Thoughts about the essence of everything around us 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 pretty soon, philosophers and clergy began to think about what, in fact, the very fabric of this being consists of. There were many theories, but in the historical perspective, atomic became dominant.
What is an atom in chemistry? This, as well as all related topics, we will discuss in the framework of this article. We hope that in it you will find answers to all your questions.
The founder of atomic theory
What does the first chemistry lesson begin with? The structure of the atom is the main topic. You probably remember that the word "atom" is translated from the ancient Greek language as "indivisible". Now many historians believe that he was the first to put forward a theory that spoke of some of the smallest particles that make up everything, Democritus. He lived in the fifth century BC.
Unfortunately, almost 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, as well as other ancient Greek thinkers.
So, our topic 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 no 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 start to cut it: into two halves, into quarters, divide them again ... In short, sooner or later you will get slices of such a meager thickness that it will no longer be possible to divide them. This will be the indivisible atom. In chemistry, this statement was considered true almost until the end of the 19th century.
From Democritus to Modern Views
It should be noted that from the ancient Greek ideas about the microworld only the word “atom” was preserved. Now every schoolchild knows that the world around us consists of much more fundamental and small particles. In addition, from the point of view of modern science, the theory of Democritus was nothing more than a purely hypothetical calculation, not supported by absolutely no evidence. However, in those days there were no electron microscopes, so the thinker would not have succeeded in proving his case in other ways.
The first suspicions that Democritus was actually right appeared among chemists. They quickly discovered that many substances break down into simpler components during reactions. In addition, it was chemists who deduced the strict laws of these processes. So, they noticed that to obtain water, eight mass fractions of oxygen and one mass of hydrogen are required (Avogadro's law).
In the Middle Ages, any materialistic teaching, including the theory of Democritus, could not be spread and developed in principle. And only in the XVIII century, scientists again returned to atomic theory. By that time, chemist A. Lavoisier, our great MV Lomonosov and the most talented English physicist D. Dalton (which 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, atomic theory was not seriously considered by many outstanding minds of that time.
Be that as it may, 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 foundation of everything.
Unfortunately, chemical experiments could not clearly demonstrate the reality of the transformation of atoms of some substances into others. But nevertheless, chemistry became the fundamental science in the study of the structure of atoms. Atoms and molecules have long been studied by one brilliant Russian scientist, without whom modern science cannot be imagined.
Teaching D.I. Mendeleev
A huge role in the formation of atomic theory was played by D. I. Mendeleev, who as early as 1869 created his ingenious periodic system. For the first time, a theory was presented to the scientific community that not only did not reject, but reasonably supplemented all the assumptions of the materialists. Already in the 19th century, scientists were able to prove the existence of electrons. All these conclusions led the best minds of the 20th century to seriously study the atom. In chemistry, this time was also marked by many discoveries.
But Mendeleev’s teaching is valuable not only for this. It remains unclear how exactly the atoms of various chemical elements formed. But the great Russian scientist was able to convincingly prove that all of them, without exception, are closely related to each other.
Dalton's discovery
But only John Dalton, whose name is forever imprinted in the law he himself discovered, is able to interpret multiple disparate data. Usually, a 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 specific atom. But the scientist, like Democritus many centuries before him, nevertheless believed that they were completely indivisible. In his drafts there are a lot of sketchy drawings in which atoms are represented as simple balls. This idea, which originated more than 2500 years ago, existed almost until our time! However, only relatively recently, a really deep atomic structure was discovered. Chemistry (grade 9 in particular) even today is largely guided by those ideas that were first voiced in the 18th century.
Experimental confirmation of atomic fissility
However, until the end of the 19th century, almost all scientists believed that the atom is the limit beyond which there is nothing. They thought that it was he who was the basis of the entire universe. Various experiments contributed to this: whatever one may say, only the molecules changed, while nothing happened to the atoms of the substances themselves, which could not be explained by simple chemistry. 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 absolutely no experimental data that at least indirectly confirmed the suspicions of some scientists that there were any more fundamental particles. Only in the 19th century (not least due to the experience of the Curie spouses) it was proved that under certain conditions the atoms of some elements can turn into others. These discoveries formed the basis of modern ideas about the world around us.
Raisins and Puddings
In 1897, J. Thomson, an English physicist, found that any atom has a certain number of negatively charged particles, which he called "electrons." Already in 1904, the scientist created the first atomic model, which is better known under the designation "pudding with raisins." The name quite accurately reflects the essence. Judging by Thomson's theory, an atom in chemistry is a kind of “vessel” with a charge and electrons evenly distributed in it.
Note that a similar model was in circulation even in the 20th century. Subsequently, it turned out that she was completely wrong. But still, this was the first conscious attempt by a person (and on a scientific basis) to recreate the microcosm surrounding him, proposing a model of the atom that is quite simple and clear.
Curie's experiences
It is believed that the spouses Pierre and Marie Curie laid the foundation for atomic physics. Of course, the contribution of these ingenious people, who actually sacrificed their health and life, can not be underestimated, but their experiences were much more fundamental. 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, spoke of exactly this.
Already at the beginning of 1898, Maria published the first article on radiation. Soon, Maria and
Pierre Curie proved that other substances, the existence of which official chemistry doubted, appeared in a mixture of chloride compounds of uranium and radium. The structure of the atom has since begun to be investigated closely.
"Planetary" approach
Finally, Rutherford decided to bombard heavy metal atoms with α particles (fully ionized helium). The scientist immediately suggested that light electrons could not change the particle trajectory. Accordingly, only some heavier elements that can be contained in the nucleus of an atom can cause scattering. Just note that initially Rutherford did not claim to change the theory of "pudding." This model of the atom was considered impeccable.
And therefore the result, in which almost all the particles passed through a thin layer of silver without problems, did not surprise him. It only soon became clear that some helium atoms were deflected immediately by 30 °. It was not at all what chemistry was talking about at that time. The atom composition according to Thomson assumed a uniform distribution of electrons. But this was clearly contrary to the observed phenomena.
Extremely rare, but still some particles flew away at an angle of even 180 °. Rutherford was deeply perplexed. After all, this sharply contradicted the “pudding”, in which the charge was supposed to be (according to Thomson's theory) evenly distributed. Consequently, unevenly charged areas that could repel ionized helium should be absent.
What conclusions did Rutherford come to?
These circumstances prompted the scientist to the idea that the atom is practically empty and only in the center is some kind of formation with a positive charge - the nucleus. And so a planetary model of the atom arose , the postulates of which are as follows:
- As we already said, the core is located in the central part, 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 located in the nucleus.
- Electrons revolve around him. It is important that their number is equal to the value of the positive charge.
The paradoxes of theory
Everything would be fine, but this model of the atom does not explain their incredible stability. It should be remembered that electrons move in their orbits with tremendous acceleration. According to all laws of electrodynamics, such an object should lose its charge over time. If we take into account the postulates of Newton and Maxwell, then the electrons in general should be strewed onto the nucleus, like a hail to the earth.
Of course, nothing of the kind happens in reality. Any atom is not only completely stable, but also can exist for absolutely unlimited time, and no radiation will come from it. This discrepancy is explained by the fact that we are trying to apply laws to the microcosm that are valid only in relation to classical mechanics. They, as it turned out, are completely inapplicable to phenomena of an atomic scale. Therefore, the structure of the atom (chemistry, grade 11), the authors of textbooks try to explain as simple as possible words.
Teaching of Bohr
It was proved by the Danish physicist Niels Bohr that the same laws cannot be applied to the microworld, the provisions of which are valid for macroscopic objects. It is he who belongs to the idea that the microworld is “guided” exclusively by quantum laws. Of course, then quantum theory itself did not exist at that time, but Bohr actually became its ancestor, expressing his thoughts in the form of three postulates that “saved” an atom that would inevitably die if it “lived” according to Rutherford's theory. It was this theory of the Dane that formed the basis of all quantum mechanics.
Postulates of Bohr
- The first of them says: any atomic system can only be in special atomic states, and each of them is characterized by a certain value of energy (E). If the state of the atom is stationary (calm), then it cannot emit.
- The second postulate says that the emission of light energy occurs only in the case of a transition from a state with a higher energy to a more moderate one. Accordingly, the energy released is equal to the difference between the two stationary states.
Niels Bohr Atom Model
The scientist proposed this semiclassical theory in 1913. It is noteworthy that he based it on the planetary model of Rutherford, who 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 should necessarily fall onto the surface of the atom.
To "get around" this contradiction, the scientist introduced a special assumption. Its essence was that electrons can radiate energy (which should have led to their fall) only by moving in certain specific orbits. When they moved along other trajectories, the supposedly chemical atoms remained in a passive state. According to Bohr's theory, such orbits were those whose quantitative moment of motion along which was equal to Planck's constants.
Quantum Theory of Atom Structure
As we have already said, today the quantum theory of the structure of the atom is in use. Chemistry of recent years is guided exclusively by it. It is based on four fundamental axioms.
1. First, the duality (particle-wave nature) of the electron itself. Simply put, this particle behaves both as a material object (corpuscle) and as a wave. As a particle, it has a certain charge and mass. The ability to diffraction makes electrons related to classical waves. The length of this wave (λ) and the particle velocity (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 absolutely impossible to measure the coordinate and particle velocity with absolute accuracy. The more accurately the coordinate is determined, the higher the uncertainty in speed. As, however, and vice versa. This phenomenon is called the Heisenberg uncertainty, which can be expressed as the following relation: ∆x ∙ m ∙ ∆v> ћ / 2. Delta X (∆x) expresses the uncertainty of the position of the coordinate in space. Accordingly, the delta V (∆v) displays the velocity errors.
3. Contrary to all previously held beliefs, electrons do not travel 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.
The part of the space around the atomic nucleus itself, in which this probability is maximum, is called the orbital. Modern chemistry studies the structure of the electron shells of atoms precisely from this point of view. Of course, in schools they teach the correct distribution of electrons across levels, but, apparently, in reality they differ quite differently.
4. The nucleus of an atom consists of nucleons (protons and neutrons). The serial 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 an atom.
The founders of quantum mechanics
We note those scientists who made the greatest contribution to the development of such an important industry: the French physicist L. de Broglie, the German V. Heisenberg, the Austrian
E. Schrodinger, the Englishman P. Dirac. All these people were subsequently awarded the Nobel Prize.
How far has chemistry gone in this regard? The structure of the atom, most chemists of those years considered quite simple: many only by 1947 finally recognized the reality of the existence of elementary particles.
Some conclusions
In general, when creating a quantum theory, mathematicians could not do without, since all these processes can only be calculated using complex calculations. But this is not the main difficulty. Those processes that are described by this theory are inaccessible not only to our senses, despite all modern scientific technology, but also to imagination.
Not a single person can even approximately imagine the processes in the microworld, since they are completely different from all those phenomena that we observe in the macrocosm. Just think: the latest discoveries give reason to believe that quarks, neutrinos, and other fundamental particles exist in the nine-dimensional (!) Dimension. How can a person living in three-dimensional space even approximately describe their behavior?
At the moment, we can only rely on the mathematics and power of modern computers, which, perhaps, will be used to simulate the microworld. Chemistry also helps significantly: the structure of the atom will probably be revised after recently scientists working in this field announced the discovery of a new type of chemical bond.
Modern idea of the structure of the atom
If you carefully read all of the above, then you yourself can surely say what is the current idea of the structure of atoms of substances. But still we will explain: this is a somewhat modified theory of Rutherford, supplemented by priceless postulates of Niels Bohr. Simply put, today it is believed that electrons move along chaotic, blurry trajectories near the nucleus, which consists of neutrons and protons. The part of the space around it in which the appearance of an electron is most likely 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 to penetrate the secrets of the microworld: LHC (Large Andron Collider), Nobel Prizes in the field of physics - all this is the result of these studies.
But even now we do not present an approximate picture of what atoms still hide. It is only clear that the atom itself on a microcosm scale is a huge apartment building, in which we examined only the first floor, and even then not completely. Almost every year there are reports about the possibility of discovering more and more elementary particles. When the process of atomic research is completely completed, today no one will undertake to predict.
Suffice it to say that our ideas about them began to change only in 1947, when the so-called V-particles were discovered. Prior to this, people only slightly deepened the theories on which chemistry has been based since the 19th century. The structure of the atom is a fascinating mystery, the best minds of mankind are busy solving it.