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

Mendeleev arranges known elements by atomic weight and notes that chemical properties repeat periodically as the atomic weight increases. We now know that chemical properties depend on the number of valence electrons. Every time a proton is added to the nucleus, the valence electron increases by one until the electron layer is filled. Although he didn't know anything about protons, Mendeleev did notice the gaps in his periodic table. He correctly interprets these blanks as four elements that have not yet been discovered, and can even predict many of their attributes.

Over time, three of these elements were discovered: scandium, gallium and germanium. But there is still one element missing, right between molybdenum and ruthenium, which we find must have a nucleus of 43 protons. For 70 years, chemists have been looking for element 43, but it is nowhere to be found in nature. But it was eventually discovered, but not in nature.

In 1937, Emilio Segr è, an Italian physicist, got some molybdenum foil that was once part of Ernest Lawrence's newly invented cyclotron. The foil is radioactive in the accelerator, and Segre and his Carlo Perrier can prove that some molybdenum has been converted into element 43 by protons. They named the new element Technetium after the Greek word "art" and technetium in Chinese. It is a silver-gray metal with chemical properties between manganese and ruthenium and below manganese and above rhenium in the periodic table.

So why do we have to produce technetium artificially when all other elements can be found in nature? In fact, technetium can also be produced in nature, which is formed in supernova explosions. But technetium is so unstable that by the time the earth gets the material from the debris of the death star, all technetium has long since disappeared.

A more common term for the idea that elements may be unstable is radioactivity, which we tend to associate with very heavy elements such as uranium and plutonium. But in fact, any element in the periodic table may be unstable. More precisely, every element in the periodic table has unstable isotopes. "isotope" refers to different versions of the same element with different numbers of neutrons.

For example, a carbon atom has six protons in its nucleus. Carbon-12 has six neutrons and it is very stable. Carbon-14, which has eight neutrons, is so unstable that it has an extra neutron that is converted into protons after emitting electrons and neutrinos, thus turning it into nitrogen. Each element has unstable isotopes, some elements have unstable isotopes, and larger atomic numbers tend to produce fewer stable isotopes and shorter half-lives. Elements with more than 118 protons decay so fast that we have never tested them in the laboratory.

In fact, stability depends on the balance between protons and neutrons in the nucleus. You might think that after a century and a half of thinking about nuclear physics, we have figured out all these rules. But in fact, the dynamics of the nucleus is so complex that it requires complex computer modeling to understand, but many mysteries remain.

The nucleus is a place where extreme power is in a delicate balance. On the one hand, we have an electromagnetic force that tries to separate all positively charged protons, and because the protons are very close, this repulsive force is very large. On the other hand, we have a stronger nuclear force to hold nucleons together, which involves sending bogus packets-mesons-between nucleons. But it is important to know that the force is a short-range effect. If the nucleus becomes too large, the force cannot hold it together, and various types of nuclear decay are inevitable.

Although the strength is very strong, it does not change much in the short distance that really works. However, the closer the two charges are, the stronger the electromagnetic force will be. This means that if the proton is too close, the electromagnetic force can overwhelm the force, which is another way to destabilize the nucleus. This is why neutrons work. They help to separate protons, making them stronger than electromagnetic forces.

For smaller nuclei (up to 20 atomic numbers), the uniform distribution of protons and neutrons is usually the most stable. But for heavier elements, more and more neutrons are needed to provide a buffer of 1.5 or more neutrons to protons. But this is only part of the stability theory, which does not explain why differences in individual neutrons may mean huge differences in stability, nor does it explain why technetium does not have stable isotopes.

To understand this, we must go beyond the idea of representing the nucleus as a chaotic mass of protons and neutrons. We must think of these nucleons as having energy levels, just like electrons. As was taught in high school chemistry class: if an electron shell has eight electrons, it is stable. Similarly, there are some magic numbers that can complete the core and shell. Neutrons are 2, 8, 20, 28, 50, 82, 126, and protons are 2, 8, 20, 28, 50, 82, 114. The closer the nucleus is to these numbers, the more stable it will be.

These magic numbers are even because nucleons are paired according to their quantum spins, and upward and downward spins lead to net zero spins. This spin coupling means that even if the number of protons or neutrons is not magic, the nucleus still tends to have even number of protons, or even number of protons plus neutrons. Having protons or neutrons that do not counteract spin does not seem to be good for stability.

Does this explain the instability of technetium? Of course, we can see that 43 is not a magic number of protons, but the odd elements around it, such as silver with 47 protons, have very stable isotopes. It doesn't help to give technetium an even number of neutrons. Technetium-96, for example, decays in less than an hour. So in addition to neutron filling, core-shell filling and spin coupling, there seem to be more mysterious forces at work.

It turns out that there is no simple set of principles to determine nuclear stability. There are so many factors at work that the only way to solve this problem is to simulate the nucleus. We have achieved some remarkable success in this area by using computational techniques such as density functional theory. These models are still not perfect, but they make a lot of predictions that we have validated, and there are some predictions that we have not yet verified, such as stability island.

When we combine the experimental data with our simulation, we can make such a chart, where we can see these magic numbers. Elements with a magic number of protons have more stable isotopes, while elements with a near magic number of neutrons tend to have more isotopes. Patterns have emerged, but they do not provide us with clear rules for the conditions needed to produce stable nuclei.

So, are there any elements that are not on the periodic table? Our calculations show that there may be more magic numbers for protons and neutrons outside the periodic table, and our computer simulations agree. We are not sure what these magic numbers are, but obviously they are near neutron 184 and proton 126, and their half-lives may be millions of years.

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

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