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Atomic structure



History of Discovery

Since the British chemist and physicist Dalton (J.John Dalton, 1766~1844) founded the atomic theory, for a long time people believed that atoms were It's like a solid glass ball too small, and there is no more tricks inside.

Since the discovery of cathode rays by German scientist Hitoff in 1869, a large number of scientists such as Crooks, Hertz, Lerner, Thomson, etc. have studied cathode rays for more than 20 years. Finally, Thomson (Joseph John Thomson) discovered the existence of electrons. Normally, atoms are uncharged. Since negatively charged electrons that are 1700 times smaller than its mass can escape from the atom, this shows that there is still a structure inside the atom, and it also shows that there are still positively charged things in the atom. They Should neutralize the negative charge carried by the electrons to make the atom neutral.

Introduction

What other things are in an atom besides electrons, how do electrons stay in the atom, what is positively charged in the atom, how is the positive charge distributed, with A lot of new questions such as how negatively charged electrons and positively charged things interact are before physicists. Based on scientific practice and experimental observations at the time, physicists used their rich imagination and proposed various atomic models.

In 1901, the French physicist Perrin (Jean Baptiste Perrin, 1870-1942) proposed a structural model. It was believed that the center of the atom was some positively charged particles, and the periphery was some revolving electrons. The revolving period corresponds to the frequency of the spectral line emitted by the atom, and the outermost electrons are ejected to emit cathode rays.

Atomic model

Neutral atom model

In 1902, German physicist Philipp Edward Anton Lenard (1862-1947) proposed neutral Particle power sub-model. Lenard’s early observations showed that cathode rays can pass through the aluminum window inside the vacuum tube to the outside of the tube. Based on this observation, his absorption experiment in 1903 proved that high-speed cathode rays can pass thousands of atoms. According to the semi-materialist view that prevailed at the time, most of the volume of an atom is empty space, while rigid matter is only about 10-9 (that is, one hundred thousandths) of its entirety. Lenard imagined that "rigid matter" is a composite of several positive and negative electricity scattered in the inner space of atoms.

Solid charged ball

The famous British physicist and inventor Kelvin (Lord Kelvin, 1824~1907) was originally named W. Thomson (William Thomson), because of the installation of the first The Atlantic submarine cable had merits. The British government knighted him in 1866 and was promoted to Lord Kelvin in 1892, beginning to use the name Kelvin. Kelvin has a wide range of research and has made contributions in the fields of heat, electromagnetics, fluid mechanics, optics, geophysics, mathematics, and engineering applications. 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 sphere atom model in 1902, which regarded atoms as uniformly positively charged spheres with negatively charged electrons buried in them, and they were in electrostatic equilibrium under normal conditions. This model was later developed by J.J. Thomson, and was later known as the Thomson atomic model.

Date cake model

Raisin cake model (date cake model)

Thomson (Joseph John Thomson, 1856-1940) continues to conduct more systematic research , Try to describe the atomic structure. Thomson thought that the atom contains a uniform positive electric sphere, and several negative electrons run 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 moving electrons must be stable. If the number of electrons exceeds this limit, they will be listed into two rings, and so on up to as many as rings. In this way, the increase of electrons has caused the similarity of the periodicity in structure, and the repeated reproduction of physical and chemical properties in Mendeleev's periodic table may also be explained.

This model proposed by Thomson, the electrons are distributed in a sphere, 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 the phenomenon of cathode rays and the phenomenon that metals can emit electrons under ultraviolet radiation. And according to this model, the size of the atom can be estimated to be about 10^-8 cm. This is an amazing thing. Because the Thomson model can explain many experimental facts at that time, it is easily accepted by many physicists.

The Saturn model

The Japanese physicist Nagaoka Hantaro (1865-1950) presented an oral presentation at the Tokyo Mathematical and Physics Society on December 5, 1903, and separately in 1904 The paper "Explaining the movement of electrons in atoms in linear and band spectra and radioactive phenomena" was published in Japanese, English, and German magazines. He criticized Thomson's model, thinking that positive and negative electricity cannot penetrate each other, and proposed a structure that he called the "Saturn model"-that is, an atomic model with an electron ring rotating around a positively charged core. A large-mass positively charged ball has a circle of equally spaced electrons moving in a circle at the same angular velocity. The radial vibration emission line spectrum of electrons, the vibration perpendicular to the ring surface, the emission band spectrum, the electrons on the ring fly out as beta rays, and the positively charged particles in the center sphere fly out as alpha rays. This Saturn model had a great influence on his later establishment of a nuclear model of the atom. In 1905, he analyzed the results of experiments such as the measurement of the charge-to-mass ratio of alpha particles, and he found that alpha particles are helium ions. In 1908, Swiss scientist Leeds proposed a magnetic atom model.

Their model can explain some experimental facts at the time to a certain extent, but it cannot explain many new experimental results that will appear in the future, 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

The British physicist Ernest Rutherford (1871-1937) came to the Cavendish Laboratory in England in 1895 to follow Thomson Study and become Thomson's first graduate student from overseas. Rutherford was studious and diligent. Under Thomson's guidance, Rutherford discovered alpha rays during his first experiment, the 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 the radiation, only a small part of the radiation is emitted from the small hole, forming a very narrow beam of radiation. Rutherford placed a strong magnet near the radiation beam and found that there is a kind of radiation that is not affected by the magnet and keeps traveling in a straight line. The second type of ray is deflected to one side by the influence of the magnet, but is not deflected too much. The third type of ray deflects severely.

Rutherford places materials of different thicknesses in the forward direction of the radiation to observe how the radiation is absorbed. The first type of radiation is not affected by the magnetic field, indicating that it is uncharged and has a strong penetrating power. Common materials such as paper and wood chips cannot block the progress of the radiation, and only thicker 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 deflect 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 by a piece of paper. This is the alpha ray discovered by Rutherford. The third type of ray is determined by the deflection direction to be negatively charged and has the same properties as fast-moving electrons, called beta rays. Rutherford was particularly interested in the alpha rays he had discovered. After intensive research, he pointed out that alpha rays are a stream of positively charged particles. These particles are ions of helium atoms, that is, helium atoms with two fewer electrons.

The "counting 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 the charged particles pass through the counter tube, the counter tube sends out an electric signal. Connect this electric signal to the alarm, the instrument will make a "click" sound, and the indicator light will also light up. The invisible rays can be recorded and measured with a very simple instrument. People call this instrument a Geiger counter. With the help of Geiger counters, the Manchester laboratory led by Rutherford has made rapid progress in research on the properties of alpha particles.

In 1910, Marsden (E. Marsden, 1889-1970) came to Manchester University. Rutherford asked him to bombard the gold foil with alpha particles, do practice experiments, and use the fluorescent screen to record those who passed through the gold foil. Alpha particles. According to Thomson’s raisin cake model, tiny electrons are distributed in a uniformly positively charged matter, and alpha particles are helium atoms that have lost two electrons, and their mass is thousands of times larger than electrons. When such a heavy artillery shell bombards atoms, the tiny electrons cannot withstand it. The positive matter in gold atoms is evenly distributed in 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 it is blocked a little, it will only change the direction of the alpha particles slightly after passing through the gold foil. Rutherford and Geiger have done this type of experiment many times, and their observations are in good agreement with Thomson's raisin cake model. The alpha particle changes its direction slightly under the influence of gold atoms, and its scattering angle is extremely small.

Marsden and Geiger repeated this experiment that had been done many times, and a miracle appeared! They not only observed the scattered alpha particles, but also the alpha particles reflected by the gold foil. Rutherford described the situation in a speech in his later years. He said: "I remember two or three days later, Geiger came to me very excitedly and said:'We got some reflected alpha particles... ....', this is the most incredible event in my life. It is as incredible as when you shoot a 15-inch cannonball at the cigarette paper, but you are hit by the reflected cannonball. After thinking about it, I know This kind of backscattering can only be the result of a single collision. After calculations, I have seen that if most of the atomic mass is concentrated in a small nucleus, it is impossible to get this order of magnitude."< /p>

What Rutherford said "after thinking" is not thinking about one or two days, but thinking about a whole year or two. After doing a lot of experiments and theoretical calculations and careful consideration, he boldly proposed a nuclear atom model, overthrowing his teacher Thomson's solid charged sphere atom model.

Rutherford verified that the reflected alpha particles in his student's experiment were indeed alpha particles, and then carefully measured the total number of alpha particles reflected. Measurements show that under their experimental conditions, one alpha particle is reflected back for every 8,000 alpha particles incident. Using Thomson's solid charged sphere atom model and the scattering theory of charged particles can only explain the small-angle scattering of alpha particles, but cannot 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 too far from the observation that one of the 8,000 alpha particles is reflected back.

The Thomson atomic model cannot explain the scattering of alpha particles. After careful calculation and comparison, Rutherford found that only if the positive charges are concentrated in a small area, when alpha particles pass through a single atom, It is possible for large-angle scattering to occur. In other words, the positive charge of the atom must be concentrated in a small nucleus in 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.

The atomic model proposed by Rutherford 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 that governs between them is the electromagnetic interaction force. He explained that the positively charged matter in the atom is concentrated in a small core, and most of the atomic mass is also concentrated in this small core. When alpha particles are shot directly at the atomic core, they may be bounced back. This satisfactorily explains the large-angle scattering of alpha 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 the structure of the atom and made an immortal feat for the development of atomic science. However, for a long time at that time, Rutherford's theory was coldly treated by physicists. The fatal weakness of the Rutherford atomic model is that the electric field force between the positive and negative charges cannot meet the requirements of stability, that is, it cannot explain how the electrons stay outside the nucleus stably. The Saturn model proposed by Nagaoka Hantaro in 1904 was unsuccessful because it could not overcome the difficulty of stability. Therefore, when Rutherford proposed a nuclear atom model, many scientists regarded it as a conjecture, or one of various models, and ignored the solid foundation on which Rutherford proposed the model. Experimental basis.

Rutherford has extraordinary insight, so he can often grasp the essence and make scientific predictions. At the same time, he has a very rigorous scientific attitude. He starts from experimental facts and draws conclusions that should be made. Rutherford believes that the model he proposed is still very imperfect and needs further research and development. He stated at the beginning of the paper: "At this stage, it is not necessary to consider the stability of the atom mentioned, because obviously it will depend on the fine structure of the atom and the movement of the charged components." He also wrote to his friends that year. Said: "I hope to have some clearer insights into atomic structure within one or two years."

Bohr Model

Rutherford’s theory attracted a person from Denmark A young man named Niels Henrik David Bohr (Niels Henrik David Bohr, 1885-1962), based on the Rutherford model, he proposed the quantized orbit of electrons outside the nucleus , Solved the problem of the stability of the atomic structure, and described a complete and convincing theory of the 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 he gave birth to his atomic theory. Bohr first extended Planck’s quantum hypothesis to the energy inside the atom to solve the difficulty in the stability of the Rutherford atomic model, assuming that the atom can only change its energy through discrete energy quantum, that is, the atom can only In the discrete stationary state, and the lowest stationary state is the normal state of the atom. Then, inspired by his friend Hansen, he reached the concept of steady state transition from the law of combination of spectral lines. In July, September and November 1913, he published the three parts of a long paper "On Atomic Structure and Molecular Structure".

Bohr’s atomic theory gives such an image of the atom: the electron moves in a circle around the nucleus on some specific possible orbits, and the farther away from the nucleus, the higher the energy; It is determined by an integer multiple of h/2π; when electrons move on these possible orbits, atoms do not emit or absorb energy, and only when electrons transition from one orbit to another, atoms emit or Absorb energy, and the emitted or absorbed radiation is single-frequency. The relationship between the frequency and energy of the radiation is given by E=hν. Bohr's theory successfully explained the stability of the atom and the law of the spectrum of hydrogen atoms.

Bohr's theory greatly expanded the influence of quantum theory and accelerated the development of quantum theory. In 1915, the German physicist Arnold Sommerfeld (1868-1951) extended Bohr’s atomic theory to include elliptical orbits, and took into account the special relativity effect that the mass of an electron changes with its speed, and derived the fineness 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 matter’s absorption and emission of radiation, and derived Planck’s law of radiation . This work of Einstein combines the achievements of the first stage of quantum theory, combining the work of Planck, Einstein, and Bohr into a whole.

Nuclear model

Among Rutherford’s students, there are more than a dozen Nobel Prize winners. The famous ones are Bohr, Chadwick, Cockcroft, After the discovery of the atomic nucleus, Kapica, Hahn, etc., Rutherford used alpha rays to bombard the nitrogen nucleus in 1919, achieving "alchemy" for the first time in human history and the first nuclear reaction. From then on, the element is not an eternal thing. Through a series of nuclear reactions, Rutherford discovered that protons, or hydrogen ions, are the components of all atomic nuclei, and predicted neutrons. The neutrons were later discovered by his student Chadwick, and finally established that protons and neutrons are Basic nuclear structure model. After the Pauli exclusion principle was established, the periodic law of the elements was also explained. Rutherford came to be known as the father of nuclear physics. Of course, at the time when Britain was vigorous and vigorous, don't forget the Curies of France, because the atomic bombs needed for Rutherford's series of discoveries were alpha particles emitted by radioactive elements (especially radium). At this time, France established the Curie Laboratory. Curie was killed in a car accident. Marie won the Nobel Prize in Chemistry for his achievements in radioactivity. The famous book "General Theory of Radioactivity" was handed down. Hosted by Joliot Curie and Ilena Curie, they are equally talented, and they are not inferior to the three holy places. Little Curie and his wife were a little less lucky. They found that the neutron was preempted by Chadwick, the positron was preempted by Anderson, and the nuclear fission was preempted by Hahn, and the opportunity was fleeting. But in the end, he won the Nobel Prize for the discovery of artificial radioactivity. Today there are thousands of radioactive isotopes, most of which are artificially produced, thanks to the little Curies.

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, due to the circular motion of electrons, electromagnetic waves are bound to be radiated. Due to the loss of energy, it will fall into the nucleus within 1 ns and emit a continuous spectrum at the same time. In other words, there is no such thing as an atom in theory. But atoms do exist and are stable, emitting linear spectra, supported by a large number of experimental facts and the entire chemistry. In 1911, a 26-year-old Danish young man came to Cambridge and then transferred to the Rutherford Laboratory in Manchester to learn about this amazing discovery of the atomic nucleus. In the end, he found a fundamental correction method of the nuclear model, which can not only explain the stability of the atom, but also calculate the radius of the atom. He is Niels Bohr who is as famous as Einstein.

In 1885, Barmer, a mathematics teacher in Switzerland, discovered an empirical formula for the visible spectrum of the hydrogen atom, which was later promoted by the Swedish physicist Derber as the Rydberg formula. In 1900, German physicist Planck proposed the concept of energy quantization, explaining the black body radiation spectrum. In 1905, Einstein proposed the concept of light quantum. These conclusions gave Bohr a lot of inspiration. Under these revelations, Bohr applied the concept of quantization to the atomic model in 1913 and proposed Bohr's hydrogen atom model. The key to this model is the three assumptions introduced by Bohr. Steady-state assumption: electrons can only move on some discrete orbits, and they will not radiate electromagnetic waves. The frequency condition assumes that the energy level difference is the same as the energy of the photon absorbed (or emitted) by the atom. The hypothesis of angular momentum quantization: the angular momentum of an electron is an integer multiple of approximately Planck's constant. Through a series of derivations, the mystery of the hydrogen spectrum gradually surfaced and achieved great success. Bohr won the Nobel Prize in 1922 for this. Although the Bohr model seems to be relatively rough now, its significance does not lie in the model itself, but in the concepts introduced when building the model: stationary state, energy level, transition, etc. Bohr introduced the principle of correspondence to harmonize the conflict between the hydrogen atom model and classical mechanics. After Bohr’s success, he rejected the invitation of his mentor Rutherford and returned to his motherland. He established a research institute in Copenhagen (later renamed the Bohr Research Institute). The Bohr Research Institute attracted a large number of outstanding young physics students from all over the world. Scientists, including Heisenberg, Pauli and Dirac, the founders of quantum theory, formed a strong academic atmosphere. At this time, Copenhagen began to explore the basic laws of physics.

Until now, physics can be roughly divided into two schools. One is the classical physics school represented by Einstein. Its members are Planck, De Broglie, Schrodinger, etc.; the other is The Copenhagen School, headed by Bohr, includes Bonn, Heisenberg, Pauli, and Dirac. Naturally, this controversy has not yet ended. So what happened to physics after the Bohr hydrogen atom? What is the focus of the dispute between the two scientific giants?

Chadwick Model

In 1935, British physicist James Chadwick (Sir James Chadwick 1891~1974) was born in England in 1891. After graduating from Manchester University, he specialized in the study of radioactive phenomena. After going to Cambridge University, under the guidance of Professor Rutherford, he made many achievements. In 1935, he won the Nobel Prize in Physics for the discovery of neutrons. During the Second World War, he went to the United States to engage in nuclear weapons research. Passed away in 1974.

He found that neutrons and protons have the same mass, but he is 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 a positively charged nucleus and negatively charged electrons orbiting the nucleus. Almost all the mass of the atom is concentrated in the nucleus. At first, it was thought that the mass of an 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 have discovered in their research that the number of positive charges in the nucleus is not equal to its mass! In other words, in addition to the positively charged protons, the nucleus should also contain other particles. So, what are those "other particles"?

The one who solved this physics problem and discovered that the "other particles" are "neutrons" is the famous British physicist James Chadwick.

In 1930, when scientists Bottle and Baker bombard beryllium with alpha particles, they found a kind of very penetrating rays. They thought it was gamma rays and ignored them. Webster even carefully identified this kind of radiation and saw its neutral nature, but it was difficult to explain this phenomenon, so he did not continue to study it in depth. Marie Curie’s daughter Elena Curie and her husband also wandered on the edge of the "beryllium ray" and eventually missed the neutron.

Chadwick was born in Cheshire, England in 1891 and graduated from Victoria University in Manchester. He didn't show his talents in middle school. He is taciturn and mediocre, but he insists on his creed: if he can do it, he must do it right and be meticulous; he can't do it and doesn't understand it, and never writes. Therefore, he sometimes cannot complete physics assignments on schedule. And it is his spirit of not vanity, seeking truth from facts, and "dealing with each other, making great achievements" that has benefited him all his life in scientific research.

Chadwick, who entered the university, quickly revealed his outstanding talents in physics research due to his solid basic knowledge. He was taken by the famous scientist Rutherford, and after graduation, he stayed in the Physics Laboratory of the University of Manchester and engaged in radiological research under the guidance of Rutherford. Two years later, he won the British National Scholarship for his successful experiment of "the alpha rays deviate when passing through the metal foil".

Just as his scientific career was beginning to dawn, the First World War put him in a civilian prisoner camp. It was not until the end of the war that he was free and returned to scientific research. In 1923, he was promoted to the deputy director of the Cavendish Laboratory of the University of Cambridge because of the outstanding results in the measurement and research of the nuclear charge, and he was engaged in particle research together with the director Rutherford.

In 1931, Joliot Curie and Mrs. Curie's daughter and son-in-law announced their new discovery that paraffin wax produces a large number of protons under the irradiation of "beryllium rays." Chadwick immediately realized that this kind of rays was probably composed of neutral particles, and this neutral particle was the key to the mystery that the positive charge of the nucleus is not equal to its mass!

Chadwick immediately set out to study the experiment done by Joliot Curie and his wife. They used a cloud chamber to measure the mass of this particle. It turned out that the mass of this particle was the same as that of a proton. Charged. He called this particle "neutron".

Neutron was discovered by him. He solved the problems encountered by theoretical physicists in atomic research and completed a breakthrough in atomic physics research. Later, the Italian physicist Fermi used neutrons as "cannonballs" to bombard the uranium nuclei and discovered nuclear fission and the chain reaction in fission, which created a new era of human use of atomic energy. Chadwick won the 1935 Nobel Prize in Physics for his outstanding contributions to the discovery of neutrons.

Quantitative relationship

The quantitative relationship between the structural particles that make up an atom

①Mass number (A) = number of protons ( Z) + number of neutrons (N)

②Number of protons = number of nuclear charges = number of electrons outside the nucleus = atomic number

Note: Neutrons determine the type of atom (isotope) and mass The number determines the approximate relative atomic mass of the atom, and the number of protons (nuclear charge number) determines the type of element; the number of electrons in the outermost layer of the atom determines the apparent insignificance of the entire atom, and also determines the chemical properties of the main group elements.

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