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 |
Ever since the British chemist and physicist Dalton (J. john dalton, 1766 ~ 1844) (pictured right) founded the atomic theory, people have always thought that an atom is like a glass solid ball that can't be smaller, and there are no more tricks in it.
After German scientist Hittorff discovered cathode ray in 1869, a large number of scientists such as crookes, Hertz, Lerner and Thomson have been studying 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 the negatively charged electrons whose mass is 1700 times smaller than their own can also run out of the atom, it shows that there is a structure inside the atom, and there is something positively charged inside the atom, which should neutralize the negative charge carried by the electron and make the atom neutral.
Besides electrons, what else is in atoms? 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? Many new problems are facing physicists. According to the scientific practice and experimental observation results at that time, physicists put forward various atomic models with rich imagination.
Atomic model of planetary structure
190 1 year, French physicist jean baptiste perrin (1870- 1942) (left) put forward a structural model, which holds that the center of the atom is some positively charged particles, and the periphery is some orbiting electrons, and the period of electron orbit corresponds to the spectral line frequency emitted by the atom, and the outermost layer.
Neutral atom model
1902, German physicist Learnard (1862-1947) (right) put forward the neutral particle dynamics sub-model. Learnard's early observation shows that cathode rays can pass through the aluminum window in the vacuum tube and reach the outside of the tube. According to this observation, he proved in 1903 that high-speed cathode rays can pass through thousands of atoms through absorption experiments. According to the popular semi-materialists at that time, the volume of atoms is mostly empty, and the rigid matter is only about 10-9 (that is, one in 100,000). Learnard assumes that "rigid matter" is the synthesis of a large number of positive and negative charges dispersed in the internal space of atoms.
Solid charged ball atom model
Lord Kelvin (1824 ~ 1907) (left), a famous British physicist and inventor, was originally named William Thomson. He was knighted by the British government in 1866 and 1892 for his meritorious service in installing the first Atlantic submarine cable. Kelvin's research has a wide range, and he has made contributions in the fields of heat, electromagnetism, fluid mechanics, optics, geophysics, mathematics and engineering applications. He published more than 600 papers in his life and obtained 70 invention patents. He enjoyed a high reputation in the scientific community at that time. Kelvin 1902 put forward the atom model of solid charged sphere, 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 called Tang Musun Atomic Model.
Raisin cake model
Joseph john thomson (1856- 1940) (right) continued his more systematic research and tried to describe the atomic structure. Thomson believed that the atom contained a uniform anode sphere in which several negative electrons moved. 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, the 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 Mendeleev's periodic table may also be explained.
In this model proposed by Thomson, the distribution of electrons in the sphere is a bit like raisins dotted 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, it can be estimated that the size of the atom is about 10-8 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
Kantaro Nagaoka (1865-1950)1903 1904 was published orally in the Tokyo Institute of Mathematical Physics, and1904 was published in Japanese, English and German magazines respectively. 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 in which electrons revolve around a positively charged core. A positively charged mass ball is surrounded by a circle of electrons distributed at equal intervals, which move 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. Electrons on the ring fly out as beta rays, and positively charged particles on the central sphere fly out as alpha rays.
This Saturn model has a great influence on his later atomic nucleation model. 1905, he analyzed the experimental results such as the measurement of the charge-mass ratio of α particles and found that α particles were helium ions.
1908, Swiss scientist Leeds proposed the magnetic atom model.
Their model can explain some experimental facts at that time to some extent, but it can't explain many new experimental results, so it has not been further developed. A few years later, Thomson's "raisin cake model" was overthrown by his student Rutherford.
Solar System Model —— Nucleated Atom Model
British physicist ernest rutherford (187 1 ~ 1937) came to the Cavendish laboratory in England on 1895 to study with Thomson, becoming the first overseas graduate student of Thomson. Rutherford is diligent and studious. 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, forming a narrow beam of radiation. Rutherford placed a strong magnet near the radiation beam, and it was found that one ray kept moving in a straight line without the influence of the magnet. The second ray is influenced by the magnet and biased to one side, but it is not biased badly. The third light is badly deflected.
Rutherford placed materials with different thicknesses in the direction of radiation and observed the absorption of radiation. The first kind of radiation is not affected by the magnetic field, which means that it is uncharged and has strong penetrating power. Ordinary paper, sawdust and other materials can't stop the progress of radiation, only thick lead plates can completely stop it, which is called gamma rays. The second ray will be influenced by the magnetic field and biased to one side. Judging from the direction of the magnetic field, this ray is positively charged. The penetration of this kind of 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 particles, which are ions of helium atoms, that is, helium atoms lacking two electrons.
The "counter tube" was invented by German student hans geiger (1882- 1945), which 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 make a "click" sound and the indicator light will light up. Invisible and invisible 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 developed rapidly.
19 10, marsden (E.Marsden, 1889- 1970) came to Manchester university. Rutherford asked him to bombard the gold foil with alpha particles, do practical experiments, and record those alpha particles passing through the gold foil with a fluorescent screen. According to Thomson's raisin cake model, tiny electrons are distributed in a uniformly positively charged substance, while α particles are nitrogen atoms that have lost two electrons, and their mass is thousands of times larger than that of electrons. Such a heavy shell bombards atoms, and small electrons can't resist it. However, the positive matter in the gold atom is uniformly distributed in the whole atomic volume and cannot resist the bombardment of alpha particles. In other words, the alpha particles will easily pass through the gold foil, and even if they are blocked a little, they will only change their direction slightly after passing through the gold foil. Rutherford and Geiger have done this experiment many times, and their observations are very consistent 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 the experiment many times, and a miracle appeared! They not only observed scattered α particles, but also observed α particles reflected by gold foil. Rutherford described this 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, but being hit by a reflected shell. After thinking about it, I realized 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 atomic masses are concentrated in a small core. "
Rutherford said "after thinking", not thinking for a day or two, but thinking for a whole year or two. After doing a lot of experiments, theoretical calculations and careful consideration, he boldly put forward the atomic model of nucleus, which overthrew his teacher Thomson's atomic model of solid charged ball.
Rutherford checked that the alpha particles reflected in his students' experiments were indeed alpha particles, and then carefully measured the total number of reflected alpha particles. The measurement shows that under their experimental conditions, one alpha particle out of every 8,000 incident alpha particles is reflected back. 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. Large angle scattering can be obtained by multiple scattering, but the calculation results show that the probability of multiple scattering is very small, which is too far from the observation reflected by one of the above 8 thousand α particles.
Thomson atomic model can't explain the scattering of α particles. After careful calculation and comparison, Rutherford found that large-angle scattering can only occur when the positive charge is concentrated in a small area and the alpha particle passes through a single atom. 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 quickly 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 all the positively charged substances in the atom are concentrated in a small nucleus, and most of the atomic mass is also concentrated in this small nucleus. Alpha particles may bounce back when they shoot directly at the nucleus (left). 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 and 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. The Saturn model proposed by Hantaro in 1904 was unsuccessful because it could not overcome the difficulty of stability. Therefore, when Rutherford put forward the model of atomic nucleus 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 has extraordinary insight, so he can often grasp the essence and make scientific predictions. At the same time, he has a very strict scientific attitude, and he wants to draw conclusions from experimental facts. Rutherford believes that his model is far from perfect and needs further research and development. At the beginning of the paper, he declared: "At this stage, it is not necessary to consider the stability of the proposed atom, because obviously it will depend on the fine structure of the atom and the movement of charged components." In a letter to a friend that year, he also said, "I hope to give some clearer views on atomic structure in a year or two."
bohr model
Rutherford's theory attracted a young man from Denmark, whose name was niels bohr (1885- 1962) (left). On the basis of Rutherford 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.
Born into a family of professors in Copenhagen, Bohr received his doctorate from the University of Copenhagen in1910/. 19 12 studied in Rutherford's laboratory from March to July, during which his atomic theory was born. Bohr first extended Planck's quantum hypothesis to the internal energy of atoms to solve the difficulty in the stability of Rutherford's atomic model. It is assumed that atoms can only change energy through discrete energy photons, that is, atoms can only be in discrete steady state, and the lowest steady state is the normal state of atoms. Then, inspired by my friend Hansen, the concept of steady-state transition is obtained from the combination law of spectral lines. In July and September of 19 13 and 1 1, he published three parts of his long article on atomic structure and molecular structure.
Bohr's atomic theory gives such an atomic image: electrons move around the nucleus in a certain possible orbit, and the farther away from the nucleus, the higher the energy; The possible orbits are determined by the fact that the angular momentum of electrons must be an integer multiple of h/2π; When electrons move in these possible orbits, atoms do not emit or absorb energy. Only when electrons jump from one orbit to another, the emitted or absorbed radiation is single frequency. The relationship between radiation frequency and energy is given by e = h ν. Bohr's theory successfully explained the stability of atoms and the law of hydrogen atomic spectral lines.
Bohr's theory greatly expanded the influence of quantum theory and accelerated its development. 19 15 years, the German physicist arnold sommerfeld (1868- 195 1) extended Bohr's atomic theory to include elliptical orbits, and considered the special relativity effect of electron mass varying with its speed. The fine structure of the derivative spectrum is consistent with the experiment.
19 1955 In 1955, Albert Einstein (1879- 1955) statistically analyzed the process of material absorbing and emitting radiation according to Bohr's atomic theory, and deduced Planck's radiation law (Bohr and Einstein on the left). Einstein's work synthesizes the results of the first stage of quantum theory and integrates the work of Planck, Einstein and Bohr into a whole.
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