Democritus believed that all things are composed of atoms, and atoms are solid balls that cannot be further divided.
Various atomic models
——The exploration process of atomic structure
|Planetary structure model|Neutral model|Solid charged ball model|Raisin cake model| Saturn model | Solar system model | Bohr model |
For a long time after the British chemist and physicist J. John Dalton (1766~1844) (pictured on the right) founded the atomic theory People in the world think that an atom is like a solid glass ball that is extremely small, and there is no more 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 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, based on this model, the size of the atom can also 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.
Solar system model - nuclear atom model
British physicist Ernest Rutherford (1871～1937) came to Cavendish, England in 1895 Laboratory, studied with Thomson and became Thomson's first graduate student from overseas. Rutherford was studious and diligent. Under Thomson's guidance, Rutherford discovered alpha rays while doing his first experiment, the radioactive absorption experiment.
In an ingenious experiment designed by Rutherford, he placed uranium, radium and other radioactive elements in a lead container, leaving only a small hole in the lead container. Because lead can block radiation, only a small portion of the radiation emerges from the hole into a very narrow beam of radiation. Rutherford placed a strong magnet near the radiation beam and found that there was a type of ray that was not affected by the magnet and kept traveling in a straight line. The second type of ray is affected by the magnet and deflects to one side, but not too much. The third type of ray is highly deflected.
Rutherford placed materials of different thicknesses in the direction of the radiation and observed how the radiation was absorbed. The first type of ray is not affected by the magnetic field, which means that it is uncharged and has strong penetrating power. Ordinary materials such as paper and wood chips cannot block the progress of the ray. Only relatively thick lead The plate can completely block it, which is called gamma rays. The second type of ray will be affected by the magnetic field and deflected to one side. It can be judged from the direction of the magnetic field that this ray is positively charged. The penetrating power of this ray is very weak, and it can be completely blocked with a piece of paper. This is the alpha ray discovered by Rutherford. The third type of ray is judged to be negatively charged based on the direction of deflection, and has the same properties as fast-moving electrons, and is called beta rays. Rutherford was particularly interested in alpha rays, which he discovered himself. After in-depth and detailed research, he pointed out that alpha rays are streams of positively charged particles, and these particles are ions of helium atoms, that is, helium atoms with two electrons missing.
The "counting tube" was invented by Hans Geiger (1882-1945), a student from Germany, and can be used to measure electrically charged particles that are invisible to the naked eye. When the charged particles pass through the counting tube, the counting tube emits an electrical signal. When this electrical signal is connected to the alarm, the instrument will make a "click" sound and the indicator light will light up. Invisible and intangible rays can be recorded and measured using very simple instruments. This instrument is called a Geiger counter. With the help of Geiger counter tubes, the Manchester laboratory led by Rutherford rapidly developed its research on the properties of alpha particles.
In 1910, E. Marsden (1889-1970) came to the University of Manchester. Rutherford asked him to use alpha particles to bombard gold foil, do practice experiments, and use a fluorescent screen to record those particles that passed through the gold foil. Alpha particles. According to Thomson's raisin cake model, electrons with tiny masses are distributed in a uniformly positively charged substance, and the alpha particle is a nitrogen atom that has lost two electrons, and its mass is thousands of times greater than the electrons. When such a heavy shell hits an atom, the tiny electrons are no match for it. The positive matter in gold atoms is evenly distributed throughout the entire atomic volume, and it is impossible to withstand the bombardment of alpha particles. In other words, alpha particles will easily pass through the gold foil. Even if they are blocked a little, it will only change the direction of the alpha particles slightly after passing through the gold foil. Rutherford and Geiger had done this type of experiment many times, and their observations were in good agreement with Thomson's raisin cake model. The alpha particle changes direction slightly due to the influence of the gold atoms, and its scattering angle is extremely small.
Marsden (pictured on the left) and Geiger repeated this experiment that had been done many times, and a miracle occurred! They not only observed scattered alpha particles, but also observed alpha particles reflected by the gold foil. Alpha particles. Rutherford described the scene in a speech in his later years. He said: "I remember that two or three days later, Geiger came to me very excited and said: 'We got some alpha particles reflected back... ....', this is the most incredible event in my life. It is as incredible as shooting a 15-inch cannonball at a cigarette paper and being hit by the reflected cannonball. After thinking about it, I realized. This kind of backscattering can only be the result of a single collision. After calculation, I saw that it is impossible to obtain this order of magnitude without considering that most of the atomic mass is concentrated in a very small nucleus." /p>
What Rutherford said "after thinking" was not thinking for one or two days, but thinking for one or two years. After doing a lot of experiments and theoretical calculations and careful consideration, he boldly proposed the nucleated atom model, overturning his teacher Thomson's solid charged ball atom model.
After Rutherford verified that the reflected alpha particles in his student's experiment were indeed alpha particles, he carefully measured the total number of reflected alpha particles. Measurements showed that under their experimental conditions, one alpha particle was reflected back for every eight thousand incident alpha particles. Thomson's solid charged sphere atomic model and the scattering theory of charged particles can only explain the small-angle scattering of α particles, but cannot explain the large-angle scattering. Multiple scattering can produce large-angle scattering, but calculation results show that the probability of multiple scattering is extremely small, which is far from the above-mentioned observation that one out of eight thousand alpha particles is reflected back.
The Thomson atomic model cannot explain alpha particle scattering. After careful calculation and comparison, Rutherford found that only assuming that the positive charges are concentrated in a small area, when alpha particles pass through a single atom, Only then can large-angle scattering occur. That is, the positive charge of the atom must be concentrated in a very small nucleus in the center of the atom. On the basis of this assumption, Rutherford further calculated some laws of alpha scattering and made some inferences. These inferences were soon confirmed by a series of beautiful experiments by Geiger and Marsden.
The atomic model proposed by Rutherford is like a solar system. The positively charged atomic nucleus is like the sun, and the negatively charged electrons are like planets orbiting the sun. In this "solar system", the force that governs them is the electromagnetic interaction force. He explained that the positively charged matter in the atom is concentrated in a very small core, and most of the atomic mass is also concentrated in this very small core. When an alpha particle is shot directly at the core of an atom, it is likely to bounce back (left picture). This satisfactorily explains the large-angle scattering of alpha particles.
Rutherford published a famous paper "Scattering of alpha and beta particles by matter and its principle structure".
Rutherford's theory opened up new ways to study atomic structure and made immortal contributions to the development of atomic science. However, for a long time at that time, Rutherford's theory was ignored by physicists. The fatal weakness of Rutherford's atomic model is that the electric field force between positive and negative charges cannot meet the stability requirements, that is, it cannot explain how electrons stay stably outside the nucleus. The Saturn model proposed by Nagaoka Hantaro in 1904 failed because it could not overcome the difficulty of stability. Therefore, when Rutherford proposed the nuclear atom model, many scientists regarded it as a conjecture or just one of various models, while ignoring the solid foundation on which Rutherford proposed the model. Experimental basis.
Rutherford had extraordinary insight and was often able to grasp the essence and make scientific predictions. At the same time, he has a very rigorous scientific attitude and makes the conclusions that should be made based on experimental facts. Rutherford believes that the model he proposed is still imperfect and needs further research and development. He stated at the beginning of his paper: "At this stage, it is not necessary to consider the stability of the atoms mentioned, because it will obviously depend on the fine structure of the atoms and the movement of the charged components." He also wrote to a friend that year Said: "I hope that within one or two years we can have some clearer insights into the structure of the atom."
Bohr Model
Rutherford's theory attracted a man from Denmark A young man named Niels Bohr (1885-1962) (pictured left), based on the Rutherford model, proposed the quantized orbit of electrons outside the nucleus and solved the atomic structure The problem of stability has been solved, and a complete and convincing theory of atomic structure has been described.
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. (Pictured left are Bohr and Einstein). 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.
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