Modern atomic and molecular theory
The primitive composition of everything in the universe has aroused great interest all over the world since ancient times. According to China's ancient theory of five elements, everything is composed of five basic elements: gold, wood, water, fire and earth. The ancient Greeks listed gas, water, fire and earth as the four basic material elements in the world. More than 2,000 years ago, the Greek philosopher democritus believed that everything in the universe had only one origin, that is, a tiny particle he called an atom. He believes that atoms are inseparable, there is no qualitative difference, only the difference between size and shape, and "atom" and "empty" are the origin of everything. With the progress of human civilization and the rise of modern science, the concept of five (or four) basic elements in ancient times does not mean that chemical research is a new phenomenon. With the development of controlled chemistry and physics, the vague concept of "atom" has gained a clearer and richer meaning.
/kloc-In the 9th century, British chemist and physicist Dalton put forward atomism. He believes that chemical elements are composed of very small and inseparable particles, that is, atoms, which are immutable. Compounds are made up of molecules, which are the smallest particles of compounds. All atoms of the same element are the same, but the atoms of different elements are different. Atoms of elements will only combine if they are combined in integer proportions. In chemical reactions, atoms are just rearranged, not produced or disappeared. Then, the Italian physicist Avo Gadereau put forward the concept of molecules, and he pointed out that all gases with equal volume, whether elements, compounds or mixtures, have the same number of molecules. The smallest particle of a gas element is not necessarily a single atom, but a single molecule composed of multiple atoms. Although the same volume of gas atoms is different for several days, the number of molecules is the same. But in the next half century, people did not pay attention to Avon Gadereau's theory, and chemists measured different relative atomic weights according to different standards. 1858, Italian chemist cannizzaro proposed that only by accepting Avogadro's law can we really solve the problems of chemical formula and atomic weight. His views have been recognized by people, and modern atomic and molecular theories have been established. Then, people discovered a large number of elements and determined their precise atomic weights. By 1869, Russian chemist Mendeleev proposed that there was a periodic change between the properties of elements and the atomic weight of elements, and gave the first periodic table of elements, which was published in 187 1 year. He also published the second revised periodic table of elements.
Discovery of radioactivity and electrons
/kloc-at the end of 0/9th century, the German physicist Roentgen discovered X-rays. Bekkerel, a French physicist, accidentally discovered that uranium could be sensitized by a negative wrapped in black paper when he was studying whether fluorescent substances were related to X-rays. After further research, he came to the conclusion that this is a new ray from the uranium atom itself, and uranium is radioactive. The discovery of radioactivity opened up a huge new research field, which not only changed the original concept of atom, but also opened people's understanding of nucleus. Then, the Curies discovered that thorium, polonium, radium and other elements are also radioactive, and discovered the quantitative law of radioactive decay, and introduced the concept of half-life: the atom of each radioactive element has a certain probability of a specific decay. After a period of time, only N/2 elements will remain because of the decay, which is called the half-life of the element. Becquerel and Curie were awarded the 1903 Nobel Prize in Physics for their contributions to radioactive research.
The discovery of X-rays not only led to the discovery of radioactive substances, but also promoted the discovery of electrons. 1897, British physicist Thomson proved that cathode rays (rays emitted by the cathode of a metal electrode in a vacuum tube when it is electrified) are particle streams, and their mass is only one thousandth of that of hydrogen ions. Thomson named it an electron, which is the smallest unit of charge, smaller than an atom and the same component of all chemical atoms. Electrons are released from a cathode composed of metal atoms. It can be seen that electrons are released from atoms.
Rutherford's atomic model and nucleus
In the study of radioactivity, it is found that the radiation emitted by radioactive substances actually belongs to different kinds, and there are three ways to release radioactivity: α, β or γ rays, which will be identified more specifically later. Alpha rays are high-speed helium nuclei with positive charges; Beta rays are electrons, negatively charged; Those electromagnetic waves that are not affected by electromagnetism are called gamma rays (actually high-energy protons).
Rutherford, a physicist in New Zealand, found that yellow lines of helium appeared in the aggregated and neutralized α particles, which confirmed the identity of α particles and helium ions and proved that helium came from other elements. With a few exceptions, radioactive elements emit alpha rays or beta rays. The element that emits alpha rays becomes the first two elements in the periodic table, and its mass decreases by 4, while the element that emits beta rays becomes the next element in the periodic table, and its mass remains unchanged. With the decay of α or β, γ rays are often emitted, which is a kind of electromagnetic radiation with high energy. Gamma rays will not change the position of elements in the periodic table of elements, but will only release excess energy inside the atoms of elements.
The discovery of radioactivity shows that atoms have a complex internal structure, and it also breaks the long-held idea that atoms are eternal, because the atoms of natural radioactive elements are constantly changing according to certain laws. However, can we change the atoms of stable elements in nature? Rutherford believed that alpha particles were released from atoms of radioactive elements. What happens if alpha particles are used as "shells" to hit atoms of stable elements?
19 10, Rutherford cooperated with other scientists to carry out the scattering experiment of α particles in gold and other metal films. According to the experimental results, Rutherford established the nucleation model of atoms: the positive charge and mass of atoms are concentrated in a small area in the center of atoms, which is called nucleus; Electrons in atoms move around the nucleus like planets around the sun; Space in atoms, like space in the solar system, is mostly empty. Because atoms are electrically neutral, the nucleus must be positively charged, and its charge is the same as that of electrons outside the nucleus.
19 14 years, Rutherford bombarded hydrogen with cathode rays. As a result, the electrons of hydrogen atoms are knocked out and become positively charged cations, which are actually the nuclei of hydrogen and the lightest nuclei. Rutherford speculated that it was an anode ray discovered before, which had a charge unit and a mass unit. Rutherford named it proton. On the basis of the new atomic model, Rutherford estimated that the radius of the nucleus is about 10- 14m, which is only about one ten thousandth of the atomic radius. Most of the mass of atoms is concentrated in such a small nucleus, so the density of substances in the nucleus is extremely high, which is about 10 12 times that of ordinary substances, and the nuclear substance with 1 cubic centimeter will weigh about 1000 tons.
19 19, Rutherford bombarded nitrogen atoms with accelerated high-energy alpha particles, and found that nitrogen nuclei were knocked out of protons and nitrogen atoms became oxygen atoms. This may be the first time that human beings have really changed one element into another, but the evolution of this element is of no value for the time being, because only one of hundreds of thousands of particles has been hit by high-energy particles. By 1924, Rutherford had knocked out the protons in the nuclei of many light elements, further confirming the existence of protons.
Rutherford established a nuclear model of atoms on the basis of experiments, which showed the deeper existence of nuclear matter. He and many physicists all over the world under his direct or indirect guidance have formed a university school. For decades, everything has proceeded from reality, and nuclear physics research and nuclear technology application have flourished. He is a pioneer in nuclear physics and a leader in exploring nuclear mysteries.
Discovery of neutrons
After the discovery of electrons and protons, people initially speculated that the nucleus was composed of electrons and protons, because both α particles and β particles were emitted from the nucleus. But mozer, a student of Rutherford, noticed that the number of positive charges in the nucleus is equal to the atomic number, but the atomic weight is greater than the atomic number. This shows that if nuclear light is composed of protons and electrons, its mass is not enough, because the mass of electrons can be ignored. On this basis, Rutherford speculated that there might be electrically neutral particles as early as 1920.
Chadwick, another student of British physicist Rutherford, is looking for this kind of electrically neutral particle in Cavendish's laboratory. He has been designing an acceleration method to make protons get high energy, thus hitting the nucleus and looking for evidence about neutral particles. 1929, he is going to bombard beryllium atoms.
Meanwhile, German physicist Bert and his student Becker have taken the lead. They cooperated with alpha particles to bombard a series of elements, and found an unknown radiation when bombarding beryllium nuclei. In order to determine some characteristics of this radiation, they tried to place various objects on the path through which the radiation passed. As a result, they found that this kind of radiation has a strong penetration ability, which can penetrate a lead plate several centimeters thick. I knew at that time that only gamma rays could have such a strong radiation ability. Therefore, they think that this kind of radiation is a kind of gamma rays.
193 1 year, French physicists Curie and his wife repeated Bert-Becker's experiment with α rays generated by the strongest radioactive polonium source at that time, and studied the "beryllium radiation" when α particles bombarded beryllium. In addition to the same results as Bert-Becker, they were surprised to find that this radiation can knock out protons in hydrogen-containing substances. People have never found that gamma rays have this property, but the Curies can't figure out what this radiation can be. They only reported that they found that alpha rays could produce a new effect.
After these results were published in 1932, seeing the experimental results of German and French colleagues, chadwick realized that this new ray might be the neutron he had been looking for for for many years. He immediately repeated the same experiment under the superior conditions in the laboratory, and proved that the so-called "beryllium radiation" is a stream of electrically neutral particles, which have almost the same mass as protons. Less than a month later, chadwick published a paper "Neutrons may exist". He pointed out that gamma rays have no mass and it is impossible to knock protons out of the nucleus. Only those particles that have roughly the same mass as protons have this possibility. He also measured the mass of neutrons and confirmed that neutrons are indeed electrically neutral.
The discovery of electrons
With the continuous progress of accelerator technology in the last century, "basic" particles poured down like a rainstorm, and then were proved to be "non-basic" by theory and experiments and eliminated one by one. Among them, there is a difference. It is the first subatomic particle known to people, but it has been sitting on the throne of "elementary particle" since it was discovered, and 100 has not wavered for more than years. The highest in this family of elementary particles is the electron.
Although the discovery of electrons was in 1897, it took nearly a century to prepare for this discovery. In 18 1 1, avogadro put forward the avogadro hypothesis that gases with the same temperature, pressure and volume contain the same number of molecules. This man is miserable. He died in 1956, but his hypothesis was not generally accepted until 1960, and was replaced by "Fa". In the history of science, there are countless examples of this kind of resurrection after death. Similarly, Galois, a mathematical genius, tasted human suffering at the age of 20 and died in a duel. His achievements were published 14 years after his death. Huygens, the inventor of wave optics, was suppressed by Newton before his death, and his achievements were not recognized until 40 years after his death ... If you are interested, you can Google it.
1833, when Arrhenius' law was still a hypothesis, Faraday put forward the law of electrolysis, saying that the total electricity of monovalent ion bands of any atom in 1 mole is the same. If this result is combined with Arrhenius hypothesis, it can be inferred that there must be a minimum charge unit. But no one has dared to make such a combination-all because of the word "hypothesis". It was not until 1874 that Arrhenius' law was vindicated that Si Tong took this step. Combining with the experimental results, he deduced the approximate value of elementary charge's electric quantity, which was officially named "electron" in 188 1.
Look, the name of the electron is there, but people haven't even seen its shadow. You may not believe it. At the end of 19, there was still considerable controversy about the existence of atoms. Therefore, people's concept of the basic structure of matter is basically blank, and there is no expectation that the discovery of electrons will be in what occasions, in what phenomena, and what kind of relationship with atoms.
Although it is generally believed that the electron was discovered by Tang Musun in 1897, in fact, more than one person had done similar experiments before him. Let's see what's different. Tang Musun finally won this honor.
The experiments these people do are all about cathode rays. So before we talk about the experiment of discovering electrons, we need to look at what cathode rays are. Cathode rays, as the name implies, are the rays emitted by the cathode. We now know that it is actually an electron beam. How are cathode rays produced? Students who have studied physics in high school all know the photoelectric effect and photocurrent, that is, electrons leave the metal surface and need photons to provide an overflow work. Of course, this overflow work is not only provided by photons, but also by energy from any source. Therefore, we can heat the metal to make it very hot. High temperature is equivalent to the intensification of particle thermal motion, and the kinetic energy of electron thermal motion can overflow from the metal surface when it is large enough. At this time, we use this metal as the cathode (negative electrode) and set a positive electrode at a certain distance, so that the electrons overflowing from the metal surface move to the anode under the action of the electric field to form an electron beam, that is, cathode ray.
Before Tang Musun's experiment, crookes had suggested that cathode rays were composed of negatively charged particles, so Tang Musun could directly measure the charge and quality of such particles under the guidance of this viewpoint without worrying about the properties of cathode rays. This is actually a key point, which I will mention below.
The principle of Tang Musun's experiment is actually very simple. Electrons will deflect in an electric field perpendicular to their moving direction, and the point where cathode rays hit the opposite screen will deviate in the experiment. It can be judged that the cathode ray is negatively charged by its offset direction. Coupled with the magnetic field opposite to the electric field, the Lorentz force on the electron is opposite to the electric field force, that is to say, when the spot offset is zero, the two forces are equal. From the ratio of these two forces, the charge-mass ratio of electrons can be obtained. On the basis of these data, Tom Musun announced the existence of electrons.
One of the two people who did similar experiments was the famous Hertz. Maxwell's electromagnetic theory was not recognized before his death (oh, this is another example of rehabilitation after death, which was forgotten), and it was not confirmed by Hertz's electromagnetic wave experiment until eight years after his death. Maxwell's equations are great, which proves that its experiments are equally great. Hertz was lucky enough to discover the photoelectric effect. But unfortunately, God didn't give him another honor. There is a so-called "sense of smell" in scientific research, which means whether you can find the right direction. Both Einstein and Yang Zhenning advocated this very much. But I don't know if this so-called "sense of smell" is in a sense that in the history of science, winners and losers are mostly lucky. After all, Einstein's sense of smell did not play a very good role in his later years (I will elaborate on the unified field theory later).
In principle, Hertz's experiment is the same as Tang Musun's, which applies an electric field in the vertical direction of cathode rays, several years earlier than Tang Musun's. I was supposed to take the lead, but I dialed the wrong number. Well, the reasons are inexcusable and can be divided into subjective and objective. Objectively, there is a fatal reason, that is, the vacuum technology is not warm enough when Hertz is doing experiments, and the residual air molecules are ionized, which offsets the external electrostatic field. As a result, the fluorescence trajectory of cathode ray seems to have no deflection at all, so Hertz thinks that cathode ray is uncharged. The subjective reason lies in Hertz's most famous achievement-electromagnetic wave. Hertz insisted that cathode ray is an electromagnetic wave, and this undeflected experiment further confirmed his view, so he kept this view for the rest of his life. Hertz's "sense of smell" has a problem with this. From the beginning, Tang Musun tended to think that cathode rays were composed of particles, which led to his ultimate success to some extent.
Compared with Hertz, Kaufman in Germany is even more unlucky. He also did a similar experiment in 1897. However, the charge-to-mass ratio of electrons measured by him is far more accurate than that of Tang Musun, which is only one percent lower than the modern value. He even got the result that this ratio changed with the change of electron speed. This is nothing more than Einstein's special relativity effect, and it was eight years before the birth of special relativity! Kaufman should have made a name for himself because of this experiment, but because of his prejudice that cathode rays can't be particles, he dared not publish his own results until 190 1 year. From this, we can see how important it is that Tom Musun believed that cathode rays were composed of particles from the beginning.
About the history of electronic discovery, let's stop here for the time being. Finally, some more modern facts about electrons. Electrons were first discovered and are the most suitable basic particles for study. Look at all the elementary particles listed in the previous chapter. The lifetime of electrons is much longer than that of other leptons (the word "duo" is too pale, just like the lifetime of muons belonging to leptons is minus 6 seconds of 10, and electrons don't decay at all), and they interact with other particles more easily than neutrinos (a neutrino can easily pass through a lead plate with a thickness of 1000 light years), compared with quarks and others (due to), but so far, Although they have observed experimentally in recent years that there are virtual particles "gas" around electrons due to vacuum polarization mechanism (which will be discussed later), obviously this cannot be regarded as the structure of electrons themselves.
In addition, we think that the electric quantity of electrons is equal to that of protons, but this is entirely based on an assumption of experimental results (at present, it is accurate to minus 2 10/power), and there is no deeper theory to deduce this point.
Finally, due to the relativistic effect, the mass of electrons increases with the increase of motion speed, but the charge of electrons is strictly constant, and the principle is still unknown.