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Why Does Everything Decay Into Lead

Mar 06, 2024
Humans have used

lead

in different ways throughout history. The ancient Romans used it to artificially sweeten their wine, modern dentists use it to protect the torso while x-raying the mouth. And of course, alchemists put a lot of effort into turning it into gold. But it turns out that nature

does

the opposite. Because if you look at the periodic table, all elements besides

lead

are radioactive. Its atoms are unstable. And the vast majority of those atoms will eventually, if given enough time,

decay

into lead. And then they will continue like this, because lead is not only stable.
why does everything decay into lead
Is magic. But I am not using that word to refer to the alchemists of the last century. Modern scientists call lead and many other elements magic. And luckily for us, it can be explained with science. Now before we get to any kind of magic, we should start with some basics of nuclear physics. The nucleus of an atom is made up of particles collectively called nucleons. You may know them better as protons and neutrons. It is the number of protons that determines what element an atom is. Any atom that has 82 protons is a lead atom, no matter how many neutrons it has.
why does everything decay into lead

More Interesting Facts About,

why does everything decay into lead...

But different isotopes of the same element have different numbers of neutrons. And when we talk about them we use both the name of the element and the total number of nucleons that an isotope has. Lead-208 has the necessary 82 protons plus 126 neutrons. Meanwhile, lead-206 only has 124 neutrons. And no matter what element you're dealing with, a given isotope is either stable, meaning you can put a proverbial chunk of it on your desk and it will remain an element forever... ...or it's radioactive, meaning it's probably you should do it. You don't have a lump of any size on your desk!
why does everything decay into lead
Sooner or later, all atoms will disintegrate. And in doing so, they release radiation and often become a completely different element. The two types of radioactive

decay

we will focus on today are called alpha and beta. Alpha decay occurs when a nucleus emits an alpha particle, which is made up of two protons and two neutrons. It is basically the same as a helium-4 core. And during beta decay, the unstable isotope emits a beta particle, which is either an electron or a positron, depending on the exact type of beta decay you're dealing with. The most common type converts a neutron into a proton and releases a couple more particles that we are not interested in for the topic of this video.
why does everything decay into lead
Both alpha and beta decay change the number of protons in the nucleus and therefore the element. Meanwhile, only alpha decay changes the total number of nucleons. But it is often discovered that a single decay, whether alpha or beta, is not enough for an unstable nucleus to become stable. You have to do it again, but you don't have to stick to the same kind of decadence every time. So, by putting together a series of alpha and beta decays, you get what scientists call decay chains. And if you know what isotope you're starting with, you can consistently predict the exact steps it will take to become stable, as well as what the final stable isotope will be.
The three main decay chains we see in nature are called the thorium series, actinium series, and radium series, which is also known as the uranium series. these end with lead-208, lead-207 and lead-206 respectively. But these names are a bit confusing because they are usually not named after the isotope they start with. And in addition to that, different isotopes of uranium, thorium and radium appear in the three series. So to clear things up a bit, let's look at the decay chain of thorium. It starts with the isotope thorium-232, which is less unstable than many radioactive elements out there.
It has a half-life of about 14 billion years. Which means that if you ignored my instructions and threw a piece of pure thorium-232 on your desk, half the atoms will no longer be thorium-232 when you check again in 14 billion years. But we don't have to wait halfway. We only need one nucleus to disintegrate. And when it

does

, it will spit out an alpha particle, becoming radium-228. It then beta decays twice, first to actinium-228 and then to thorium-228. …and then, four alpha decays later, it reaches lead-212. But we're not done yet, because that's radioactive lead! So if you beta decay to bismuth-212, then you have a proverbial choice.
Sometimes it follows the alpha decay route, sometimes it follows the beta decay route. But either way, the core does the other thing next and we end up at lead-208. Now, having analyzed all that, I must admit that there is a fourth chain of decay that does not end with lead: the neptunium chain. But the isotopes in this chain have such short half-lives that they have already gone through most of the steps. Scientists say it's "extinct in the wild"...except for the last step. The penultimate isotope in the chain is bismuth-209, which has a half-life of almost 20 quintillion years, or 1.4 billion times the age of the universe.
But if we fast forward to the very distant future, the last step in this chain is thallium-205. And there are a couple more exceptions to lead also being the final destination of a radioactive nucleus. Like all atoms that start with fewer nucleons than any isotope of lead. Or the fact that sometimes a supermassive nucleus that could turn into lead doesn't even care about a decay chain. Instead, it undergoes this really fun reaction known as spontaneous fission, where it effectively splits in two instead of just emitting little particles in an attempt to achieve stability. That seems fine to me.
But that brings us to an important question: why are some isotopes radioactive while others are stable? If we get closer to the nucleus, all the protons are positively charged, so they repel each other. Adding neutral neutrons can compensate. But the more protons you have, the more neutrons you need to keep things stable. This leads to a pattern called stability valley. If we look at the number of neutrons and protons of all the known isotopes and plot them on a fancy graph, we can see that all the stable ones lie on the same line. But increasing the ratio of neutrons to protons only works for so long.
Finally, the valley ends. Once you get past lead-208,

everything

is unstable. Everything is radioactive. But even within the valley of stability, you'll see that there are some strange patterns. For example, why does indium, with atomic number 49, only have 2 stable isotopes... but its neighbor on the periodic table, tin, has a whopping 10? Well, back in the 1940s, this particular quirk did not go unnoticed by a chemist named Maria Goeppert Mayer. While she was working on a project to map the abundance of various isotopes, she noticed that isotopes with a particular number of protons or neutrons were more likely to be stable.
These numbers were 2, 8, 20, 28, 50, 82 and 126. She proposed that this was evidence of the nuclear shell model, where protons and neutrons occupy specific energy levels within a nucleus. You can think of them as shells, and each one can contain a particular number of nucleons. When the outermost shell is completely filled, the atom holds its nucleons more tightly. This means that the isotope has a better chance of being stable. Now, if you're wondering why this all sounds a little familiar, it's probably because you learned that something very similar happens with electrons. Chemistry textbooks often describe electrons orbiting the nucleus in “shells.” And when the outermost layer is full, the element is less reactive.
This gives us the noble gases on the far right of the periodic table. They are inert because they have complete outer electronic shells. But while electron shells were accepted when Goeppert Mayer proposed the nuclear shell model, many physicists were reluctant to accept her ideas. Instead, they used the liquid drop model, which suggested that a nucleus is basically a mass of protons and neutrons. A physicist named Eugene Wigner was particularly skeptical, but he could not deny the patterns that Goeppert Mayer had discovered. That's why he called these stability numbers magic numbers, because the liquid drop model couldn't explain them.
But it wasn't magic, it was a revelation in nuclear physics. And Goeppert Mayer published her findings in 1948, almost exactly at the same time that German physicist Hans Jensen was discovering the same thing. But rather than this leading to academic rivalry or an attempt to erase a certain person's contributions to the world of science, this story has a happy ending. They continued to work together and in 1963 they shared the Nobel Prize in Physics. Despite not being magical, the term magical numbers stuck. And today, the accepted ones remain the ones Goeppert Mayer first identified: 2, 8, 20, 28, 50, 82 and, at least for neutrons, 126.
Some scientists think that 114 is more likely to be the seventh magic number. for protons, but that has not been demonstrated experimentally. Either way, the magic numbers explain why tin has more stable isotopes than indium. Tin has 50 protons, one of our magic numbers. But some isotopes are doubly magical because both their proton and neutron counts are correct. The simplest is helium-4, with two protons and two neutrons. And the extreme stability of this isotope is partly the reason why radioactive elements like thorium-232 undergo alpha decay. Because remember, alpha particles are identical to helium-4 nuclei. Now, having a magic number does not guarantee stability, it simply makes it more likely given other factors, such as the physical size of a nucleus.
For example, all isotopes of lead have 82 protons, so they are all magic. But many radioactive forms still exist. And then there's lead-208, which has 126 neutrons, making it doubly magical. That makes it more stable than it would otherwise be given its large size, and it ends up being the heaviest stable isotope... as far as we know. In short, many radioactive elements decay into lead because lead is magic, and an isotope of lead is doubly magic. But that's not the end of the story, because physicists are still searching for new magic numbers. And which ones are truly magical is a matter of debate!
Magic numbers can be predicted through theoretical calculations, but scientists have to perform experiments to actually test the real-world properties and stability of a given isotope. For example, in 2013, separate studies suggested that both 32 and 34 were magic numbers. But a follow-up study in 2021 looked at 32 by measuring the size of specific potassium nuclei. If a potassium nucleus started with 32 neutrons, and 32 were a magic number, you would measure a huge jump in its size if you added one more neutron. Because remember, the magic numbers refer to filled outer shells, so the 33rd neutron would have to go to a new outer shell that enlarges the entire nucleus.
But when the team tested this, they didn't see that change. So they concluded that there was nothing magical about the number 32. But potassium isn't exactly new territory to explore. That's why it's worth noting that magic numbers can also help scientists expand the periodic table and predict the stability of elements yet to be discovered. Anything heavier than lead-208 is radioactive, but as we get further and further away, towards heavier and heavier elements, things become very unstable. I'm talking about half-lives that last only milliseconds or microseconds, at most. But somewhere beyond the known elements, there is a hypothetical island of stability.
A group of isotopes that could be stabilized by unconfirmed magic numbers. They would still be radioactive, but their half-life would be longer than the other superheavy isotopes around them. Predictions range from minutes to millions of years. The center of the island could be Flerovium-298, which some scientists believe would be doubly magical with 114 protons and 184 neutrons. However, we still have a long way to go. As of 2023, the heaviest element we have synthesized is Oganesson-294, composed of 118 protons and 176 neutrons. So while they may not be “real” magic, magic numbers can tell us a lot about how the fundamental components of our universe work.
They may be the key to expanding the periodic table. And they explain how a piece of gray metal that makes you sick from radiation poisoning can become a piece of another gray metal that makes you sick from lead poisoning, with no alchemist in sight. Thank you for watching this episode of SciShow. And special thanks to everyone who supports us on Patreon. We couldn't do this without you. But is there a magic number of Patreon followers? Well, this is a science channel, so we should probably do an experiment to find out. ANDIf you'd like to help us test that hypothesis, consider signing up today!
Just head over to Patreon.com/SciShow for more information.

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