Shulou(Shulou.com)11/24 Report--

Atoms are made up of positively charged nuclei and negatively charged electrons, while nuclei are made up of protons and neutrons. Because all protons have the same charge, they produce electrostatic forces that repel each other. Here we can use the classical Coulomb law to calculate the magnitude of the force, which is inversely proportional to the square of the distance "r". Because the positively charged protons are very close to each other in the nucleus, the electrostatic repulsion between them is very large.

But what binds them together to resist this huge repulsive force is a more powerful force called "force", which is about 100 times stronger than the electromagnetic force. This force is one of the four fundamental forces in the universe, but unlike gravitational and electromagnetic forces, it acts at a very small distance, only the width of a proton. So it only operates within and between protons and neutrons, and electrons, photons and neutrinos are not affected by it.

This means that the nucleus should not be too large. Strength works only over short distances, so as the nucleus becomes larger, the attraction of a given proton or neutron decreases rapidly. But because the range of the electromagnetic force is infinite, the repulsive force it produces will accumulate with more and more protons. With the addition of more protons, this repulsive force quickly overwhelms the strong attraction and destroys the stability of the nucleus.

The heaviest stable element is lead, which has 82 protons. The number of protons in an element is important because it completely determines the atomic properties of the element. A given element or atom can have the same number of protons but different numbers of neutrons, which are called isotopes of elements. They have exactly the same chemical properties, only of different mass. A stable nucleus consists of roughly the same number of protons and neutrons, which are used to provide the extra strength needed to maintain nuclear stability. Without neutrons, neither proton can be combined to resist their repulsion.

Free protons are stable, but free neutrons are unstable. Free neutrons decay into a proton, an electron and an antineutrino in about 15 minutes. But in the nucleus, they remain stable because they are not conducive to their decay in terms of energy. In other words, if there are a large number of other protons around, the energy from neutron decay to protons is less than the additional energy needed to retain an extra proton in the nucleus. But this type of neutron decay in the nucleus is not impossible and may occur in neutron-rich isotopes. There should not be too few neutrons in a nucleus, otherwise the repulsion force is too large and the nucleus will be unstable. Nor can it have too many neutrons, because eventually the force can no longer stop them from decaying and will form different elements.

At this point, we have to talk about three forms of natural radioactivity: α decay, β decay and gamma decay. If the nucleus is very large, it is almost impossible to gather it together, in which case they emit alpha particles (essentially helium nuclei made up of two protons and two neutrons). Beta particles are high-energy electrons, that is, the neutrons we mentioned above decay into protons. Gamma particles are high-energy photons, also known as gamma rays. Gamma rays are usually emitted by excited nuclei produced after α or β decay.

These three forms of radioactivity are very different in their ability to penetrate matter. Alpha particles can be blocked by a piece of thin paper; beta particles can penetrate the skin but can be blocked by thin sheets of metal such as aluminum foil; and gamma rays are difficult to block and can even penetrate 2.5 centimeters of lead. Why is there such a difference? The alpha particles composed of two protons and two neutrons are relatively large and heavy, so they are slow and easy to block. Beta particles are faster and smaller, so they travel much faster and penetrate matter more easily. Gamma rays are completely uncharged and travel at the speed of light, so the penetration is so strong that these photons almost have to hit the nucleus directly before they can stop.

Radioactive nuclei have a characteristic called "half-life". A certain number of radioactive atoms, say 16 atoms, have a half-life of one week. And then a week later, there will be eight atoms left. But in fact, half-life is a statistical concept. If we have these 16 atoms, we can't be sure how long they will decay. We can only have a 50% chance of decay of any atom in a week.

Although we don't know when they decay, we can control their fission. If a large nucleus, such as the uranium isotope Umur235, is hit by a neutron, it splits into two smaller nuclei, which is called nuclear fission. If the total mass of the two smaller nuclei is less than the mass before the impact of the uranium, the lost mass is converted into energy through E=mc ².

For some isotopes of uranium and plutonium, when they are hit by neutrons, three neutrons are thrown out in addition to splitting into two lower-mass nuclei. If there is a sufficient concentration of fissionable nuclei, then three thrown neutrons may hit other nuclei, resulting in further fission, resulting in a so-called chain reaction. This is the mechanism behind the atomic bombs dropped on Hiroshima and Nagasaki during World War II.

The opposite of fission is fusion. Fusion occurs when two small nuclei are close enough to merge into a single nucleus. Fusion is difficult to achieve because protons strongly repel each other. At Earth Entropy, only gases heated to hundreds of millions of degrees Celsius can make atoms move fast enough that they can get close enough to achieve fusion, where quantum tunneling also plays a role. The hydrogen bomb uses the heat of the fission bomb to trigger hydrogen fusion to achieve a very high level of energy release.

The sun is powered by hydrogen fusion, but its mechanism is very different from that of a hydrogen bomb. It hardly needs high temperature because its mass exerts great pressure on its core, pushing hydrogen nuclei together.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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