Matter is made up of atoms. You all know that. But where do atoms come from? When and how were they made? And what is the “island of stability”? That’s what we’ll talk about today.
At first glance, it doesn’t seem that difficult to make an atom. All you need are some neutrons and protons for the nucleus, then put electrons around them until the whole thing is electrically neutral. Sounds easy. But it is not like that.
The electrons are the easy part. Once you have a positively charged nucleus, it will attract electrons and they will automatically form shells around the nucleus. For more information on atomic electron shells, see my previous video.
However, it is not easy to make an atomic nucleus. The problem is that the protons are all positively charged and repel one another. Now, when you have them really, very close together, the strong nuclear force will step in and hold them together – if the mixture contains an adequate amount of neutrons. But to bring the protons close enough together, you need very high temperatures. We’re talking hundreds of millions of degrees.
Such high temperatures, in fact much higher temperatures, existed in the early Universe shortly after the Big Bang. At that time, however, the density of matter was very high everywhere in the universe. It was a mostly structureless soup of subatomic particles called plasma. This soup did not contain any seeds, just a mixture of the ingredients in the seeds.
It was only when this plasma expanded and cooled that some of these particles managed to stick together. This created the first atomic nuclei, which could then trap electrons to form atoms. From this you get hydrogen and helium as well as some other chemical elements with their isotopes up to atomic number 4. Incidentally, the process of making atomic nuclei is called “nucleosynthesis”. And this part of nucleosynthesis, which took place a few minutes after the Big Bang, is called “Big Bang Nucleosynthesis”.
However, the expansion of the plasma after the Big Bang took place so quickly that only the lightest atomic nuclei were able to form. Making the heavier ones requires more patience, hundreds of millions of years in fact. During this time the universe continued to expand, but the cores of light gathered under gravity and formed the first stars. In these stars, gravitational pressure increased the temperature again. Eventually the temperature became high enough to press the small atomic nuclei into one another and fuse them with larger ones. This nuclear fusion creates energy and is why stars are hot and glow.
Nuclear fusion in stars can continue up to atomic number 26, which is iron, but then it stops. This is because iron is the most stable of the chemical elements. Its binding energy is the greatest. So when you join small nuclei, energy is released in the process until you hit iron. After that, pushing more into the core begins to absorb energy.
With nuclear fusion within the stars, we now have elements down to iron. But where do the elements heavier than iron come from? They come from a process called “neutron capture”. Some fusion processes produce free neutrons, and because the neutrons have no electrical charge, they have a much easier time to enter an atomic nucleus than protons. And once they are in the nucleus, they can decay into a proton, an electron, and an electron antineutrino. If you do, you have created a heavier element. Many of the nuclei so created are unstable isotopes, but they spit out pieces until they hit a stable configuration.
Neutron capture can happen randomly in stars from time to time. Therefore, over time, ancient stars grow some of the elements that are heavier than iron. But the stars eventually run out of nuclear fuel and die. Many of them collapse and then explode. These supernovae distribute the nuclei in galaxies or even blow them out of galaxies. Some of the lighter elements that exist today actually arise from the splitting of these heavier elements by cosmic rays.
However, neutron capture in old stars is slow and stars only live that long. This process just doesn’t produce enough of the heavy elements that we have here on earth. This requires a process called “Rapid Neutron Capture”. This requires an extreme environment with very high pressure and many neutrons that bombard the small atomic nuclei. Again, some of the neutrons enter the nucleus and then decay, leaving a proton that creates heavier elements.
For a long time, astrophysicists thought that supernovae were characterized by rapid neutron capture. But that turned out not to be very good. Their calculations showed that supernovae would not produce enough neutrons fast enough. The idea didn’t go well with observations either. For example, if the heavy elements that astrophysicists observe in some small galaxies – so-called “dwarf galaxies” – had been created by supernovae, it would have required so many supernovae that these small galaxies would have been blown apart and we would not have observed them in the first place.
Astrophysicists now believe that the heavy elements are most likely not created in supernovae, but rather in fusions of neutron stars. Neutron stars are one of the remnants of supernovae. As the name suggests, they contain a lot of neutrons. They don’t actually contain any nuclei, just a large clump of super-dense nuclear plasma. However, when they collide, the collision spits out many nuclei and creates conditions suitable for rapid neutron capture. This can create all the heavy elements that we find on earth. A recent analysis of the light emitted during a neutron star fusion supports this hypothesis, as the light contains clues to the presence of some of these heavy elements.
You may have noticed that we haven’t checked off the heaviest elements in the periodic table and that some are missing in between. That’s because they’re unstable. They disintegrate into smaller nuclei in times between a few thousand years and a few microseconds. Those that were produced in stars have long since disappeared. We only know their properties because they were made in laboratories by shooting smaller cores at high energy.
Are there any other stable nuclei that we haven’t discovered yet? May be. It has been a long-standing hypothesis in nuclear physics that there are heavy nuclei with a certain number of neutrons and protons that should have a lifespan of up to a few hundred thousand years. It’s just that we haven’t been able to create them before. Nuclear physicists call it the “island of stability” because it looks like an island when you put each nucleus on a graph where one axis is the number of protons and the other axis is the number of neutrons.
However, the exact location of the Isle of Stability is not clear, and the predictions have shifted somewhat over time. Nuclear physicists currently believe that in order to reach the island of stability, more neutrons will have to be pushed into the heaviest nuclei that they previously produced.
But perhaps the most amazing thing about atoms is how much complexity, look around you, is made up of just three components, neutrons, protons and electrons.
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