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 |
The atom was founded by British chemist and physicist J.John Dalton (1766 ~ 1844) (right).
Since the German scientist Hittorff discovered cathode ray in 1869, a large number of scientists such as crookes, Hertz, Lerner and Thomson have studied cathode ray for more than twenty years. Finally, Joseph John Thomson discovered the existence of electrons (please visit the "mysterious green fluorescence" in the Science Park). Under normal circumstances, atoms are uncharged. Since negatively charged electrons 17 times smaller than their mass can run out of atoms, it shows that there are structures inside atoms, and there are positively charged things in atoms, which should be neutralized with the negative charges carried by electrons to make atoms neutral.
what is there in an atom besides electrons? How do electrons stay in atoms? What is positively charged in an atom? How is the positive charge distributed? How do negatively charged electrons interact with positively charged things? A lot of new problems are in front of physicists. According to scientific practice and experimental observation results at that time, physicists used their rich imagination and put forward various atomic models.
Atomic model of planetary structure
In p>191, the French physicist Jean Baptiste Perrin (187-1942) (left) put forward a structural model, which holds that the center of an atom is some positively charged particles, and the periphery is some orbiting electrons, and the cycle of the electrons corresponds to the spectral line frequency emitted by the atom, and the outermost electrons are thrown out to emit cathode rays.
neutral atom model
In p>192, the German physicist Learnard (1862—1947) (pictured right) proposed the neutral particle dynamical submodel. Learnard's early observation shows that cathode rays can pass through the aluminum window in the vacuum tube to the outside of the tube. According to this observation, he proved in 193 that high-speed cathode rays can pass through thousands of atoms by absorption experiments. According to the prevailing semi-materialists, most of the volume of atoms is empty, while rigid matter is only about 1-9 (that is, one in 1,). Learnard imagined that "rigid matter" was a synthesis of a number of positive and negative charges scattered in the internal space of atoms.
solid charged ball atomic model
Lord Kelvin (1824 ~ 197) (pictured left), a famous British physicist and inventor, was originally named William Tang Musun. Because of his meritorious service in installing the first Atlantic submarine cable, the British government knighted him in 1866 and promoted him to Lord Kelvin in 1892, and began to use Kelvin. Kelvin has a wide range of research, and has made contributions in the fields of heat, electromagnetism, fluid mechanics, optics, geophysics, mathematics and engineering applications. He published more than 6 papers in his life and obtained 7 invention patents. He enjoyed a high reputation in the scientific community at that time. Kelvin put forward the model of solid charged sphere atom in 192, that is, the atom is regarded as a uniformly positively charged sphere with negatively charged electrons buried in it, which is in electrostatic equilibrium under normal conditions. This model was later developed by J.J. Tang Musun, and later became known as Tang Musun Atomic Model.
raisin cake model
Joseph John Thomson (1856-194) (right) continued to make more systematic research, trying to describe the atomic structure. Thomson thought that the atom contained a uniform anode sphere, and several negative electrons were running in this sphere. According to Alfred Mayer's research on the balance 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 be listed as two rings, and so on. In this way, the increase of electrons leads to the periodic similarity in structure, and the repeated reappearance 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 in a cake. Many people call Thomson's atomic model "raisin cake model". It can not only explain why atoms are electrically neutral and how electrons are distributed in atoms, but also explain cathode ray phenomenon and the phenomenon that metals can emit electrons under ultraviolet radiation. Moreover, according to this model, the size of atoms can be estimated to be about 1-8 cm, which is an amazing thing. Because Thomson model can explain many experimental facts at that time, it is easily accepted by many physicists.
Saturn model
Nagaoka Hantaro (1865-195), a Japanese physicist, gave an oral presentation at the Tokyo Institute of Mathematical Physics on December 5, 193, and published a paper "Electronic Motion in Atoms Explaining Linear and Banded Spectra and Radioactivity Phenomenon" in magazines in Japan, Britain and Germany in 194. He criticized Thomson's model, thinking that positive and negative electricity can't penetrate each other, and put forward a structure he called "Saturn model"-an atomic model with an electron ring rotating around a positively charged core. A massive positively charged ball is surrounded by a circle of electrons distributed at equal intervals and moving in a circle at the same angular velocity. The radial vibration of electrons emits a line spectrum, and the vibration perpendicular to the torus emits a band spectrum. The electrons on the ring fly out as beta rays, and the positively charged particles on the central sphere fly out as alpha rays.
This Saturn model has a great influence on his later model of atomic nucleation. In 195, he analyzed the experimental results such as the measurement of the charge-mass ratio of alpha particles, and found that alpha particles were helium ions.
in p>198, Swiss scientist Leeds proposed the magnetic atom model.
Their models can explain some experimental facts at that time to a certain extent, but they can't explain many new experimental results, so they have not been further developed. A few years later, Thomson's "raisin cake model" was overthrown by his student Rutherford.
solar system model-nuclear atom model
Ernest Rutherford (1871 ~ 1937), a British physicist, came to the Cavendish laboratory in England in 1895 to study with Thomson and became the first overseas graduate student. Rutherford was studious and diligent. Under the guidance of Thomson, Rutherford discovered alpha rays when he was doing his first experiment-radioactive absorption experiment.
Rutherford designed an ingenious experiment. He put radioactive elements such as uranium and radium in a lead container, leaving only a small hole in the lead container. Because lead can block radiation, only a small part of the radiation comes out of the small hole and forms a narrow beam of radiation. Rutherford placed a strong magnet near the radiation beam, and it was found that there was a ray that was not affected by the magnet and kept moving in a straight line. The second ray is influenced by the magnet and deflected to one side, but it is not deflected badly. The third ray is deflected badly.
Rutherford put materials with different thicknesses in the direction of the radiation, and observed the absorption of the radiation. The first kind of radiation is not affected by the magnetic field, which means that it is uncharged and has strong penetrating power. General materials such as paper and wood chips can't stop the progress of the radiation, and only a thick lead plate can completely block it, which is called gamma rays. The second ray will be influenced by the magnetic field and biased to one side. From the direction of the magnetic field, it can be judged that this ray is positively charged. The penetration 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 kind of ray is negatively charged according to the deflection direction, and its properties are the same as those of fast-moving electrons, so it is called beta ray. Rutherford was particularly interested in the alpha rays he discovered himself. After in-depth and meticulous research, he pointed out that alpha rays are positively charged particle streams, and these particles are ions of helium atoms, that is, helium atoms with two electrons missing.
The "counter tube" was invented by Hans Geiger (1882-1945), a student from Germany, and can be used to measure charged particles invisible to the naked eye. When charged particles pass through the counter tube, the counter tube sends out a telecommunication signal. When this telecommunication signal is connected to the alarm, the instrument will give a "click" sound and the indicator light will light up. Invisible and intangible rays can be recorded and measured with very simple instruments. People call this instrument Geiger counter. With the help of Geiger counter, the research on the properties of α particles in Manchester laboratory led by Rutherford has developed rapidly.
In p>191, marsden (1889-197) came to the University of Manchester. Rutherford asked him to bombard the gold foil with alpha particles, do practice experiments, and record those alpha particles passing through the gold foil with a fluorescent screen. According to Thomson's model of raisin cake, tiny electrons are distributed in a uniformly positively charged substance, while alpha particles are nitrogen atoms that have lost two electrons, and their mass is several thousand times larger than that of electrons. When such a heavy shell bombards atoms, small electrons can't resist. However, the positive matter in gold atoms is evenly distributed in the whole atomic volume, and it is impossible to resist the bombardment of alpha particles. That is to say, the alpha particle will easily pass through the gold foil, even if it is blocked a little, it will only change its direction slightly after passing through the gold foil. Rutherford and Geiger have done this kind of experiment many times, and their observation results are in good agreement with Thomson's raisin cake model. Influenced by the gold atom, the α particle slightly changed its direction, and its scattering angle was extremely small.
marsden (left) and Geiger repeated this experiment which had been done many times, and a miracle appeared! They not only observed the scattered alpha particles, but also observed the alpha particles reflected by the gold foil. Rutherford described the scene in a speech in his later years. He said, "I remember Geiger came to me very excitedly two or three days later and said,' We got some reflected alpha particles ...', which was the most incredible event in my life. It's as incredible as shooting a 15-inch shell at a cigarette paper, only to be hit by a reflected shell. After thinking about it, I realize that this backscattering can only be the result of a single collision. After calculation, I see that it is impossible to get this order of magnitude without considering that most of the atomic mass is concentrated in a small core. "
when Rutherford said "after thinking", he did not think for a day or two, but for a whole year or two. After doing a lot of experiments, theoretical calculations and careful consideration, he boldly put forward the nuclear atom model, overthrowing his teacher Thomson's solid charged ball atom model.
Rutherford carefully measured the total number of reflected alpha particles after checking that it was indeed alpha particles in his students' experiments. The measurement shows that under their experimental conditions, one alpha particle is reflected back for every 8, incident alpha particles. Thomson's solid charged ball atom model and the scattering theory of charged particles can only explain the small angle scattering of α particles, but can't explain the large angle scattering. Multiple scattering can get large-angle scattering, but the calculation results show that the probability of multiple scattering is extremely small, which is too far from the observation that one of the above 8 thousand α particles reflects back.
Thomson atomic model can't explain the scattering of α-particles. After careful calculation and comparison, Rutherford found that only when the positive charges are concentrated in a small area and α-particles pass through a single atom can large-angle scattering occur. In other words, the positive charge of an atom must be concentrated in a small nucleus at the center of the atom. On the basis of this assumption, Rutherford further calculated some laws of α scattering and made some inferences. These inferences were soon confirmed by a series of beautiful experiments by Geiger and marsden.
Rutherford's atomic model is like a solar system, with positively charged nuclei like the sun and negatively charged electrons like planets orbiting the sun. In this "solar system", the force between them is electromagnetic interaction. He explained that positively charged substances in atoms are concentrated in a small core, and most of the atomic mass is also concentrated in this small core. When the alpha particle is shooting directly at the atomic core, it may be bounced back (left picture). This satisfactorily explains the large angle scattering of α particles. Rutherford published a famous paper "Scattering of α and β particles by matter and its principle structure".
Rutherford's theory opened up a new way 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 given a cold shoulder by physicists. The fatal weakness of Rutherford's atomic model is that the electric field force between positive and negative charges can't meet the requirements of stability, that is, it can't explain how electrons stay outside the nucleus stably. In 194, the Saturn model put forward by Hantaro Changgang failed because it could not overcome the difficulty of stability. Therefore, when Rutherford put forward the nuclear atom model again, many scientists regarded it as a conjecture or one of various models, ignoring the solid experimental basis on which Rutherford put forward the model.
Rutherford had extraordinary insight, so he was often able to grasp the essence and make scientific predictions. At the same time, he has a very strict scientific attitude, and he should make conclusions from the experimental facts. Rutherford thinks that his model is far from perfect and needs further research and development. At the beginning of his paper, he declared: "At this stage, it is unnecessary to consider the stability of the proposed atom, because obviously it will depend on the fine structure of the atom and the movement of the charged components." In a letter to a friend that year, he also said, "I hope I can give some clearer views on atomic structure in one or two years."
Bohr model
Rutherford's theory attracted a young man from Denmark, whose name was Niels Bohr (1885-1962) (left). On the basis of Rutherford's model, he put forward the quantized orbit of electrons outside the nucleus, solved the stability problem of atomic structure and described a complete and convincing theory of atomic structure.
Bohr was born in a professor's family in Copenhagen, and received his doctorate from the University of Copenhagen in 1911. From March to July 1912, he studied in Rutherford's laboratory, during which his atomic theory was born. Bohr first extended Planck's quantum hypothesis to the internal energy of atoms to solve the problem of Rutherford's atomic model in stability