How is the atom found? From the discovery process to the use of nuclear energy

Beijing December 2 news, according to foreign media reports, the atom is a very magical particle, it has a complex structure, naturally there will be magical changes. The entire world is made up of a large number of tiny atoms, which in turn are made up of neutrons, protons, and electrons. For more than two hundred years, scientists have conducted numerous experiments in order to confirm the existence, internal structure and radioactivity of atoms. Only by continuously understanding the atomic structure and its changes can we better understand the use of nuclear energy and detect and control nuclear radiation.

The atom is very small. Physics tells people that matter consists of a large number of tiny atoms that interact and form the entire world, but atoms are invisible to the naked eye. For many people, knowing this theory alone is not enough. One of the great achievements of science is that it can solve the mysteries of the universe through real observations. So how do people come to the conclusion of the existence of an atom? How much do we know about these tiny structures?

It seems simple to prove the existence of atoms: they are placed under a microscope for observation. However, this method does not work. In fact, even the most powerful condenser microscope cannot see individual atoms. The principle behind the visible object is that it reflects visible light waves, but the atoms are smaller than the wavelength of visible light, so that the two cannot interact. In other words, atoms are invisible to light. However, atoms will have observable effects on objects that people can see.

In 1785, Dutch scientist Jane Engerhouse studied a strange phenomenon that he could not understand: In the laboratory, some tiny particles of coal dust flew around on the surface of alcohol. In about 1827, about fifty years later, the Scottish ecologist Robert Brown also described phenomena that had striking similarities. As he moved the microscope to observe the pollen grains, Brown noticed that some pollen grains released tiny particles randomly scattered. At first, Brown speculated that these particles may be some unknown microorganism. He then repeated the experiment with other inorganic substances, such as rock dust, and observed the same strange movement. In order to solve the mystery of this phenomenon, scientists spent more than a century. As Einstein developed a set of mathematical formulas, he realized the prediction of this type of special movement called "Brownian movement." Einstein's theory is that the particles produced by pollen grains keep doing random movements because they are constantly colliding with millions of even smaller water molecules, which are made up of atoms.

Harry Cliff, the director of the Science Museum in London, explained: “Einstein's explanation for this movement was that these dust particles were affected by the impact of a single water molecule.

By 1908, computationally validated observational experiments confirmed the real existence of atoms. In the following decade, physicists conducted deeper research. By separating individual atoms, scientists began to learn more about the internal structure of the atom. Surprisingly, atoms can also be separated because the word "atom" comes from the Greek word "atomos," which means "inseparable." However, physicists now know that the atom is not a solid pellet, but it should be seen as a tiny charged “planetary” system consisting mainly of three parts: protons, neutrons, and electrons. Together, protons and neutrons form a "sun", the nucleus, surrounded by electrons orbiting the planet. If the atoms are too small to imagine, those sub-atoms are even smaller. Interestingly, among the three atom components, the smallest electron was found first. The protons in the nucleus are 1830 times larger than electrons. An analogy is like a small cobblestone roundabout a hot air balloon.

However, how do you prove that these particles exist? The answer is that even if these particles are tiny, they can produce huge impacts. In 1897, British physicist Thomson used a special and wonderful method to prove the existence of electrons. This special device is the Cruz electrode tube, which is a strange and interesting glass tube, the air inside is almost evacuated by a machine. Next, put a negative charge at one end of the glass tube, enough to remove electrons from the gas molecules in the tube. The electrons are negatively charged so they flow from one end to the other along the glass tube. Due to the internal vacuum, these electrons can pass through the pipeline without being blocked by atoms. The charge caused the electrons to move rapidly, about 59,500 kilometers per second, and hit the other end of the glass tube, crashing into atoms with more electrons. What is even more amazing is that the impact of this tiny particle generates tremendous energy, emitting a dazzling yellow-green light. Clive said: "This is some form of the first particle accelerator that accelerates electrons from one end of the tube to the other, and emits this phosphor when the electrons hit the other end." Because Thomson found it possible With the aid of the electromagnetic field, the direction of the electron beam is changed, so he can determine that this is not a strange light but a charged particle.

People may wonder how these electrons orbit around atoms alone. The answer is ionization. Ionization refers to an atom or molecule that becomes positively or negatively charged due to the impact of high-energy particles. In fact, because the electrons are easy to control, they can move in the circuit. The movement of electrons in a copper line is similar to that of a train movement, that is, moving from one copper atom to the next, so that the charge is carried from one end of the copper line to the other. As a result, atoms are no longer solid, small masses of material, but systems that can improve or change the structure. The discovery of electrons means that people can learn more about atoms. Thomson's research shows that electrons are negatively charged, but he knows that the atom itself is not charged. Therefore, he reasoned that atoms must have some magic positively charged particles to counteract the negative charge of electrons. At the beginning of the 20th century, scientists conducted a large number of experiments to determine positively charged particles, and at the same time revealed the internal structure of atoms similar to the solar system.

Ernest Rutherford and colleagues conducted an experiment where they placed thin metal foil under a beam of positively charged rays. As a result, it was found that most of the rays passed through the metal foil. However, what surprises researchers is that some rays are bounced back by the metal foil. Rutherford speculates that the reason is that the atoms in the metal foil must contain some tiny, densely packed areas that are positively charged. There is nothing else that can reflect the radiation so strongly. He discovered the positive charge in the atom, and at the same time proved that these positive charges are different from the discrete electrons. They are bound in a compact substance. In other words, Rutherford confirmed that there is a dense core in the atom.

However, a new problem has emerged. Although scientists have been able to estimate the atomic mass, even if one knows the weight of a certain particle in the nucleus, the idea that they all have a positive charge cannot be justified. Clif explained: "The carbon atom has six electrons, so there are also six protons in the nucleus, six positive and six negative. But the weight of a carbon nucleus is not only the weight of six protons, it has twelve Protons are so heavy.” Early scientists thought that there are six other particles in the nucleus. They are the same mass as protons, but they do not carry charge: neutrons. However, no one can demonstrate this. It was not until the 1930s that scientists really discovered neutrons. James Chadwick, a physicist at the University of Cambridge, made unremitting efforts for the discovery of protons and it was only in 1932 that breakthroughs were made in this area. Prior to this, other physicists used ray to experiment. They tried to radiate positively charged rays on helium atoms in a way similar to Rutherford's discovery of nuclei. Helium atoms emit their own rays, which are neither positive nor negative, and can penetrate substances. During this period, other scientists have discovered that gamma rays are neutral and very penetrating, so scientists believe that helium atoms emit gamma rays. However, Chadwick expressed deep doubts. He fired some new rays and aimed the rays at proton-rich substances. Surprisingly, these protons were as if they were struck by particles of the same mass and left the original material to fly into the air. Gamma rays could not cause protons to deviate. In this way, Chadwick realized that there must be some kind of uncharged particles with the same proton mass—this is neutron. So far, all the key questions about the atom have been solved, but the story is not yet finished.

Although people's understanding of the atom has greatly improved, it is still not easy to observe the atom. Around 1930, no one could directly image atoms. However, many people want to observe atoms directly to truly understand and accept their existence. The scientific research methods used by scientists such as Thomson, Rutherford, and Chadwick have provided important reference for later generations of atomic research. Among them, the Cruz Electrode Tube Experiment developed by Thomas is most useful. Today, many electron beams are emitted by electron microscopes, and the most powerful microscope can generate images of individual atoms. Since the electron beam wavelength is several thousand times shorter than the beam wavelength, the electron beam can be deflected by the influence of tiny atoms to generate an image, which cannot be achieved by the beam. Neil Skippa of University College London points out that this kind of image is very useful for people who are studying the atomic structure of special substances, such as those used to make electric car batteries. The more we understand the structure of atoms, the more efficient and reliable the design of materials can be.

At present, scientists use atomic force microscopy to study the atomic structure. Atomic force microscopy is an analytical instrument that can be used to study the surface structure of solid materials, including insulators. A pair of micro-cantilever arms with extremely sensitive weak forces is fixed at one end and the small needle tip at the other end is close to the sample. At this time, it will interact with it, and the force will cause the micro-cantilever to deform or change its motion state, thereby obtaining a single molecule image. Using this method, researchers have recently published a series of molecular images before and after a wonderful chemical reaction. Skipper added: "Many current atom studies are exploring how the structure of matter will change under high pressure or high temperature. Many people know that when a substance is heated, it will usually swell. If you heat liquid You will find that the atom has a more chaotic structure. All this can be seen directly from the atomic map. With the help of the neutron beam Chadwick used in the 1930s, we often do experiments. To launch a neutron beam toward many materials, it can be inferred from the scattering pattern that many neutrons are scattered in the nucleus to calculate the mass and approximate volume of the material that is scattering."

However, the atom is not moving at all and is waiting for the test to be quiet and stable. Many times, atoms decay, which means that they are radioactive. There are many naturally occurring radioactive elements in the natural world. They generate energy during radiation, forming nuclear energy and nuclear bombs. The main research content of nuclear physicists is to understand thoroughly the basic changes that occur in nuclear reactions. Laura Harkness-Brennan from the University of Liverpool is a gamma ray researcher. She said that different types of radioactive atoms produce different gamma-ray modalities, which means that atoms can be detected by detecting gamma-ray energy. distinguish.

Breinan explained: "With the aid of the detector we can measure the presence of radiation and the energy stored by the radiation, because all the nuclei have their own special fingerprints." Because there are various differences in the radiation detection zone, especially in the large nuclear reaction zone. The atoms, so it is very important to know exactly which radioactive isotopes exist. Scientists usually perform this kind of detection in nuclear power plants or in areas where nuclear disasters occur. Currently, Harkness-Brenan and colleagues are working on the exploration of the detection system. She said: "What we need to do is to develop scientific and technological equipment and means to image a three-dimensional space and discover the area where radiation is present." Cloud Room It is a nuclear radiation detector and is also the earliest charged particle track detector. It uses pure vapor adiabatic expansion, the temperature is reduced to the state of supersaturation, when charged particles are injected, ions are generated in the passing path, and the saturated gas condenses into small droplets with the ions as the core, thus showing the track of the particles. Take a photo shoot. The experimental results of this detection method are indeed amazing. Harkness-Brennan elaborated the atom in one sentence: "Although the atom is very small, we can obtain a great deal of physics knowledge from it."

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