190 1 year, the French physicist jean baptiste perrin (1870- 1942) put forward a structural model, which holds that the center of an atom is some positively charged particles and the periphery is some electrons orbiting. The cycle of the electron's detour corresponds to the spectral line frequency emitted by the atom, and the outermost electron is thrown out to emit cathode rays. Raisin cake model (jujube cake model)
Joseph john thomson (1856- 1940) continued his 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 10-8cm, which is an amazing thing. Because Thomson model can explain many experimental facts at that time, it is easily accepted by many physicists. 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. 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 model of raisin cake, tiny electrons are distributed in a uniformly positively charged substance, while alpha particles are helium atoms that have lost two electrons, and their mass is several thousand 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 and Geiger repeated the experiment that had been done 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. When alpha particles shoot directly at the nucleus, they may bounce back. 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." Rutherford's theory attracted a young man from Denmark, whose name was niels henrik david bohr niels henrik david bohr (1885- 1962). 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=hν. 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. 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. There are more than a dozen Nobel Prize winners among Rutherford's students, such as Bohr, chadwick, Cockcroft, Kapicha and Hahn. After discovering the nucleus, Rutherford bombarded the nitrogen nucleus with alpha rays in 19 19, realizing the first alchemy and nuclear reaction in human history. From now on, elements are not eternal things. Rutherford discovered that protons, that is, hydrogen ions, are the components of all nuclei through a series of nuclear reactions, and predicted neutrons, which were later discovered by his student chadwick, and finally established a nuclear structure model based on protons and neutrons. After the establishment of Pauli exclusion principle, the periodic law of elements has also been explained. Rutherford was later called the father of nuclear physics. Of course, when Britain is booming, don't forget the French Curie couple, because the atomic shell needed by Rutherford's series of discoveries is α particles released by radioactive elements (especially radium). At this time, the French set up the Curie laboratory, and Curie was killed in a car accident. Mary won the Nobel Prize in chemistry for her achievements in radioactivity. The famous book General Theory of Radioactivity has been handed down from generation to generation. After the Curie Laboratory, it was presided over by young Curies: Aurio Curie and Elena Curie, both of whom were equally talented and not inferior to the three holy places. Little Curie and his wife were a little unlucky. They found that neutrons were robbed by chadwick, positrons by Anderson and nuclear fission by Hahn. Opportunities are fleeting. But in the end, he won the Nobel Prize for discovering artificial radioactivity. Now there are thousands of radioisotopes, most of which are artificially made, thanks to the little Curies.
The nuclear model experiment was successful, but it was in serious conflict with the basic theory at that time. According to classical electrodynamics, due to the circular motion of electrons, electromagnetic waves will be radiated, and due to the loss of energy, they will fall into the nucleus within 1ns, and at the same time emit a continuous spectrum. In other words, there is no such thing as an atom in theory. But atoms do exist, and they are stable, emitting a linear spectrum, which is supported by a lot of experimental facts and the whole chemistry. 19 1 1 year, a 26-year-old Danish young man came to Cambridge and then transferred to Rutherford Laboratory in Manchester, thus learning about the amazing discovery of atomic nuclei. Finally, he found a fundamental correction method of the nuclear model, which can not only explain the stability of atoms, but also calculate the radius of atoms. He is niels bohr, as famous as Einstein.
1885, balmer, a Swiss math teacher, discovered an empirical formula for the visible spectrum of hydrogen atoms, which was later extended to the Rydberg formula by the Swedish physicist Rydberg. 1900, the German physicist Planck put forward the concept of energy quantization and explained the spectrum of blackbody radiation. 1905, Einstein put forward the concept of light quantum. These conclusions gave Bohr great inspiration. Under these inspirations, Bohr applied the concept of quantization to the atomic model in 19 13 and put forward Bohr's hydrogen atomic model. The key of this model is three hypotheses put forward by Bohr. Steady-state assumption: electrons can only move in some discrete orbits and will not radiate electromagnetic waves. The frequency condition assumes that the energy level difference is the same as the photon energy absorbed (or emitted) by the atom. The quantization of angular momentum assumes that the angular momentum of electrons is an integer multiple of Planck's constant. Through a series of deduction, the mystery of hydrogen spectrum gradually surfaced and achieved great success. Bohr therefore won the Nobel Prize of 1922. Although Bohr model looks rough now, its significance lies not in the model itself, but in the concepts introduced when establishing the model: steady state, energy level, transition and so on. Bohr introduced correspondence principle to coordinate the conflict between hydrogen atom model and classical mechanics. After Bohr succeeded, he refused the invitation of his mentor Rutherford, returned to his motherland, and established a research institute in Copenhagen (later renamed Bohr Institute). Bohr Institute attracted a large number of outstanding young physicists from all over the world, including Heisenberg, Pauli and Dirac, the founders of quantum theory, and formed a rich academic atmosphere. At this time, Copenhagen began to explore the basic laws of physics.
Until now, physics can still be roughly divided into two schools. One school is a classical physics school represented by Einstein, and its members include Planck, De Broglie, Schrodinger, etc. One school is the Copenhagen School headed by Bohr, and its members are Bonn, Heisenberg, Pauli, Dirac, etc. Naturally, this debate is inconclusive. So what happened to physics after Bohr's hydrogen atom? What is the focus of the debate between the two scientific giants? British physicist james chadwick (189 1 ~ 1974) was born in England on 189 1 year. After graduating from Manchester University, he specialized in radioactive phenomena. Later, I went to Cambridge University and made many achievements under the guidance of Professor Rutherford. 1935 won the nobel prize in physics for discovering neutrons. During World War II, he went to the United States to study nuclear weapons. 1974 is dead.
He found that neutrons and protons have the same mass, but they are not charged. The existence of neutrons explains why the mass of atoms is greater than the total mass of protons and electrons. He also won the 1935 Nobel Prize for discovering neutrons.
Atoms are composed of positively charged nuclei and negatively charged electrons that revolve around the nuclei. Almost all atomic mass is concentrated on the nucleus. At first, people thought that the mass of the nucleus (according to Rutherford and Bohr's atomic model theory) should be equal to the number of positively charged protons it contains. However, some scientists found in their research that the number of positive charges in the nucleus is not equal to its mass! That is to say, the nucleus should contain other particles besides positively charged protons. So what are those "other particles"? James chadwick, a famous British physicist, solved this physical problem and found that "other particles" are "neutrons". 1930, when scientists Bert and Baker bombarded beryllium with alpha particles, they found a penetrating ray, which they thought was gamma ray and ignored. Webster even carefully identified this radiation and saw its neutrality, but it was difficult to explain this phenomenon, so he did not continue to study it in depth. Madame Curie's daughter, Irina Curie, and her husband also lingered on the edge of "Beryllium ray" and finally missed the neutron. Chadwick 189 1 was born in Cheshire, England, and graduated from Victoria University in Manchester. No talent in middle school. He is taciturn and his grades are average, but he insists on his creed: if you can do it, you must do it right and be meticulous; Never write unless you can do and understand it. So sometimes he can't finish his physics homework on time. It is his spirit of not being vain, seeking truth from facts and "recalling a horse ten times and making contributions" that has benefited him in scientific research for life. Chadwick, who entered the university, immediately showed his outstanding talent in physics research because of his solid basic knowledge. He was attracted by the famous scientist Rutherford. After graduation, he stayed in the physics laboratory of Manchester University and engaged in radioactive research under the guidance of Rutherford. Two years later, he won the British National Scholarship for his successful experiment of "Alpha rays deviate when passing through metal foil". Just as the dawn of his scientific research career dawned, he was put into a civilian prison camp in World War I, and it was not until the end of the war that he regained his freedom and returned to his scientific research post. 1923, due to his outstanding achievements in nuclear power charge measurement and research, he was promoted to be the deputy director of Cavendish Laboratory of Cambridge University, and worked with Director Rutherford on particle research. 193 1 year, Curie, the daughter and son-in-law of Aurio and Madame Curie, announced their new discovery that paraffin wax produced a large number of protons under the irradiation of beryllium rays. Chadwick immediately realized that this ray is probably composed of neutral particles, which is the key to solve the mystery that the positive charge of the nucleus is not equal to its mass! Chadwick immediately set out to study the experiments made by Aurio Curie and his wife, and measured the mass of this particle with a cloud chamber. It is found that the mass of this particle is the same as that of proton, and there is no charge. He called this particle a neutron. Neutrons were thus discovered by him. He solved the problems encountered by theoretical physicists in atomic research and completed the breakthrough in atomic physics research. Later, Italian physicist Fermi bombarded uranium nuclei with neutrons as a "shell", discovered nuclear fission and chain reaction in fission, and initiated a new era of human utilization of atomic energy. Chadwick won the 1935 Nobel Prize in Physics for his outstanding contribution to the discovery of neutrons.