atomic structure

Terminology of atomic physics

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Atomic structure (also called atomic model) refers to the composition of atoms and the collocation and arrangement of parts^[1] 。 The atom is very small. Take the carbon (C) atom as an example, its diameter is about 140pm (picometer), but it is usually recorded in radius. In the case of millimeter (mm), the diameter is 1.4X10 ^ - 7mm, which is formed by the Nucleus And some tiny Electronics These electrons move around the center of the atomic nucleus, just like the planets of the solar system move around the sun. And the atom is the same as any black particle in the universe. The latest research on atomic nuclei shows that protons or neutrons in atomic nuclei may be spherical vibrating energy layers composed of internal and external balance forces. Based on this principle, various stable atomic nuclei can be constructed by using energy stacks of different sizes. The entry details neutral atom model, solid charged sphere model, jujube cake model, Saturn model, solar system model, Bohr model, nuclear model and Chadwick model.

Chinese name: atomic structure
Foreign name: atomic structure

Alias: Atomic model
Concept proposer: Dalton
Discipline: Atomic physics

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Discovery history

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From British chemists and physicists Dalton (J. John Dalton, 1766-1844) Atomic theory Later, for a long time, people believed that the atom was like a glass that could not be smaller Solid ball , there is nothing in it anymore^[2] 。

German scientists since 1869 Hittorf find Cathode ray Later, Krux hertz A large number of scientists, such as Lerner and Thomson, have studied cathode rays for more than 20 years. Finally, Joseph John Thomson discovered the existence of electrons. In general, the atom is uncharged. Since it can produce negative electrons 1700 times smaller than its mass from the atom, it means that there is structure in the atom, and there is also band in the atom positive electricity They should be neutralized with the negative charge of the electron to make the atom neutral.

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What else is there in an atom except electrons? How does an electron stay in an atom? What does an atom contain positive charge How the positive charge is distributed, how the negatively charged electron interacts with the positively charged thing, and a lot of new problems have been posed to physicists. According to scientific practice and experimental observation results at that time, physicists played their rich imagination and proposed various atomic models.

1901 French physicist Perrin (Jean Baptiste Perrin, 1870-1942.

Atomic model

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Neutral atom model

1902 German physicist Leonard (Philip Edward Anton Lenard, 1862-1947) proposed Neutral particle dynamo Model. Leonard's early observations show that, Cathode ray It can pass through the aluminum window inside the vacuum tube to the outside of the tube. According to this observation, in 1903, he proved that high-speed cathode rays could pass through thousands of atoms through absorption experiments. According to the prevailing semi materialism at that time, most of the volume of atoms is empty space, while rigid matter is only about 10-9% of its total volume (that is, 1/100000). Leonard imagined that "rigid matter" is scattered in the inner space of atoms Several positive electricity and negative electricity Synthesis of^[3] 。

Solid charged ball

Schematic diagram of atomic internal structure

Famous British physicist and inventor Kelvin (Lord Kelvin, 1824-1907) Formerly known as W Thomson (William Thomson), for 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, beginning to use the name Kelvin. Kelvin's research scope is extensive heat 、 electromagnetics , fluid mechanics, optics, geophysics, mathematics engineering application And other aspects have made contributions. 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 put forward the model of solid charged sphere atom in 1902, that is, he regarded the atom as a uniform band positive electricity A sphere with negatively charged electrons embedded in it. Under normal conditions Electrostatic balance 。 This model was later developed by J J. Thomson developed it and later became known as Thomson Atomic model.

Jujube cake model

Raisin cake model (date cake model)^[4]

Joseph John Thomson (1856-1940) continued to carry out more systematic research, trying to describe the atomic structure. Thomson thought that the atom contained a uniform anode ball, Several negative electrons Run inside the ball. He followed Mayer (Alfred Mayer) About floating magnet The balance research proves 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, they will be listed in two rings, and so on, to multiple rings. In this way, the increase of electrons results in periodic similarity in structure Mendeleyev periodic table The repetition of physical and chemical properties in may also be explained.

This model proposed by Thomson, Electron distribution In the sphere, it is a little 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 the cathode ray phenomenon and the phenomenon that metals can emit electrons under ultraviolet radiation. Moreover, according to this model, the size of the atom can be estimated to be about 10-8 cm, which is a great thing. Just because Thomson model can explain many experimental facts at that time, it is easy to be accepted by many physicists.

Saturn model

Japanese physicist Bantaro Nagaoka (Nagaoka Hantaro, 1865-1950) December 5, 1903^[5] In 1904, it was published in Japanese, English and German magazines Electronic Motion in Atoms Explaining Linear and Band Spectra and Radioactive Phenomena Thesis of. He criticized Thomson He proposed a structure he called the "Saturn model", that is, the surrounding belt positive electricity At its core is an atomic model of electron ring rotation. A large, positively charged ball with a circle of electrons equally spaced around it angular velocity do Circular motion 。 Radial vibration emission line spectrum of electron, perpendicular to torus The vibration of Band spectrum The electrons on the ring fly out of beta rays, Central ball The positive particles of Alpha ray 。 This Saturn like model had a great influence on his later model of atomic nuclei. In 1905, he went from Alpha particle The analysis of experimental results such as the measurement of the charge mass ratio of Helium ion 。 In 1908, Swiss scientist Leeds (Leeds) proposed Magnetic atom model 。

Their models can explain some experimental facts at that time to a certain extent, but can not explain many new experimental results emerging in the future, so they have not been further developed. Years later, Thomson's“ Eccles cake The model "is used by its own students rutherford Overthrew.

Solar system model

British physicist Ernest Rutherford (Ernest Rutherford, 1871-1937) came to England in 1895 Cavendish Laboratory , followed Thomson and became Thomson's first graduate student from overseas. Rutherford is studious and diligent. Under the guidance of Thomson, Rutherford is doing his first experiment—— radioactivity Found during absorption experiment Alpha ray 。

Experiments to verify Thomson's model

The ingenious experiment designed by Rutherford uranium 、 radium etc. radioactive element Put it in a lead container, leaving only a small hole on the lead container. Because lead can block the radiation, only a small part of the radiation comes out of the hole, forming a very narrow beam of radiation. Rutherford Radiation A strong magnet was placed near the beam, and it was found that there was a kind of ray that was not affected by the magnet and kept moving in a straight line. The second kind of ray is affected by the magnet and deflects to one side, but it does not deflect too much. The third kind of ray deflects severely.

rutherford Put materials of different thicknesses in the direction of radiation and observe the absorption of radiation. The first kind of ray is not affected by the magnetic field, which means that it is not charged and has strong penetration. General materials, such as paper and wood chips, cannot block the ray's progress. Only thick lead plates can completely block it, which is called Gamma ray 。 The second kind of ray will deflect to one side due to the influence of the magnetic field. It can be judged from the direction of the magnetic field that this kind of 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 what Rutherford found Alpha ray 。 The third kind of ray is determined to be negatively charged from the deflection direction, and its property is the same as that of fast moving electrons. It is called Beta ray 。 Rutherford was particularly interested in his own discovery of alpha rays. After thorough and careful research, he pointed out that alpha rays are positively charged Particle Flow , these particles are Helium atom An ion of, that is, a helium atom with two electrons missing.

"Counter tube" is a student from Germany Hans Geiger (Hans Geiger, 1882-1945), which can be used to measure charged particles invisible to the naked eye. When charged particles pass through Counter tube The counter will send out an electrical signal. Connect this electrical signal to the alarm, the instrument will send out a "click" sound, and the indicator light will also light up once. The invisible and intangible rays can be recorded and measured with very simple instruments. This instrument is called Geiger counter tube. With the help of Geiger counter, the Manchester laboratory led by Rutherford Alpha particle The research on properties has developed rapidly.

1910 Marsden (E. Marsden, 1889-1970) came University of Manchester Rutherford asked him to bombard with alpha particles Gold foil , do practice experiments, and use the fluorescent screen to record those alpha particles passing through the gold foil. According to Thomson Eccles cake Model, electrons with small mass are distributed in uniform band positive electricity The alpha particle is a helium atom that has lost two electrons, and its mass is thousands of times larger than the electron. When such a heavy shell strikes an atom, small electrons cannot resist it. The positive matter in the gold atom is evenly distributed in the entire atomic volume, and it is impossible to resist the bombardment of alpha particles. in other words, Alpha particle It will easily pass through the gold foil. Even if it is slightly blocked, it is only a slight change of direction after the alpha particles pass through the gold foil. Rutherford and Geiger have done such experiments for many times, and their observation results are in good agreement with Thomson's raisin cake model. The alpha particle changes its direction slightly due to the influence of the gold atom, and its scattering angle is very small.

Marsden and Geiger Repeat the experiment that has been done many times, and a miracle appears! They not only observed the scattering alpha particles, but also the alpha particles reflected back by the gold foil. In a speech in Rutherford's later years, he once described the scene at that time. He said: "I remember that two or three days later, Geiger came to me excitedly and said: 'We got some reflected alpha particles...' This is the most incredible event in my life. It's just like you shot a 15 inch shell at a cigarette paper, but it was hit by a reflected shell. After thinking, I realized that this backscattering can only be the result of a single collision. After calculation, I see that if we do not consider Atomic mass Most of them are concentrated in a very small nucleus, which is impossible to obtain Order of magnitude Of the. "

When Rutherford said "after thinking", he did not think for a day or two, but for a whole year or two. After a lot of experiments, theoretical calculations and careful consideration, he boldly put forward the idea of nuclear Atomic model , overthrowing his teacher Thomson's model of solid charged sphere atoms.

Rutherford carefully measured the total number of alpha particles reflected back after checking that they were really alpha particles in his students' experiments. The measurement shows that under their experimental conditions, one alpha particle is reflected back for every eight thousand alpha particles incident. Thomson's solid charged sphere atomic model and charged particle scattering theory can only explain the small angle scattering of alpha particles, but can not explain the large angle scattering. Multiple scattering can obtain large angle scattering, but the calculation results show that the probability of multiple scattering is extremely small, which is far from the observation that one of the 8000 alpha particles reflected back.

Thomson atomic model can not explain the scattering of alpha particles. Rutherford found that only assuming that the positive charges are concentrated in a small area and alpha particles pass through a single atom can large angle scattering occur after careful calculation and comparison. In other words, the positive charge of the atom must be concentrated in a very small nucleus at the center of the atom. On the basis of this assumption, Rutherford further calculated some laws of alpha scattering and made some deductions. 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 a positively charged nucleus 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 the positively charged matter in the atom is concentrated on a very small core, and most of the atomic mass is also concentrated on this very small core. When alpha particles shoot directly at the atomic core, they may bounce back. This satisfactorily explains the large angle scattering of alpha particles. Rutherford published a famous paper, "Scattering of Matter on α and β Particles and Its Principle Structure".

Rutherford's theory has 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 ignored by physicists. Rutherford atomic model The existing fatal weakness is between positive and negative charges Electric field force The requirement of stability cannot be met, that is, it is impossible to explain how the electrons are stable outside the nucleus. The Saturn model proposed by Nagaoka Bantaro in 1904 failed because of the difficulty of stability. Therefore, when Rutherford proposed the nuclear atom model again, many scientists regarded it as a guess, or just one of various models, and ignored the solid experimental basis on which Rutherford proposed the model.

Rutherford has extraordinary insight, so he can often grasp the essence to make scientific predictions. At the same time, he has a very rigorous scientific attitude, and he draws the conclusions that should be made from the experimental facts. Rutherford believes that the model proposed by himself is still imperfect 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 atom in question, because obviously it will depend on the fine structure of the atom and the movement of the charged components." In his letter to a friend that year, he also said: "I hope that in one or two years, I can say something more specific about the atomic structure."

Bohr model

Rutherford's theory attracted a young man from Denmark, his name is Nils Henrik David· Bohr Niels Henrik David Bohr，1885-1962), On the basis of Rutherford's model, he proposed that quantized orbit , solved the stability problem of atomic structure, and described a complete and convincing theory of atomic structure^[6] 。

Born into a professor's family in Copenhagen, Bohr was awarded the title of University of Copenhagen doctorate. From March to July 1912, he studied in Rutherford's laboratory, during which time he gave birth to his atomic theory. Bohr first Plonk The quantum hypothesis of is extended to the energy inside the atom to solve the difficulty in the stability of Rutherford's atomic model. It is assumed that the atom can only pass the discrete Energon To change its energy, that is, atoms can only be separated Stationary state And the lowest stationary state is the normal state of the atom. Then he went to his friend Hansen Inspired by Spectral line In July, September and November 1913, he published three parts of his long paper "On Atomic Structure and Molecular Structure".

Bohr's atomic theory gives the atomic image that electrons orbit around the nucleus in some specific possible orbits Circular motion , the farther away from the nucleus, the higher the energy; The angular momentum of the electron must be h/2π Is determined by integral multiple of; When electrons move in these possible orbits, atoms do not emit or absorb energy. Only when electrons transition from one orbit to another can atoms emit or absorb energy. Moreover, the radiation emitted or absorbed is of single frequency. The relationship between the frequency of radiation and energy is E=hν give. Bohr's theory successfully explains the stability and Atomic hydrogen spectroscopy Line rule.

Bohr His theory has greatly expanded the influence of quantum theory and accelerated the development of quantum theory. In 1915, German physicist sommerfeld (Arnold Sommerfeld, 1868-1951) extended Bohr's atomic theory to include Elliptical orbit , and taking into account the special relativistic effect that the mass of the electron changes with its speed, the spectral fine structure It is consistent with the experiment.

In 1916, Einstein (Albert Einstein, 1879-1955) Based on Bohr's atomic theory, the process of absorption and emission of radiation of matter was analyzed statistically, and the Planck radiation law 。 Einstein's work synthesizes the achievements of the first stage of quantum theory, and integrates the work of Planck, Einstein and Bohr into a whole.

Nucleated model

Among Rutherford's students, there are more than a dozen Nobel Prize winners, including Bohr 、 Chadwick 、 Cocroft 、 Kapitza 、 Hahn After the discovery of atomic nuclei, Rutherford bombarded nitrogen atomic nuclei with alpha rays in 1919, realizing "alchemy" and nuclear reaction for the first time in human history. Since then, elements are not eternal. Rutherford discovered the proton through a series of nuclear reactions Hydrogen ion It is the component of all atomic nuclei, and predicted neutrons, which were later discovered by his student Chadwick, and finally established the proton and neutron based Nuclear structure Model. Pauli exclusion principle Once established, Periodic law of elements It was also explained. Rutherford was later called nuclear physics Father. Of course, don't forget France's Curie Couple, because the atomic shells needed for Rutherford's series of discoveries were emitted by radioactive elements (especially radium) Alpha particle 。 Curie Laboratory was set up in France at this time. Curie was killed in a car accident, and Mary was rewarded for her achievements in radioactivity Nobel Prize in Chemistry , famous works General Theory of Radiology It was handed down from generation to generation that after Curie Lab, the little Curie couple: Julio Curie and Elena Curie The host is also full of talents, which is no inferior to the three holy places. Little Curie and his wife were a bit unlucky. They found that the neutron was preempted by Chadwick, and the positron was Anderson First, we found that nuclear fission was Hahn Take the lead, the opportunity is fleeting. But finally, because Artificial radioactivity And obtained Nobel Prize 。 nowadays radio isotope There are thousands of them, most of which are artificially generated, thanks to the Little Curie couple.

The nuclear model has achieved success in experiments, but there is a serious conflict with the basic theory at that time. According to classical electrodynamics, because electrons Circular motion , it will radiate electromagnetic waves, because of the loss of energy, it will fall into the atomic nucleus within 1ns, and emit at the same time Continuous spectrum 。 In other words, there is no such thing as atom in theory. But atoms do exist and are stable, emitting Linear spectrum There are a lot of experimental facts and the support of the whole chemistry. In 1911, a 26 year old Danish young man came Cambridge , then go to Manchester Rutherford's laboratory, thereby understanding the amazing discovery of atomic nucleus. Finally, he found a fundamental correction method for 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.

In 1885, a mathematics teacher in Switzerland Balmer An empirical formula for the visible spectrum of hydrogen atom was discovered Rydberg Promoted as Rydberg formula 。 In 1900, German physicist Planck proposed energy quantization The concept of Blackbody radiation spectrum 。 In 1905, Einstein proposed Light quanta Concept. These conclusions give Bohr great inspiration. Under these inspirations, Bohr used the concept of quantization in 1913 Atomic model Bohr's hydrogen atom Model. The key to this model is the three assumptions introduced by Bohr. Stationary hypothesis : Electrons can only move in some discrete orbits and will not radiate electromagnetic wave 。 Frequency condition assumption: energy level Differing from atomic absorption (or emission) Photon energy Same. angular momentum Quantization hypothesis: the angular momentum of the electron is about Planck constant Integer multiple of. Through a series of derivation, Hydrogen spectrum The mystery gradually surfaced and achieved great success. Bohr won the Nobel Prize in 1922. although Bohr model It seems rough now, but its significance lies not in the model itself, but in the concepts introduced when building the model: Stationary state , energy levels, transitions, etc. Bohr introduced Correspondence principle The conflict between the hydrogen atom model and classical mechanics is reconciled. After Bohr's success, he refused the invitation of his mentor Rutherford, returned to his motherland, and set up a research institute in Copenhagen (later renamed Bohr Institute ）Bohr Institute has attracted a large number of outstanding young physicists from all over the world, including the founder of quantum theory heisenberg 、 Pauli and dirac A strong academic atmosphere was formed, and Copenhagen began to explore the basic laws of physics.

Up to now, physics can still be roughly divided into two schools, one is the classical school of physics represented by Einstein, and the members are roughly Plonk 、 De Broglie 、 Schrodinger Etc; One school is led by Bohr Copenhagen School , members are roughly bonn Heisenberg, Pauli, Dirac, etc. Naturally, this debate has not yet come to an end. So that is Bohr hydrogen atom What changes have taken place in physics since then? What is the focus of the dispute between the two giants of science?

Chadwick model

In 1935, Sir James Chadwick, a British physicist, was born in England in 1891. After graduating from Manchester University, he specialized in the study of radioactive phenomena. Later, under the guidance of Professor Rutherford, he made many achievements in Cambridge University. In 1935, due to the discovery of neutrons The nobel prize in physics 。 the Second World War China once went to the United States to engage in nuclear weapons research. Died in 1974^[7] 。

He found that neutrons and protons have the same mass, but they are not charged. The existence of neutrons explains why the atomic mass is greater than the total mass of protons and electrons. He also won the 1935 Nobel Prize for the discovery of neutrons.

An atom is composed of a positively charged nucleus and negatively charged electrons running around the nucleus. Almost all the mass of an atom is concentrated in the nucleus. At first, people thought that the mass of the atomic 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 of the atomic nucleus is not equal to its mass! In other words, the nucleus should contain other particles besides the positively charged protons. So, what are those "other particles"?

The famous British physicist James Chadwick solved this physical problem and found that "other particles" are "neutrons".

In 1930, when scientists Bott and Beck bombarded beryllium with alpha particles, they found a kind of ray with strong penetrability. They thought it was gamma ray and ignored it. Webster even made a careful identification of this radiation and saw its neutral nature, but it was difficult to explain this phenomenon, so he did not continue to study it in depth. Madame Curie's daughter, Elena Curie, and her husband also wandered on the edge of "beryllium ray", and finally lost touch with neutrons.

Chadwick was born in Cheshire in 1891 and graduated from Victoria University in Manchester. Middle school did not show great talent. He is silent and his achievements are mediocre, but he insists on his credo: if you can do it, you must do it correctly and meticulously; Can't do it and don't understand it, never write. So sometimes he can't finish his physics homework on time. It is his spirit of not focusing on vanity, seeking truth from facts, and "the weak horse will never give up his contribution" that has benefited him in the scientific research career all his life.

After entering the university, Chadwick immediately showed his outstanding talent in physical research because of his solid basic knowledge. He was liked by Rutherford, a famous scientist, and stayed in the physics laboratory of Manchester University after graduation to engage in radiation research under Rutherford's guidance. Two years later, he won the British National Scholarship due to his successful experiment of "alpha rays deviate when passing through metal foil".

Just at the dawn of his scientific research career, World War I put him in a civilian prison camp. Until the end of the war, he was free and returned to scientific research. In 1923, he was promoted to be the deputy director of Cavendish Laboratory of Cambridge University and engaged in particle research together with Rutherford, the director, because of his outstanding achievements in the measurement and research of nuclear charge.

In 1931, the Jolio Curie couple, Madame Curie's daughter and son-in-law, announced their new discovery that paraffin produced a large number of protons under the irradiation of beryllium rays. Chadwick immediately realized that this ray might be composed of neutral particles, which is the key to solving the mystery of the difference between the positive charge of the atomic nucleus and its mass!

Chadwick immediately set out to study the experiment carried out by Jolio Curie and his wife, and measured the mass of this particle with a cloud chamber. It was found that the mass of this particle was the same as that of the proton, and it was not charged. He called this particle "neutron".

So he found the neutron. He solved it Theoretical physicist The difficulties encountered in atomic research have completed a breakthrough in atomic physics research. Later, Italian physicist Fermi used neutrons as "cannonballs" to bombard uranium atomic nuclei, and discovered nuclear fission and chain reaction in fission, opening a new era for mankind to use atomic energy. Chadwick won the 1935 Nobel Prize in Physics for his outstanding contribution to the discovery of neutrons.

Quantitative relation

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Quantitative relationship between structural particles constituting atoms

① Mass number (A)=proton number (Z)+neutron number (N)

② Proton number= Nuclear charge number =Number of extranuclear electrons=atomic number

Note: Neutron determines the atomic type (isotope), the number of masses determines the approximate relative atomic mass of atoms, and the number of protons (nuclear charge) determines the element type; The number of the outermost electrons of an atom determines whether the whole atom is apparent or not, and also determines Main family elements Chemical properties of.

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