions considered by Dalton.) Based on these four postulates, Dalton's atomic theory gave the simplest explanation of the laws of constant and multiple ratios. However, she did not give any idea about the structure of the atom itself.
Brownian motion
Scottish botanist Robert Brown conducted pollen studies in 1827. He, in particular, was interested in how pollen participates in the fertilization process. Once he examined elongated cytoplasmic grains suspended in water, isolated from pollen cells under a microscope. Suddenly, Brown saw that the smallest solid grains, which could hardly be seen in a drop of water, were constantly trembling and moving from place to place. He found that these movements, in his words, "are not associated either with flows in the liquid, or with its gradual evaporation, but are inherent in the particles themselves." The phenomenon observed by Brown was called "Brownian motion." The explanation of Brownian motion by the motion of invisible molecules was given only in the last quarter XIX century, but it was far from immediately accepted by all scientists. In 1863, the teacher of descriptive geometry Ludwig Christian Wiener (1826-1896) suggested that the phenomenon is associated with the oscillatory movements of invisible particles.
Discovery of the electron
The real existence of molecules was finally confirmed in 1906 by experiments on studying the laws of Brownian motion of the French physicist Jean Perrin.
During the period when Perrin carried out his research on cathode and x-rays, a consensus has not yet been reached regarding the nature of the cathode rays emitted by the negative electrode (cathode) in a vacuum tube during an electric discharge. Some scientists believed that these rays were a form of light radiation, but in 1895 Perrin's research showed that they are a stream of negatively charged particles. Atomic theory held that elements are made up of discrete particles called atoms, and that chemical compounds are made up of molecules, larger particles containing two or more atoms. By the end of the XIX century. atomic theory has gained wide acceptance among scientists, especially among chemists. However, some physicists believed that atoms and molecules were nothing more than fictitious objects, which were introduced for reasons of convenience and useful in the numerical processing of the results of chemical reactions.
Joseph John Thomson, modifying Perrin's experiment, confirmed his conclusions and in 1897 determined the most important characteristic of these particles by measuring the ratio of their charge to mass by the deviation in the electrical and magnetic fields... The mass turned out to be about 2 thousand times less than the mass of the hydrogen atom, the lightest of all atoms. It soon became widely believed that these negative particles, called electrons, were component part atoms.
The process of cognition develops in such a way that brilliant guesses and great theories, the appearance of which we owe to creative geniuses, after a while become almost trivial facts that most people take on faith. How many of us could independently, on the basis of observation and reflection, guess that the Earth is round or that the Earth revolves around the Sun, and not vice versa, and finally, that there are atoms and molecules? From high modern science the main provisions of the atomic-molecular theory seem to be common truths. Let us, however, digress from the long-known scientific results, put ourselves in the place of scientists of the past and try to answer two main questions. First, what are substances made of? Secondly, why are substances different and why can some substances turn into others? Science has already spent over 2,000 years solving these complex issues. As a result, the atomic-molecular theory appeared, the main provisions of which can be formulated as follows.
- 1. All substances are made up of molecules. A molecule is the smallest particle of a substance that has its chemical properties.
- 2. Molecules are made up of atoms. An atom is the smallest particle of an element in chemical compounds. Different atoms correspond to different elements.
- 3. Molecules and atoms are in continuous motion.
- 4. During chemical reactions, the molecules of some substances are converted into molecules of other substances. Atoms do not change during chemical reactions.
How did scientists figure out the existence of atoms?
Atoms were invented in Greece in the 5th century. BC NS. The philosopher Leucippus (500-440 BC) wondered if every particle of matter, no matter how small, could be divided into even smaller particles. Leucippus believed that as a result of such a division it is possible to obtain such a small particle that further division would become impossible.
A student of Leucippus, the philosopher Democritus (460-370 BC) called these tiny particles "atoms" (atomos - indivisible). He believed that the atoms of each element have special sizes and shapes and that this explains the differences in the properties of substances. Substances that we see and feel are formed when atoms join together. various elements, and by changing the nature of this compound, one can transform one substance into another.
Democritus created the atomic theory in an almost modern form. However, this theory was only the fruit of philosophical reflections not related to natural phenomena and processes. It was not confirmed experimentally, since the ancient Greeks did not conduct experiments at all, they put thinking above observation.
The first experiment confirming the atomic nature of matter was carried out only 2,000 years later. In 1662, the Irish chemist Robert Boyle (1627-1691), while compressing air in a U-shaped tube under the pressure of a column of mercury, found that the volume of air in the tube is inversely proportional to the pressure:
The French physicist Edm Marriott (1620-1684) confirmed this relationship 14 years after Boyle and noted that it only holds at constant temperature.
The results obtained by Boyle and Mariotte can be explained only if it is recognized that air is composed of atoms, between which there is empty space. Compression of air is caused by the approach of atoms and a decrease in the volume of empty space.
If gases are composed of atoms, it can be assumed that solids and liquids are also composed of atoms. For example, when heated, water boils and turns into steam, which, like air, can be compressed. This means that water vapor consists of atoms. But if water vapor is made of atoms, why can't liquid water and ice be made of atoms? And if this is true for water, it may be true for other substances as well.
Thus, the experiments of Boyle and Mariotte confirmed the existence of the smallest particles of matter. It remained to find out what these particles are.
Over the next 150 years, the efforts of chemists were mainly aimed at establishing the composition of various substances. Substances that decomposed into less complex substances were called compounds (complex substances), for example, water, carbon dioxide, iron scale. Substances that cannot be decomposed are called elements (simple substances), for example hydrogen, oxygen, copper, gold.
In 1789, the great French chemist Antoine Laurent Lavoisier (1743-1794) published the famous book "Elementary Course in Chemistry" (Traite elementaire de chimie), in which he systematized the knowledge of chemistry accumulated by that time. In particular, he gave a list of all known elements, which contained 33 substances. Two names in this list were fundamentally erroneous (light and caloric), and eight later turned out to be complex substances (lime, silica, and others).
The development of techniques for quantitative measurements and methods of chemical analysis made it possible to determine the ratio of elements in chemical compounds. The French chemist Joseph Louis Proust (1754-1826), after careful experiments with a number of substances, established the law of constancy of composition.
I All compounds, regardless of the method of preparation, contain ele-. cops in strictly defined weight proportions.
So, for example, sulfur dioxide obtained by burning sulfur, the action of acids on sulfites, or in any other way, always contains 1 part by weight (mass fraction) of sulfur and 1 part by weight of oxygen.
Proust's opponent, the French chemist Claude Louis Berthollet (1748-1822), on the contrary, argued that the composition of the compounds depends on the method of their preparation. He believed that if in the reaction of two elements one of them is taken in excess, then in the resulting compound the weight fraction of this element will also be greater. Proust, however, proved that Berthollet received erroneous results due to inaccurate analysis and the use of insufficiently pure substances.
Surprisingly, Berthollet's idea, erroneous for its time, is currently the basis of a large scientific direction in chemistry - chemical materials science. The main task of materials scientists is to obtain materials with specified properties, and the main method is to use the dependence of the composition, structure and properties of a material on the production method.
The law of constancy of composition, discovered by Proust, was of fundamental importance. He led to the idea of the existence of molecules and confirmed the indivisibility of atoms. Indeed, why is the weight (mass) ratio of sulfur and oxygen always 1: 1, and not 1.1: 0.9 or 0.95: 1.05 in sulfur dioxide S0 2? It can be assumed that during the formation of a particle of sulfur dioxide (later this particle was called a molecule), a sulfur atom combines with a certain number of oxygen atoms, and the mass of sulfur atoms is equal to the mass of oxygen atoms.
But what happens if two elements can form several chemical compounds with each other? This question was answered by the great English chemist John Dalton (1766-1844), who from the experiment formulated law of multiple relations (Dalton's law).
I If two elements form several connections between themselves, then. in these compounds, the masses of one element per unit mass of the other element are referred to as small whole numbers.
So, in three iron oxides, per unit weight (mass) of oxygen, there are 3.5, 2.625 and 2.333 weight parts (mass fractions) of iron, respectively. The ratios of these numbers are as follows: 3.5: 2.625 = = 4: 3; 3.5: 2.333 = 3: 2.
From the law of multiple ratios, it follows that the atoms of the elements are combined into molecules, and the molecules contain a small number of atoms. Measurement of the mass content of elements allows, on the one hand, to determine molecular formulas compounds, and on the other - to find the relative masses of atoms.
For example, when water is formed, one part by weight of hydrogen combines with 8 parts by weight of oxygen. If we assume that a water molecule consists of one hydrogen atom and one oxygen atom, it turns out that an oxygen atom is 8 times heavier than a hydrogen atom.
Consider the inverse problem. We know that an iron atom is 3.5 times heavier than an oxygen atom. From the ratio
it follows that in this compound there are three oxygen atoms per two iron atoms, that is, the formula of the compound is Fe 2 0 3.
Reasoning in this way, Dalton compiled the first ever table of atomic weights of elements. Unfortunately, it turned out to be incorrect in many ways, since Dalton often proceeded from incorrect molecular formulas in determining atomic weights. He believed that the atoms of the elements almost always (with rare exceptions) are combined in pairs. Dalton water formula - NO. In addition, he was sure that the molecules of all simple substances contain one atom each.
The correct formulas for water and many other substances were determined by studying chemical reactions in the gas phase. The French chemist Joseph Louis Gay-Lussac (1778-1850) discovered that one volume of hydrogen reacted with one volume of chlorine to produce two volumes of hydrogen chloride; during the electrolytic decomposition of water, one volume of oxygen and two volumes of hydrogen are formed, etc. This rule of thumb was published in 1808 and received the name the law of volumetric relations.
I The volumes of the reacting gases refer to each other and to the volumes of the gas. shaped reaction products as small whole numbers.
The meaning of the law of volumetric relations became clear after the great discovery of the Italian chemist Amedeo Avogadro (1776-1856), who formulated a hypothesis (assumption), which was later called Avogadro's law.
| In equal volumes of any gases at constant temperature and pressure? lelen contains the same number of molecules.
This means that all gases behave in a certain sense in the same way and that the volume of a gas under given conditions does not depend on the nature (composition) of the gas, but is determined only by the number of particles in a given volume. By measuring the volume, we can determine the number of particles (atoms and molecules) in the gas phase. The great merit of Avogadro is that he was able to establish a simple connection between the observed macroscopic quantity (volume) and the microscopic properties of gaseous substances (the number of particles).
Analyzing the volumetric ratios found by Gay-Lussac, and using his hypothesis (which was later called Avogadro's law), the scientist found that the molecules of gaseous simple substances (oxygen, nitrogen, hydrogen, chlorine) are diatomic. Indeed, when hydrogen reacts with chlorine, the volume does not change; therefore, the number of particles does not change either. If we assume that hydrogen and chlorine are monoatomic, the initial volume should be halved as a result of the addition reaction. But after the reaction, the volume does not change, which means that the hydrogen and chlorine molecules contain two atoms each and the reaction proceeds according to the equation
Molecular formulas can be established similarly complex substances- water, ammonia, carbon dioxide and other substances.
Oddly enough, but contemporaries did not appreciate and did not recognize the conclusions drawn by Avogadro. The leading chemists of that time J. Dalton and Jens Jacob Berzelius (1779-1848) objected to the assumption that the molecules of simple substances could be diatomic, since they believed that molecules are formed only from different atoms (positively and negatively charged). Under pressure from such authorities, Avogadro's hypothesis was rejected and gradually forgotten.
Only almost 50 years later, in 1858, the Italian chemist Stanislao Cannizzaro (1826-1910) accidentally discovered Avogadro's work and realized that it clearly distinguishes between the concepts of "atom" and "molecule" for gaseous substances. It was Cannizzaro who proposed the definitions of the atom and the molecule, which are given at the beginning of this paragraph, and made the concepts of "atomic weight" and "molecular weight" completely clear. In 1860, the First International Chemical Congress was held in Karlsruhe (Germany), at which, after long discussions, the main provisions of the atomic-molecular theory were universally recognized.
Let's summarize. Three fundamental stages can be distinguished in the development of atomic-molecular teaching.
- 1. The birth of atomic doctrine, the emergence of an idea (hypothesis) about the existence of atoms (Leucippus and Democritus).
- 2. First experimental confirmation atomic theory in experiments with compressed air (Boyle-Mariotte law).
- 3. The discovery of an important regularity that atoms of different elements are present in a molecule in certain weight ratios (Dalton's law of multiple ratios), and the establishment of formulas for gaseous simple substances (Avogadro's hypothesis).
Interestingly, when the assumption was made about the existence of atoms, the theory was ahead of the experiment (first atoms were invented, and after 2000 years it was proved). In the case of molecules, experiment outpaced theory: the idea of the existence of molecules was put forward to explain the experimental law of multiple ratios. In this sense, the history of atomic-molecular theory is a typical example that reflects different paths of scientific discoveries.
Albert Einstein
Often, only the creation of the theory of relativity is considered to be the merit of Albert Einstein. From the point of view of the history of science, such an assessment is incorrect and unfair in relation to his remarkable achievements in other areas of physics. The "father of the theory of relativity" was a scientist with extremely multifaceted interests.
In the Bernese years, at the time of the most stormy creative activity of Einstein, almost simultaneously the first results of his research appeared, which were of great importance for further development physics. The year 1905 turned out to be especially fruitful, when Einstein was 26 years old. Chronologically, the first was his research in molecular physics.
Einstein's work on thermal motion was mainly devoted to the problem of the statistical description of the motion of atoms and molecules and the relationship between motion and heat. In these works, Einstein came to conclusions that significantly expand the results obtained by the brilliant Austrian physicist Ludwig Boltzmann and the American Willard Gibbs. Einstein's main merit was not so much in overcoming mathematical difficulties as in a deeper formulation of physical problems. He was guided in this by Boltzmann's idea that the concept of probability ("Boltzmann's principle") should underlie the mathematical interpretation of the theory of heat.
All these questions were developed by Einstein independently, so we have the right, together with Max Born, to say that "Einstein rediscovered all the essential features of statistical mechanics." The young researcher began his work in molecular physics with firm intention to confirm with reliable results the atomistic theory, in the correctness of which he was convinced, although at that time it seemed to many to be controversial.
Einstein's focus in his research work according to the theory of heat, Brownian molecular motion was found. In 1827, the English botanist Robert Brown observed pollen under a microscope; at the same time, he discovered that particles suspended in a drop of liquid continuously perform irregular, zigzag movements. This movement of particles - later named after the scientist who discovered it "Brownian motion" - occurs the more intensively, the smaller the mass of the particles and the warmer the liquid in which they are located.
For several decades, scientists have tried unsuccessfully to find an explanation for this mysterious phenomenon. In the 1880s - two decades before Einstein - a French physicist suggested that Brownian motion was the result of random impacts by suspended particles from liquid molecules invisible under a microscope. However, this ingenious explanation had no mathematical justification or experimental confirmation.
In the article "On the motion of particles suspended in a fluid at rest, which follows from the molecular kinetic theory", Einstein with the help of statistical methods showed that there is a quantitative relationship between the speed of movement of suspended particles, their size and the coefficient of viscosity of the liquid used, which can be experimentally verified.
Einstein, who at that time was not yet familiar with the previous work on Brownian motion, believed that the movement of particles visible under a microscope is a manifestation of the movement of microscopically invisible molecules of a liquid. Einstein gave a complete mathematical form to the statistical explanation of this phenomenon, already formulated before him by the Polish physicist Marian von Smoluchowski. "Einstein's law of Brownian motion" was fully confirmed in 1908 by the experiments of the French physicist Jean Perrin, who received the Nobel Prize for this work in 1926.
Einstein's work on molecular physics proved the correctness of the idea that heat is a form of energy in the disordered motion of molecules. At the same time, they reinforced the atomistic hypothesis that matter — in the physical sense — consists of molecules and atoms.
Einstein's proposed method for determining the size of molecules and his formula for Brownian motion make it possible to determine the number of molecules. Prior to this, physicists had to get by with approximate methods proposed in 1865 by the Austrian physicist Loschmidt; now, thanks to Einstein's research, they could operate with precise mathematical methods.
Along with the purely scientific value, Einstein's research on thermal motion was of great theoretical and cognitive significance. They showed that the negative or skeptical attitude of some natural scientists to the atomistic theory is not justified in any way. Einstein's proof of the correctness of atomic views was so convincing that the chemist Wilhelm Ostwald, who had previously been a stubborn opponent of the doctrine of atoms with Ernst Mach, now, in his own words, "was converted to the atomic faith."
The decisive contribution that Einstein made to the victory of atomism should be considered one of his greatest scientific achievements. In this he is a worthy successor to the great materialists of antiquity: Democritus, Epicurus and Lucretius.
Friedrich Gerneck, 1984
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