Introduction
Electrons are negatively charged subatomic particles. It can be free (not belonging to any atom), or it can be bound by the nucleus. The electrons in atoms exist in spherical shells of various radii and describing energy levels. The larger the spherical shell, the higher the energy contained in the electron.
In electrical conductors, current is generated by the independent movement of electrons between atoms, and usually flows from the cathode to the anode of the electrode. In semiconductor materials, current is also generated by moving electrons. But sometimes, it is more illustrative to think of current as an electron-deficient movement from atom to atom. The electron-deficient atoms in semiconductors are called holes. Generally, holes "move" from the positive electrode to the negative electrode of the electrode.
Electrons belong to the class of leptons in subatomic particles. Lepton is considered to be one of the basic particles that make up matter. It has 1/2 spin, which is another kind of fermion (according to Fermi-Dirac statistics). The charge of the electron is e=-1.6×10-19C (Coulomb), and the mass is 9.11×10-31kg (0.51MeV/c2< /sup>), the energy is 5.11×105eV, usually expressed as e⁻. The antiparticle of an electron is a positron, which has the same mass, energy, spin and the same amount of positive charge as the electron (the charge of a positive electron is +1, and the charge of a negative electron is -1).
The basic unit of matter-atom is composed of electrons, neutrons and protons. Neutrons are not charged, protons are positively charged, and atoms are not electrically charged to the outside. Compared with the nucleus composed of neutrons and protons, the mass of electrons is extremely small. The mass of a proton is approximately 1,840 times that of an electron.
When an electron leaves the nucleus and moves freely in other atoms, the net flow phenomenon it produces is called electric current.
Various atoms have different abilities to bind electrons, so they lose electrons and become positive ions, and gain electrons and become negative ions.
Static electricity refers to the situation where an object has more or less electricity than the nucleus, resulting in an imbalance of positive and negative electricity. When there are excess electrons, it is said that the object is negatively charged; when there are not enough electrons, it is said that the object is positively charged. When the positive and negative electricity balances, the object is said to be electrically neutral. There are many application methods of static electricity in our daily lives, among which laser printers are examples.
Research History
The electron was discovered in 1897 by Joseph John Thomson of the Cavendish Laboratory of the University of Cambridge while studying cathode rays. Joseph John Thomson proposed the jujube cake model.
In 1897, Joseph John Thomson of the Cavendish Laboratory at the University of Cambridge, England, redone Hertz's experiment. Using a vacuum tube with a higher degree of vacuum and a stronger electric field, he observed the deflection of the negative rays and calculated the mass-charge ratio of the particles (electrons) of the negative rays. Therefore, he won the Nobel Prize in Physics in 1906. Thomson adopted the name given by George Stoney in 1891-electrons to call this particle. So far, electrons have been discovered by Thomson as the first subatomic particle discovered by mankind and the door to the atomic world.
More than 100 years ago, when the American physicist Robert Millikan first measured the charge of electrons as 1.602×10-19C through experiments, this charge value was widely used. Think of it as the basic unit of charge. However, if according to the classical theory, the electron is regarded as a "whole" or "elementary" particle, which will make us extremely confused about the behavior of the electron in certain physical situations, such as the non-integral quantum that appears when the electron is placed in a strong magnetic field. Hall Effect.
Researchers from the University of Cambridge and colleagues from the University of Birmingham completed a study in collaboration. The communiqué stated that electronics are generally considered indivisible. Researchers from the University of Cambridge placed extremely thin "quantum metal wires" above a metal plate, controlled the distance between them to be about 30 atoms wide, and placed them in an ultra-low temperature environment near absolute zero. Then, they changed the external magnetic field and found the metal The electrons on the board split into spinons and holes when they jump to the wire through the quantum tunneling effect.
In order to solve this problem, in 1980, American physicist Robert Laughlin proposed a new theory to solve this puzzle. The theory also succinctly explained the complex interaction between electrons. However, accepting this theory does cost the physics community: the bizarre inferences derived from the theory show that current is actually made up of one-third of the electronic charge.
But in 1981, physicists proposed that electrons can be split into magnetic spinons and charged holes under certain special conditions.
On November 16, 2018, the International Conference on Weights and Measures passed a resolution that 1 ampere is defined as "the current corresponding to 6.24146×1018 electronic charges in 1s."
Characteristics
Electrons are classified as leptons in subatomic particles. Lepton is a class of matter classified as elementary particles. The electron has one-half of the spin and satisfies the condition of fermions (according to the Fermi-Dirac statistics). The charge of the electron is about -1.6×10-19 Coulomb, and the mass is 9.10956×10-31kg (0.51MeV/c2) . Usually expressed as e⁻. Particles that have the opposite electrical properties of electrons are called positrons, which have the same mass, spin and equal positive charge as electrons. The electron moves around the nucleus in the atom. The greater the energy, the farther away the trajectory of the nucleus is. The space where electrons move is called the electron layer, and the first layer can have up to 2 electrons. The second layer can have up to 8 electrons, the nth layer can hold up to 2n2 electrons, and the outermost layer can hold up to 8 electrons. The number of electrons in the last layer determines whether the chemical properties of the substance are active. Electrons 1, 2, and 3 are metallic elements, 4, 5, 6, 7 are non-metallic elements, and 8 are rare gas elements.
The electrons of a substance can be lost or obtained. The property of a substance to gain electrons is called oxidizing, and the substance is an oxidizing agent; the property of a substance to lose electrons is called reducing, and the substance is a reducing agent. The oxidizing or reducing property of a substance is determined by the difficulty of gaining or losing electrons, and has nothing to do with the number of electrons gaining or losing.
Atoms composed of electrons, neutrons, and protons are the basic units of matter. Compared with the nucleus composed of neutrons and protons, the mass of electrons is extremely small. The mass of a proton is approximately 1,842 times the mass of an electron. When the number of electrons in an atom is not equal to the number of protons, the atom will be charged and the atom is called an ion. When an atom gets extra electrons, it has a negative charge, called anion, when it loses electrons, it has a positive charge, called cation. If an object has more or less electrons than the nucleus of the charge, which results in an imbalance between the positive and negative charge, the object is said to be static. When the positive and negative electricity are in balance, the electrical property of the object is called electrical neutrality. Static electricity has many uses in daily life. For example, the electrostatic paint system can spray enamel paint (English: enamelpaint) or polyurethane paint evenly on the surface of objects.
The attractive Coulomb force between the electron and the proton causes the electron to be bound to the atom. This electron is called bound electron. Two or more atoms exchange or share their bound electrons, which is the main cause of chemical bonds. When an electron is free from the bondage of the nucleus and can move freely, it is renamed as a free electron. The net flow of many free electrons moving together is called electric current. In many physical phenomena, like electrical conduction, magnetism or thermal conduction, electrons play an important role. Moving electrons generate a magnetic field and are also deflected by an external magnetic field. Electrons that move at an acceleration emit electromagnetic radiation.
The ultimate carrier of charge is the tiny electrons that make up atoms. In a moving atom, each electron moving around the nucleus has a unit of negative charge, and the proton in the nucleus has a unit of positive charge. Under normal circumstances, the number of electrons and protons in a substance is equal, the charges they carry are balanced, and the substance is neutral. After friction, matter either loses electrons, leaving more positive charges (more protons than electrons). Either add electrons and gain more negative charge (more electrons than protons). This process is called triboelectricity.
Arrangement law
1. The electrons are arranged hierarchically on different electron layers from near and far from the nucleus to the nucleus, with energy from low to high.
2. The maximum number of electrons contained in each layer is 2n2 (n represents the number of electron layers).
3. The number of electrons in the outermost layer does not exceed 8 (the first layer does not exceed 2), the secondary outer layer does not exceed 18, and the penultimate layer does not exceed 32.
4. Generally, electrons are always arranged in the electron layer with the lowest energy first, that is, the first layer is arranged first, when the first layer is full, then the second layer is arranged, and after the second layer is full , And then ranked third.
The electron cloud is a visual description of the probability density distribution of electrons in the space outside the nucleus. Electrons appear in a certain area of the space outside the nucleus, like a negatively charged cloud covering the nucleus. People call it vividly. As the "electronic cloud". It is an Austrian scholar Schrödinger in 1926 on the basis of De Broglie's equation, who did appropriate mathematical treatment of the movement of electrons, and proposed the famous Schrödinger equation of second-order partial differential. The solution of this equation, if represented graphically in three-dimensional coordinates, is an electronic cloud.
Atomic Theory
In different times, people have had various speculations about the way electrons exist in atoms.
The earliest atomic model is Thomson’s plum pudding model. Published in 1904, Thomson believed that electrons are arranged uniformly in atoms, just like negatively charged plums in a positively charged pudding. In 1909, the famous Rutherford scattering experiment completely overthrew this model.
Rutherford designed the Rutherford model in 1911 based on the results of his experiments. In this model, most of the mass of an atom is concentrated in a small atomic nucleus, and most of the atom is in a vacuum. The electrons orbit the nucleus like a planet orbits the sun. This model has had a huge impact on future generations. Until now, many high-tech organizations and units still use atomic images of electrons surrounding the nucleus to represent themselves.
Under the framework of classical mechanics, the planetary orbit model has a serious problem that cannot be explained: electrons moving at an acceleration will generate electromagnetic waves, and generating electromagnetic waves will consume energy; ultimately, electrons that run out of energy It will hit the nucleus head-on (like an artificial satellite that runs out of energy will eventually enter the earth’s atmosphere). In 1913, Niels Bohr proposed the Bohr model. In this model, electrons move in a specific orbit outside the nucleus. The farther away from the nucleus, the higher the orbital energy. When an electron jumps to an orbit closer to the nucleus, it releases energy in the form of photons. Conversely, energy will be absorbed from the low-level orbital to the high-level orbital. With these quantized orbitals, Bohr correctly calculated the spectrum of the hydrogen atom. However, using the Bohr model cannot explain the relative intensities of the spectral lines, nor can it calculate the spectra of more complex atoms. These problems have yet to be explained by quantum mechanics.
In 1916, American physical chemist Gilbert Louis successfully explained the interaction between atoms. He suggested that a pair of shared electrons between two atoms form a covalent bond. In 1923, Walter Heitler and Fritz London applied the theory of quantum mechanics to fully explain the reasons for the generation of electron pairs and the formation of chemical bonds. In 1919, Owen Langmuir put Louis's cubic atom model cubeicalatom. To make use of it, it is recommended that all electrons are distributed in layers of concentric (near concentric) spherical shells of equal thickness. He divided these spherical shells into several parts, each of which contained a pair of electrons. Using this model, he was able to explain the periodic chemical properties of every element in the periodic table.
In 1924, Austrian physicist Wolfgang Bubble used a set of parameters to explain the shell structure of atoms. The four parameters in this group determine the quantum state of the electron. Each quantum state can only allow one electron to occupy. (This rule that prohibits more than one electron from occupying the same quantum state is called the Pauli exclusion principle). The first three parameters of this group of parameters are the main quantum number, angular quantum number and magnetic quantum number. The fourth parameter can have two different values. In 1925, Dutch physicists Samuel Abraham Goudsmit and George Uhlenbeck proposed the physical mechanism represented by the fourth parameter. They believe that electrons, in addition to the angular momentum of the orbital motion, may have inherent angular momentum, called spin, which can be used to explain the mysterious spectral line splitting previously observed with a high-resolution spectrometer in the experiment. This phenomenon is called fine structure splitting.
Mass measurement
The mass of electrons appears in many basic laws in the subatomic field, but because the mass of the particle is extremely small, it is very difficult to directly measure it. A team of physicists overcame these challenges and came up with the most accurate measurement of electronic mass to date.
An electron is bound in a hollow carbon nucleus, and the synthesized atom is placed in a uniform electromagnetic field called a Penning ion trap. In the Penning ion trap, the atom began to oscillate at a stable frequency. The research team used microwaves to shoot the trapped atom, causing the electron spin to flip up and down. By comparing the frequency of the atom's rotational motion with the frequency of the spin-flip microwave, the researchers used quantum electrodynamics equations to obtain the mass of the electron.
Positrons against electrons
Among the many theories explaining the early evolution of the universe, the Big Bang theory is a scientific theory that can be widely accepted by the physics community. In the first few seconds of the Big Bang, the temperature was much higher than 10 billion K. At that time, the average energy of photons was much more than 1.022 MeV, and there was enough energy to create electron and positron pairs.
At the same time, anti-electron and positron pairs are also annihilating each other on a large scale and emitting high-energy photons. In this brief stage of universe evolution, electrons, positrons and photons struggle to maintain a delicate balance. However, because the universe is rapidly expanding, the temperature continues to cool down. In 10 seconds, the temperature has dropped to 3 billion K, which is lower than the lower limit of 10 billion K for the electron-positron creation process. Therefore, photons no longer have enough energy to create electron and positron pairs, large-scale electron-positron creationThe incident no longer occurs. However, the anti-electron and the positron continue to annihilate each other continuously, emitting high-energy photons. Due to some unidentified factors, in the process of leptogenesis (physics), more positrons are created than anti-electrons. Otherwise, if the number of electrons is equal to the number of positrons, there will be no electrons! About every 1 billion electrons, one positron will survive the annihilation process. Not only that, because of a condition called baryon asymmetry, there are more protons than antiprotons. Coincidentally, the number of positrons remaining is exactly the same as the number of positive protons more than antiprotons. Therefore, the net charge of the universe is zero, which is neutral.
Application fields
There are many application fields for electrons, such as electron beam welding, cathode ray tubes, electron microscopes, radiation therapy, lasers and particle accelerators, etc. In the laboratory, sophisticated sophisticated instruments, like quadrupole ion traps, can confine electrons for a long time for observation and measurement. Large tokamak facilities, like the International Thermonuclear Fusion Experimental Reactor, achieve controlled nuclear fusion by confining electrons and ion plasma. Radio telescopes can be used to detect electron plasma in outer space.
In a wind tunnel test conducted by the National Aeronautics and Space Administration, an electron beam was shot at a miniature model of the space shuttle to simulate the free gas surrounding the space shuttle when it returned to the atmosphere.
Astronomical observation
The long-distance observation of various phenomena of electrons mainly depends on detecting the radiation energy of electrons. For example, in a high-energy environment such as the corona of a star, free electrons form a plasma that radiates energy by braking radiation. Plasma oscillation of electron gas. It is a kind of fluctuation, which is caused by the rapid oscillation of electron density. This fluctuation will cause energy emission. Astronomers can use radio telescopes to detect this energy.
Welding application
Electron beam technology, used in welding, is called electron beam welding. This welding technology can focus heat energy up to 107W·cm2 energy density to a small area with a diameter of 0.3 to 1.3 mm. Using this technique, a craftsman can weld deeper objects, restricting most of the heat energy to a narrow area, without changing the texture of nearby materials. In order to avoid the possibility of substances being oxidized, electron beam welding must be carried out in a vacuum. For conductive materials that are not suitable for welding by ordinary methods, electron beam welding can be considered. In nuclear engineering and aerospace engineering, some high-value welding parts cannot tolerate any defects. At this time, engineers often choose to use electron beam welding to complete their tasks.
Printed Circuits
Electron beam lithography is a method of etching semiconductors with a resolution of less than one millimeter. The disadvantages of this technique are high cost, slow procedures, must be operated in a vacuum, and the electron beam will quickly disperse in a solid, and it is difficult to maintain focus. Finally, this shortcoming limits the resolution to be no less than 10nm. Therefore, electron beam lithography is mainly used to prepare a small number of special integrated circuits.
Radiation therapy
Technology uses electron beams to irradiate substances. In this way, the physical properties of substances can be changed or the microorganisms contained in medical articles and food can be eliminated. As a kind of radiation therapy, linear accelerator. The prepared electron beam is used to irradiate superficial tumors. Since the electron beam only penetrates a limited depth before being absorbed (the electron beam with an energy of 5 to 20 MeV can usually penetrate 5 cm of organisms), electron beam therapy can be used to treat skin diseases like basal cell carcinoma . Electron beam therapy can also assist in the treatment of areas that have been irradiated by X-rays.
Particle accelerators use electric fields to increase the energy of electrons or positrons, so that these particles have high energy. When these particles pass through a magnetic field, they emit synchrotron radiation. Since the intensity of the radiation is related to the spin, it causes the polarization of the electron beam. This process is called the Soklov-Tnov effect. Many experiments require the use of polarized electron beams as the particle source. Synchrotron radiation can also be used to lower the temperature of the electron beam and reduce the momentum deviation of the particles. Once the particle reaches the required energy, the electron beam and the positron beam collide and annihilate each other, which will cause high-energy radiation to be emitted. Detecting the distribution of these energies, physicists can study the physical behavior of collisions and annihilations between electrons and positrons.
Imaging technology
The low-energy electron diffraction technology (LEED) irradiates a collimated electron beam on the crystal material, and then infers the material structure based on the observed diffraction pattern. The electron energy used in this technology is usually between 20 and 200 eV. Reflected High Energy Electron Diffraction (RHEED) technology irradiates a collimated electron beam on the crystal material at a low angle, and then collects the reflection pattern to infer the data on the crystal surface. The energy of the electrons used in this technology is between 8-20 keV, and the incident angle is 1 to 4°.
The electron microscope incidents a focused electron beam on the sample. Due to the interaction of the electron beam with the sample, the properties of the electrons will change, such as the direction of movement, relative phase and energy. By carefully analyzing these data, a sample image with an atomic size resolution can be obtained. Using blue light, the resolution of ordinary optical microscopes is limited by diffraction, about 200nm; compared with each other, the resolution of electron microscopes is limited by the de Broglie wavelength of electrons. For electrons with energy of 100keV, the resolution is The rate is approximately 0.0037nm. Aberration correction transmission electron microscope. The resolution can be reduced to less than 0.05nm, which is clear enough to observe individual atoms. This ability makes the electron microscope an indispensable instrument for high-resolution imaging in the laboratory. However, electron microscopes are expensive and difficult to maintain; and because of the need to maintain a vacuum in the sample environment during operation, scientists cannot observe living organisms.
There are two main types of electron microscopes: penetration and scanning. The operating principle of the penetrating electron microscope is similar to that of an overhead projector. The electron beam is aimed at the sample slice to emit, and the penetrating electrons are then projected onto the film or charge-coupled element using a lens. A scanning electron microscope scans the sample with a focused electron beam, just like a raster scan in a display. The magnification of these two electron microscopes can range from 100 times to 1,000,000 times or even higher. Using the quantum tunneling effect, the scanning tunneling microscope tunnels electrons from the sharp metal tip to the sample surface. In order to maintain a stable current, the needle tip will move with the height of the sample surface, so that an image of the sample surface with an atomic size resolution can be obtained.
Free laser
The free electron laser passes the relativistic electron beam through a pair of undulators. Each undulator is composed of a row of magnetic dipole moments of a magnetic field in alternating directions. Due to the action of these magnetic fields, electrons emit synchrotron radiation; and this radiation interacts with the electrons in a coherent manner. When the frequency matches the resonance frequency, it will cause a strong amplification of the radiation field. Free electron lasers can emit coherent high-emissivity electromagnetic radiation, and the frequency range is quite wide, from microwaves to soft X-rays. In the near future, this instrument can be used in manufacturing, communications, and various medical applications, such as soft tissue surgery.