Since the British chemist and physicist J. John Dalton (1766 ~ 1844) (pictured on the right) founded the atomic theory, for a long time people have believed that the atom is like a small The solid glass ball cannot be any smaller, and there will no longer be any tricks inside.
Since the German scientist Hitov discovered cathode rays in 1869, a large number of scientists such as Crookes, Hertz, Lerner, and Thomson have studied cathode rays for more than 20 years. Finally, Joseph John Thomson discovered the existence of electrons (please visit the Science and Technology Forum "Mysterious Green Fluorescence"). Normally, atoms are uncharged. Since negatively charged electrons that are 1,700 times smaller than their mass can escape from the atom, this shows that there is still structure inside the atom, and it also shows that there are still positively charged things in the atom. It should neutralize the negative charge carried by the electrons, making the atoms neutral.
What else is there besides electrons in an atom? How do electrons stay in an atom? What has a positive charge in an atom? How is the positive charge distributed? Negatively charged electrons and positively charged ones How do things interact? A host of new questions confront physicists. Based on scientific practice and experimental observations at that time, physicists used their rich imagination to propose various atomic models.
Atomic model of planetary structure
The structural model proposed by French physicist Jean Baptiste Perrin (1870-1942) (pictured left) in 1901 believed that the center of the atom is a belt Positively charged particles are surrounded by some orbiting electrons. The orbiting period of the electrons corresponds to the frequency of the spectral lines emitted by the atoms. When the outermost electrons are thrown out, they emit cathode rays.
Neutral atom model
In 1902, German physicist Philipp Edward Anton Lenard (1862-1947) (pictured on the right) proposed the neutral particle kinetic atom model. Leonard's early observations showed that cathode rays could pass through the aluminum windows in the vacuum tube and reach the outside of the tube. Based on this observation, he used absorption experiments to prove in 1903 that high-speed cathode rays can pass through thousands of atoms. According to the prevailing semi-materialist views at the time, most of the volume of atoms is empty space, and rigid matter is only about 10-9 (that is, one hundred thousandth) of its total volume. Leonard imagined that "rigid matter" was a composite of several positive and negative charges scattered in the inner space of atoms.
Solid charged sphere atomic model
The famous British physicist and inventor Lord Kelvin (1824～1907) (pictured left) was originally named W. Thomson. For his contribution to the installation of the first Atlantic submarine cable, the British government knighted him in 1866 and was promoted to Lord Kelvin in 1892, and he began to use the name Kelvin. Kelvin's research range is wide-ranging and he has made contributions in thermal, electromagnetic, fluid mechanics, optics, geophysics, mathematics, engineering applications, etc. He published more than 600 papers in his lifetime and obtained 70 invention patents. He enjoyed a high reputation in the scientific community at that time. Kelvin proposed the solid charged ball atomic model in 1902, which regarded atoms as uniformly positively charged spheres with negatively charged electrons buried in them. Under normal conditions, they are in electrostatic equilibrium. This model was later developed by J.J. Thomson and was later known as the Thomson atomic model.
Raisin Cake Model
Joseph John Thomson (1856-1940) (pictured on the right) continued more systematic research in an attempt to depict atomic structure. Thomson believed that atoms contain a uniform positive sphere with a number of negative electrons running within this sphere. Following Alfred Mayer's research on the equilibrium of floating magnets, he proved that if the number of electrons does not exceed a certain limit, a ring formed by these running electrons will be stable.
If the number of electrons exceeds this limit, it will form two rings, and so on to multiple rings. In this way, the increase in electrons results in periodic structural similarities, and the repeated recurrence of physical and chemical properties in Mendeleyev's periodic table may also be explained.
In this model proposed by Thomson, the distribution of electrons in a sphere is a bit like raisins dotted in a piece of cake. Many people call Thomson's atomic model the "raisin cake model." It can not only explain why atoms are electrically neutral and how electrons are distributed in atoms, but also explain the phenomenon of cathode rays and the phenomenon that metals can emit electrons under ultraviolet irradiation. Moreover, according to this model, the size of the atom can be estimated to be about 10-8 centimeters, which is a great thing. Because the Thomson model can explain many experimental facts at that time, it is easily accepted by many physicists.
Saturn model
Japanese physicist Nagaoka Hantaro (1865-1950) gave an oral presentation at the Tokyo Mathematical Physics Society on December 5, 1903, and presented it separately in 1904 The paper "Electron Movements in Atoms Explaining Linear and Band Spectroscopy and Radioactive Phenomena" was published in Japanese, British and German magazines. He criticized Thomson's model, believing that positive and negative charges could not penetrate each other, and proposed a structure he called the "Saturn model"-that is, an atomic model with a ring of electrons rotating around a positively charged core. A massive positively charged ball has a circle of equally spaced electrons on the periphery that move in a circle at the same angular velocity. The radial vibration of electrons emits a line spectrum, and the vibration perpendicular to the ring surface emits a band spectrum. The electrons flying out of the ring are beta rays, and the positively charged particles flying out of the central sphere are alpha rays.
This Saturn-like model was very influential in his later establishment of the nuclear model of atoms. In 1905, he analyzed experimental results such as the measurement of the charge-to-mass ratio of alpha particles and concluded that alpha particles were helium ions.
In 1908, Swiss scientist Leeds proposed the magnetic atom model.
Their models can explain some experimental facts at that time to a certain extent, but they cannot explain many new experimental results that will appear in the future, so they have not been further developed. A few years later, Thomson's "raisin cake model" was overturned by his own student Rutherford.
Bohr Model
Rutherford’s theory attracted a young man from Denmark, his name was Niels Bohr (1885-1962) (left Figure), based on the Rutherford model, he proposed the quantized orbit of electrons outside the nucleus, solved the problem of the stability of the atomic structure, and sketched a complete and convincing theory of atomic structure.
Bohr was born in a family of professors in Copenhagen and received a doctorate from the University of Copenhagen in 1911. He studied in Rutherford's laboratory from March to July 1912, during which his atomic theory was conceived. Bohr first extended Planck's quantum hypothesis to the energy inside the atom to solve the stability difficulties of the Rutherford atomic model. It was assumed that an atom can only change its energy through discrete energy quantons, that is, an atom can only It is in discrete stationary states, and the lowest stationary state is the normal state of the atom. Then, inspired by his friend Hansen, he reached the concept of stationary transition from the combination law of spectral lines. He published three parts of a long paper "On Atomic Structure and Molecular Structure" in July, September and November 1913.
Bohr's atomic theory gives such an atomic image: electrons move in a circle around the nucleus in some specific possible orbits, and the farther away from the nucleus, the higher the energy; the possible orbits are determined by the angular momentum of the electron. It is determined by an integer multiple of h/2π; when the electron moves in these possible orbits, the atom does not emit or absorb energy. Only when the electron jumps from one orbit to another orbit does the atom emit or absorb energy, and emit or absorb energy. The radiation is single frequency, and the relationship between the frequency and energy of the radiation is given by E=hν. Bohr's theory successfully explained the stability of atoms and the regularity of the spectral lines of hydrogen atoms.
Bohr's theory greatly expanded the influence of quantum theory and accelerated the development of quantum theory.
In 1915, German physicist Arnold Sommerfeld (1868-1951) extended Bohr's atomic theory to include elliptical orbits, and considered the effect of special relativity in which the mass of electrons changes with their speed, and derived the fine details of the spectrum. The structure is consistent with the experiment.
In 1916, Albert Einstein (1879-1955) started from Bohr’s atomic theory and used statistical methods to analyze the process of material absorbing and emitting radiation, and derived Planck’s radiation law. (Bohr and Einstein pictured on the left). This work of Einstein synthesized the achievements of the first stage of quantum theory and combined the work of Planck, Einstein and Bohr into a whole.