For a long time, the structure of the atom was a debatable topic among physicists until the model created by the Danish scientist Niels Bohr appeared. He was not the first to attempt to describe the motion of subatomic particles, but it was his work that made it possible to create a consistent theory with the possibility of predicting the location of an elementary particle at a given point in time.
Life path
Niels Bohr was born on October 7, 1885 in Copenhagen and died there on November 18, 1962. He is considered one of the greatest physicists and is not surprising: he was the one who managed to build a consistent model of hydrogen-like atoms. According to legend, he saw in a dream how something like planets revolved around a luminous rarefied center. Then this system sharply decreased to microscopic dimensions.
Since then, Bohr has persistently searched for a way to translate the dream into formulas and tables. By carefully studying modern literature on physics, experimenting in the laboratory and thinking, he was able to achieve his goal. Even innate shyness did not prevent him from publishing the results: he was shy about speaking to a large audience, he began to get confused, and the audience did not understand anything from the scientist's explanations.
Predecessors
Before Bohr, scientists tried to create an atom model based on the postulates of classical physics. The most successful attempt belonged to Ernest Rutherford. As a result of numerous experiments, he came to the conclusion that there is a massive atomic nucleus around which electrons move in orbits. Since graphically such a model was similar to the structure of the solar system, the name planetary was strengthened behind it.
But there was a significant drawback: the atom corresponding to the Rutherford equations proved to be unstable. Sooner or later, electrons moving with acceleration in orbits around the nucleus would have to fall on the nucleus, and their energy would be spent on electromagnetic radiation. For Bohr, the Rutherford model was the starting point in building his own theory.
The first postulate of Bohr
Bohr's main innovation was the rejection of the use of classical Newtonian physics in the construction of the theory of the atom. Having studied the data obtained in the laboratory, he came to the conclusion that such an important law of electrodynamics as uniformly accelerated motion without wave radiation does not work in the world of elementary particles.
The result of his thoughts was a law that sounds like this: an atomic system is stable only if it is in one of the possible stationary (quantum) states, each of which corresponds to a certain energy. The meaning of this law, otherwise called the postulate of quantum states, is to recognize the absence of electromagnetic radiation when the atom is in such a state. Also a consequence of the first postulate is the recognition of the presence of energy levels in an atom.
Frequency rule
However, it was obvious that the atom could not be in the same quantum state all the time, since stability denies any interaction, which means that there would be no Universe, no movement in it. The apparent contradiction allowed the second postulate of the model of the structure of the Bohr atom, known as the frequency rule. An atom is capable of moving from one quantum state to another with a corresponding change in energy, emitting or absorbing a quantum whose energy is equal to the difference in energy between stationary states.
The second postulate also contradicts classical electrodynamics. According to Maxwell's theory, the nature of the motion of an electron cannot affect the frequency of its radiation.
Atom spectrum
The quantum Bohr model was made possible by a careful study of the spectrum of the atom. For a long time, scientists were confused that instead of the expected continuous color region obtained by studying the spectra of celestial bodies, the atomic spectrogram was discontinuous. Lines of bright color did not pass into each other, but were separated by impressive dark areas.
The theory of the transition of an electron from one quantum state to another explained this oddity. When an electron passed from one energy level to another, where less energy was required from it, it emitted a quantum, which was reflected in the spectrogram. Bohr's theory immediately demonstrated the ability to predict further changes in the spectra of simple atoms like hydrogen.
disadvantages
Bohr's theory did not completely break with classical physics. She still retained the idea of the orbital motion of electrons in the electromagnetic field of the nucleus. The idea of quantization during the transition from one stationary state to another successfully complemented the planetary model, but still did not solve all the contradictions.
Although in the light of Bohr's model the electron could not move in a spiral motion and fall onto the nucleus, continuously emitting energy, it remained unclear why it could not consistently rise to higher energy levels. In this case, all the electrons would sooner or later be in the lowest energy state, which would lead to the destruction of the atom. Another problem was anomalies in atomic spectra, which theory did not explain. Back in 1896, Peter Zeeman conducted an interesting experiment. He placed an atomic gas in a magnetic field and took a spectrogram. It turned out that some spectral lines split into several. Such an effect has not been explained in Bohr's theory.
Boron hydrogen atom model building
Despite all the shortcomings of his theory, Niels Bohr was able to build a model of the hydrogen atom corresponding to reality. In doing so, he used the rule of frequencies and the laws of classical mechanics. Bohr's calculations to determine the possible radii of the orbits of the electron and to calculate the energy of quantum states turned out to be quite accurate and were confirmed experimentally. The frequencies of radiation and absorption of electromagnetic waves corresponded to the location of dark gaps in the spectrograms.
Thus, using the example of a hydrogen atom, it was proved that each atom is a quantum system with discrete energy levels. In addition, the scientist was able to find a way to combine classical physics and his postulates using the principle of correspondence. It says that quantum mechanics includes the laws of Newtonian physics. Under certain conditions (for example, if the quantum number was large enough), quantum and classical mechanics converge. This was proved by the fact that with an increase in the quantum number, the length of the dark gaps in the spectrum decreased until it completely disappeared, as was expected in the light of Newtonian ideas.
Value
The introduction of the correspondence principle has become an important intermediate stage on the way to recognizing the existence of special quantum mechanics. The model of the Bohr atom has become for many a starting point in the construction of more accurate theories of motion of subatomic particles. Niels Bohr could not find the exact physical interpretation of the quantization rule, but he could not do it, since only with time the wave properties of elementary particles were discovered. Louis de Broglie, adding to the theory of Bohr new discoveries, proved that every orbit along which an electron moves, is a wave propagating from the nucleus. From this point of view, the stationary state of the atom was considered to be such that it forms in the case when the wave, having made a complete revolution around the nucleus, is repeated.