Education, Secondary education and schools
The atom in chemistry is ... The model of the atom. The structure of the atom
Thoughts about the essence of the whole environment began to visit humanity long before the heyday of modern civilization. At first people thought about the existence of certain higher forces, which, they believed, predetermined all being. But very soon philosophers and clergymen began to think about what, in fact, is the very fabric of this being. There were many theories, but in the historical perspective the atomic one became dominant.
The founder of the atomic theory
Where does the first chemistry lesson start? The structure of the atom is the main theme. You probably remember that the word "atom" is translated from the ancient Greek language as "indivisible." Now many historians believe that the first to put forward a theory that said about some of the smallest particles, of which everything is composed, Democritus. He lived in the fifth century BC.
To the great regret, practically nothing is known about this outstanding thinker. Not a single written source of those times has reached us. Therefore, we have to learn about the ideas of the greatest scientist of our time exclusively from the works of Aristotle, Plato, and also other ancient Greek thinkers.
So, our theme is "The structure of the atom." In chemistry, not everyone had high marks, but many remember that all the conclusions of ancient scientists were built solely on inferences. Democritus was not an exception.
How did Democritus reason?
His logic was extremely simple, but at the same time brilliant. Imagine that you have the sharpest knife in the whole world. You take an apple, for example, and then begin to cut it: into two halves, to quarters, they are divided again ... In a word, sooner or later you will get slices of such scanty thickness that you can not further divide them. This will be an indivisible atom. In chemistry, this claim was considered true almost before the end of the 19th century.
From Democritus to modern ideas
It should be noted that only the word "atom" has survived from the ancient Greek notions of the microworld. Now every schoolboy knows that the world around us consists of much more fundamental and smaller particles. Moreover, from the point of view of modern science the theory of Democritus was nothing more than a purely hypothetical compilation, not supported by any evidence at all. However, in those days there were no electron microscopes, so to prove their case in other ways, the thinker would still fail.
The first suspicions that Democritus is actually right, appeared in chemists. They quickly discovered that many substances break down into simpler components during the reactions. In addition, it was the chemists who deduced the strict laws of these processes. So, they noticed that eight parts of oxygen and one hydrogen are required to produce water (Avogadro's law).
In the Middle Ages, any materialistic teaching, including the theory of Democritus, propagation and development could not be obtained in principle. And only in the XVIII century, scientists again return to the atomistic theory. By that time, the chemist A. Lavoisier, our great MV Lomonosov, and the talented English physicist D. Dalton (whom we will discuss separately) have already convincingly proved to their colleagues the reality of the existence of atoms. It should be emphasized that even in the enlightened 18th century, the atomic theory was not seriously considered by many outstanding minds of that time for a long time.
Whatever it was, but even these great scientists have not yet put forward theories about the structure of the atom itself, since it was considered a single and indivisible particle, the basis of everything.
Unfortunately, chemical experiments could not clearly demonstrate the reality of the transformation of the atoms of certain substances into others. But still, chemistry was the fundamental science in studying the structure of atoms. Atoms and molecules have long been studied by one brilliant Russian scientist, without which one can not imagine modern science.
The doctrine of DI Mendeleyev
But Mendeleyev's teaching is valuable not only for this. Until now, it remains unclear how the atoms of various chemical elements were formed. But the great Russian scientist was able to convincingly prove that all of them without exception are closely related to each other.
The discovery of Dalton
But to be able to interpret multiple disparate data was able only John Dalton, whose name is forever imprinted in an open by himself law. Usually the scientist studied only the behavior of gases, but his range of interests was much wider. In 1808, he began publishing his new fundamental work.
It was Dalton who suggested that each chemical element corresponds to a certain atom. But the scientist, like Democritus for many centuries before him, still believed that they are completely indivisible. In his drafts a lot of schematic drawings, on which the atoms are presented in the form of simple balls. This idea, which originated more than 2500 years ago, existed almost to our time! However, only very recently a really deep structure of the atom was discovered. Chemistry (9th grade in particular) even today is largely guided by ideas that were first voiced in the 18th century.
Experimental confirmation of the divisibility of atoms
However, until the end of the 19th century, almost all scientists believed that the atom - the limit beyond which there is nothing. They thought that the basis of the whole universe is he. Various experiments contributed to this: no matter how cool, only the molecules changed, while nothing was happening with the atoms of the substances, which the simplest chemistry could not explain. The structure of the carbon atom, for example, remains completely unchanged even in different allotropic states.
In a word, for a long time there was not exactly any experimental data, which at least indirectly confirmed the suspicions of some scientists that there are some more fundamental particles. Only in the 19th century (not least thanks to the experiments of the Curie couple) it was proved that under certain conditions the atoms of some elements can be transformed into others. These discoveries formed the basis for modern ideas about the world around us.
Raisins and puddings
In 1897, J. Thomson, an English physicist, found that in any atom there is a certain amount of negatively charged particles, which he called "electrons." Already in 1904, the scientist created the first atomic model, which is better known as "pudding with raisins." The name accurately reflects the essence. Judging by Thomson's theory, an atom in chemistry is a kind of "vessel" with a uniformly distributed charge and electrons in it.
Note that such a model had circulation even in the 20th century. Later it turned out that she was completely wrong. But still it was the first conscious attempt of a man (and on a scientific basis) to recreate the surrounding microcosm, suggesting an atomic model that is simple enough and intuitive.
The Curie Experiments
It is generally accepted that the couple Pierre and Marie Curie initiated atomic physics. Of course, the contribution of these brilliant people, who actually sacrificed their health and life, can not be underestimated, but their experiments were of far more fundamental importance. Almost simultaneously with Rutherford, they proved that the atom is a much more complex and heterogeneous structure. The very phenomenon of radioactivity, which they investigated, it is about this and said.
"Planetary" approach
Finally, Rutherford decided to bombard the atoms of heavy metals with α-particles (fully ionized helium). The scientist immediately assumed that light electrons can not change the particle trajectory in any way. Accordingly, the scattering can only cause some heavier elements, which can be contained in the nucleus of the atom. Just note that initially Rutherford did not pretend to change the theory of "pudding". This model of the atom was considered irreproachable.
So the result, when almost all the particles passed through a thin layer of silver without problems, it did not surprise him. But it soon became clear that some helium atoms were deflected immediately by 30 °. This was not at all what chemistry was talking about at the time. The composition of the atom according to Thomson assumed a uniform distribution of electrons. But this was clearly contradicted by the observed phenomena.
Extremely rare, but still some particles flew at an angle of even 180 °. Rutherford was in the deepest perplexity. After all, this sharply contradicted the "pudding", the charge in which it was supposed to be (according to Thomson's theory) distributed uniformly. Consequently, unevenly charged sections that could repel ionized helium should have been absent.
What conclusions did Rutherford come to?
These circumstances also led the scientist to the idea that the atom is practically empty and only in the center is concentrated some formation with a positive charge - the nucleus. So there arose the planetary model of the atom, the postulates of which are as follows:
- As we have already said, in the central part the core is located, and its volume (relative to the size of the atom itself) is negligible.
- Almost the entire atomic mass, as well as the entire positive charge, is in the nucleus.
- Around it rotate electrons. It is important that their number is equal to the value of the positive charge.
The Paradoxes of Theory
Of course, nothing like this happens in reality. Any atom is not only completely stable, but there can be absolutely unlimited time, and no radiation from it will not go. This discrepancy is due to the fact that we try to apply laws to the microworld that are valid only with respect to classical mechanics. They, as it turned out, are completely inapplicable to phenomena of atomic scale. Therefore, the authors of the textbooks try to explain the structure of the atom (chemistry, 11th grade) in as simple a way as possible.
The Doctrine of Bohr
The Danish physicist Niels Bohr proved that it is impossible to spread the same laws to the microcosm, the provisions of which are valid for macroscopic objects. It is he who owns the idea that the microcosm is "guided" exclusively by quantum laws. Of course, then there was no quantum theory at all, but Bohr actually became its ancestor, expressing his thoughts in the form of three postulates that "saved" an atom that would inevitably have died if it had "lived" according to Rutherford's theory. It was this theory of the Dane that formed the basis of all quantum mechanics.
Posterates of Bora
- The first of them says: any atomic system can only be in special atomic states, and for each of them a certain value of energy (E) is characteristic. If the state of the atom is stationary (calm), then it can not radiate.
- The second postulate says that the emission of light energy occurs only in the case of transition from a state with a higher energy to a more moderate one. Accordingly, the energy released is equal to the difference in values between two stationary states.
Model of the Niels Bohr atom
This semiclassical theory was proposed by the scientist in 1913. It is noteworthy that it was based on Rutherford's planetary model, which shortly before him described the atom of matter. We have already said that classical mechanics contradicted Rutherford's calculations: proceeding from it, it was assumed that with time the electron must necessarily fall to the surface of the atom.
To "get around" this contradiction, the scientist introduced a special assumption. Its essence was that to emit energy (which should lead to their fall), electrons can only move in some definite orbits. While moving them along other trajectories, the allegedly chemical atoms remained in a passive state. According to Bohr's theory, those orbits were those whose quantitative motion was equal to Planck's constants.
Quantum theory of atomic structure
1. First, the duality (corpuscular-wave nature) of the electron itself. Simply put, this particle behaves like a material object (corpuscle), and like a wave. As a particle, it has a certain charge and mass. The ability to diffraction is related to electrons with classical waves. The length of this wave (λ) and the velocity of the particle (v) can be related to each other by a special De Broglie relation: λ = h / mv. As you might guess, m is the mass of the electron itself.
2. It is completely impossible to measure the coordinate and velocity of a particle with absolute accuracy. The more accurately the coordinate is determined, the higher the uncertainty in speed. As, however, and vice versa. This phenomenon was called the Heisenberg uncertainty, which can be expressed as the following relationship: Δx ∙ m ∙ Δv> ћ / 2. The delta X (Δx) expresses the uncertainty of the position of the coordinate in space. Accordingly, the delta V (Δv) displays the velocity errors.
3. Contrary to all the previously prevalent opinions, electrons do not pass in strictly defined orbits, like trains on rails. Quantum theory says that an electron can be anywhere in space, but the probability of this is different for each segment.
That part of space around the atomic nucleus itself, in which this probability is maximum, is called the orbital. Modern chemistry studies the structure of electron shells of atoms precisely from this point of view. Of course, schools teach the correct distribution of electrons by levels, but, in all likelihood, in reality they differ quite differently.
4. The nucleus of an atom consists of nucleons (protons and neutrons). The ordinal number of an element in a periodic system indicates the number of protons in its nucleus, and the sum of protons and neutrons is equal to the atomic mass. This is how the chemistry of modernity explains the structure of the nucleus of the atom.
The founders of quantum mechanics
How far did chemistry go? The structure of the atom most of the chemists of those years was considered quite simple: many only by 1947 finally recognized the reality of the existence of elementary particles.
Some conclusions
In general, the creation of quantum theory has not been without mathematicians, since all these processes can be calculated only using the most complicated calculations. But the main difficulty is not this. Those processes that are described by this theory are inaccessible not only to our senses, despite all modern scientific techniques, but also to the imagination.
No one can even even approximately imagine the processes in the microcosm, since they are completely unlike all those phenomena that we observe in the macrocosm. Just think: recent discoveries suggest that quarks, neutrinos and other fundamental particles exist in a nine-dimensional (!) Dimension. How can a person living in a three-dimensional space even roughly describe their behavior?
At the moment, we can only rely on the mathematics and power of modern computers, which, perhaps, will be used to model the microcosm. Chemical chemistry also helps a lot: the structure of the atom is likely to be reconsidered after recently scientists working in this field reported on the discovery of a new type of chemical bond.
Modern concept of the structure of the atom
If you carefully read all of the above, then for sure you yourself can tell what the current view of the structure of atoms of substances is. But still we will explain: this is a slightly modified theory of Rutherford, supplemented by the priceless postulates of Niels Bohr. Simply put, today it is believed that the electrons move along chaotic, blurred trajectories near the nucleus, which consists of neutrons and protons. That part of the space around it, in which the appearance of the electron is most likely, is called the orbital.
It is not yet possible to say exactly how our ideas about the structure of the atom will change in the future. Every day scientists work on penetrating the secrets of the microworld: the LHC (the Large andron Collider), the Nobel Prizes in physics - all this is the result of these studies.
But even now we do not even present an approximate picture of what the atoms are still hiding. It is clear only that the atom itself in the scale of the microworld is a huge apartment building, in which we examined only the first floor, and even then not completely. Almost every year there are reports of the possibility of discovering more and more elementary particles. When the process of studying the atoms will be completely finished, today nobody will predict.
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