After a long period of exploration, it was not until the early 20th century that people had a clear understanding of the structure and internal motion laws of atoms, and modern atomic physics was gradually established. Around 1897, scientists gradually determined the basic characteristics of electrons and established that electrons are the common components of various atoms. Usually, atoms are electrically neutral. Since all atoms have negatively charged electrons, there must be positively charged substances among them. At the beginning of the 20th century, two different hypotheses were put forward. In 1904, Thomson proposed that the positive charges in atoms are distributed in a sphere with uniform volume density, while the negatively charged electrons are distributed in different positions of the sphere, which vibrate at a certain frequency respectively, thus emitting electromagnetic radiation. This model was vividly compared to the "nut bread" model, but the model theory contradicted the experimental results and was soon abandoned.

In 19 1 1, Rutherford proposed that the center of an atom is a positively charged heavy nucleus, which is very small compared with the size of the whole atom. Electrons revolve around the nucleus, similar to planets revolving around the sun. This model is called the nuclear model of atoms, also known as the planetary model. The conclusions drawn from this model are in good agreement with the experimental results and are quickly recognized.

According to the classical electromagnetic theory, electrons should automatically radiate energy, so that the energy of atoms gradually decreases and the frequency of radiation gradually changes, so the emission spectrum should be continuous. Due to the decrease of energy, electrons gradually approach the nucleus along the spiral and finally fall on the nucleus, so the atom should be an unstable system.

But in fact, atoms are stable, and the spectrum emitted by atoms is linear, not continuous. These facts show that the classical electrodynamics established from the study of macroscopic phenomena is not applicable to the microscopic processes in atoms. This requires further analysis of atomic phenomena, exploration of the regularity of internal movement of atoms, and establishment of atomic theory suitable for microscopic processes.

19 13 years, Danish physicist Bohr proposed that the energy possessed by an atom forms discontinuous energy levels on the basis of the nuclear model proposed by Rutherford, combined with the empirical law of atomic spectrum, and applied the quantum hypothesis proposed by Planck in 1900 and the photon hypothesis proposed by Einstein in 1905. When the energy levels jump, atoms will

Bohr hypothesis can explain some atomic phenomena such as hydrogen atom spectrum, and successfully established the theory of hydrogen atom structure for the first time. The establishment of Bohr theory is a great progress in the theory of atomic structure and atomic spectrum, but the further study of atomic problems shows the deficiency of this theory, which can only be regarded as a very rough approximation theory.

1924, de Broglie put forward the hypothesis that microscopic particles have wave-particle duality, and later observations proved that microscopic particles have fluctuation. On this basis, Schrodinger established wave mechanics in 1926. At the same time, other scholars, such as Heisenberg, Born, Dirac and others, established the equivalence theory from another way. This theory, now called quantum mechanics, can explain atomic phenomena well.

In the first 30 years of the 20th century, atomic physics was at the forefront of physics and developed rapidly, which promoted the establishment of quantum mechanics and initiated a new era of modern physics. Because quantum mechanics has successfully solved some atomic physics problems encountered at that time, many physicists believe that the basic laws of atomic motion are clear, leaving only some details.

Due to the limitation of understanding and the attraction of studying atomic nuclei and elementary particles, many physicists have focused on studying atomic nuclei and elementary particles, except for some spectrologists who have made in-depth research on the fine structure and ultra-fine structure of atomic energy levels and achieved some results. For a long time, they failed to conduct a comprehensive and in-depth study of atomic physics, which affected the development of atomic physics to some extent.

In the late 1950s, due to the development of space technology and space physics, engineers and scientists found that it was not enough to use the existing knowledge of atomic physics to solve the problems of space science and space technology. In the past, people have accurately measured the wavelengths of many spectral lines, deeply studied the energy levels of atoms, and accurately explained spectral lines and energy levels.

However, we know little about the basic knowledge of space science such as spectral line intensity, transition probability and collision cross section, and even only know the order of some parameters of these physical quantities. Many problems encountered in nuclear testing are also related to this knowledge. Therefore, it is necessary to conduct new experiments and theoretical discussions on atomic physics.