For a long time, the structure of the atom was a debatable topic among physicists, until a model created by the Danish scientist Niels Bohr appeared. He was not the first who tried to describe the movement of subatomic particles, but it was his developments that made it possible to create a consistent theory with the ability to predict the location of an elementary particle at one time or another.
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 no wonder: it was he 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 certain luminous rarefied center. This system then drastically shrank to microscopic size.
Since then, Bohr has been searching hard for a way to translate the dream into formulas and tables. By carefully studying the modern literature on physics, experimenting in the laboratory and thinking, he was able to achieve hisgoals. Even congenital shyness did not prevent him from publishing the results: he was embarrassed to speak in front of a large audience, he began to get confused, and the audience did not understand anything from the scientist's explanations.
Precursors
Before Bohr, scientists tried to create a model of the atom 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 about the existence of 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 of the planetary one was strengthened behind it.
But it had a significant drawback: the atom corresponding to the Rutherford equations turned out to be unstable. Sooner or later, the electrons, moving with acceleration in orbits around the nucleus, had to fall on the nucleus, and their energy would be spent on electromagnetic radiation. For Bohr, the Rutherford model became the starting point in building his own theory.
Bohr's first postulate
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 reflections 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 an atom is in such a state. Also, a consequence of the first postulate is the recognition of the presence of energy levels in the atom.
Frequency rule
However, it was obvious that an atom cannot always be in the same quantum state, since stability denies any interaction, which means that there would be neither the Universe nor movement in it. The apparent contradiction was resolved by the second postulate of Bohr's atomic structure model, known as the frequency rule. An atom is able to move from one quantum state to another with a corresponding change in energy, emitting or absorbing a quantum, the energy of which is equal to the difference between the energies of the 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
Bohr's quantum model was made possible by careful study of the spectrum of the atom. For a long time, scientists were embarrassed that instead of the expected continuous color region obtained by studying the spectra of celestial bodies, the spectrogram of the atom was discontinuous. Lines of bright color did not flow into each other, but were separated by impressive dark areas.
Theory of electron transition from one quantum state toanother explained this oddity. When an electron moved from one energy level to another, where less energy was required of 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.
Flaws
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 resolve all contradictions.
Although in the light of Bohr's model, the electron could not go into a spiral motion and fall into the nucleus, continuously radiating energy, it remained unclear why it could not successively rise to higher energy levels. In this case, all electrons would sooner or later end up in the lowest energy state, which would lead to the destruction of the atom. Another problem was anomalies in atomic spectra that the theory did not explain. Back in 1896, Peter Zeeman conducted a curious 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 was not explained in Bohr's theory.
Building a model of the hydrogen atom according to Bohr
Despite all the shortcomings of his theory, Niels Bohr was able to build a realistic model of the hydrogen atom. In doing so, he used the frequency rule and the laws of classicalmechanics. Bohr's calculations to determine the possible radii of electron orbits and calculate the energy of quantum states turned out to be quite accurate and were confirmed experimentally. The frequencies of emission and absorption of electromagnetic waves corresponded to the location of dark gaps on the spectrograms.
Thus, using the example of the 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 correspondence principle. It states 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 dark gaps in the spectrum decreased up to complete disappearance, as expected in the light of Newtonian concepts.
Meaning
The introduction of the correspondence principle has become an important intermediate step towards the recognition of the existence of special quantum mechanics. Bohr's model of the atom has become for many a starting point in constructing more accurate theories of the motion of subatomic particles. Niels Bohr was unable to find an exact physical interpretation of the quantization rule, but he could not do this either, since the wave properties of elementary particles were discovered only over time. Louis de Broglie, supplementing Bohr's theory with new discoveries, proved that each orbit, according towhich the electron moves is a wave propagating from the nucleus. From this point of view, the stationary state of the atom began to be considered such that it is formed in the case when the wave, having made a complete revolution around the nucleus, repeats.
