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Uranium keeps decaying, so why is there still uranium on Earth after 4.55 billion years?

Reasons why uranium can decay

By ApostolakisPublished 2 years ago 4 min read

Why does uranium decay? Uranium keeps decaying, so why is there still uranium on Earth after 4.55 billion years?

Uranium is an important raw material for the use of nuclear energy, but it is also a radioactive element that decays. Why does uranium decay? It starts with the microscopic structure of the atom.

As we all know, an atom is composed of a nucleus and electrons, while the nucleus is composed of a certain number of protons and neutrons. Inside the nucleus, which has multiple protons, there is a battle between two forces: the strong interaction force, which binds protons and neutrons together, and the electromagnetic force, which repels protons when there are multiple protons in the nucleus because they are positively charged.

The strong interaction force is the strongest of the four fundamental forces of the universe, but it is a short-range force, acting at a distance of only 10^(-15) meters, while the electromagnetic force is a long-range force, theoretically infinite, which means that the repulsive force between protons can be superimposed, while the strong interaction force can only " alone".

Because of this, when the number of protons inside the nucleus reaches a certain level, the repulsive force between the protons can reach a strength that can counteract the strong interaction force, and the nucleus becomes unstable. This is the case of the uranium nucleus, which is the heaviest element we can find in nature, its atomic number is 92, that is, the number of protons in the nucleus of uranium is as high as 92, so many protons, of course, the nucleus of uranium is unstable, and this is the reason why uranium will decay.

We know that everything in the universe spontaneously tends to a stable state, and the nucleus of uranium is certainly no exception. One of the most effective ways is to reduce the number of protons.

However, for a heavy nucleus like uranium, which has a large number of protons and neutrons, it is almost impossible to release protons alone, because there is a "clumping effect" inside the nucleus, which simply means that protons and neutrons are not evenly distributed inside the nucleus, but are combined into a clump. The most easily formed clump is the "alpha clump", which consists of two protons and two neutrons.

So the nucleus of uranium always tends to release an "alpha cluster" outward. When the nucleus of uranium undergoes "alpha decay", it will lose two protons and two neutrons at once, and its atomic number will be reduced by 2, which will turn into thorium, the 90th element, for example, uranium-238 will turn into thorium-234 after "alpha decay".

As we know, the "age" of the Earth is about 4.55 billion years, so the question arises since uranium keeps decaying, then why is there still uranium on the Earth after 4.55 billion years? This is quite understandable.

Just because a uranium nucleus always tends to decay doesn't mean it will decay right away; it's probabilistic. For a single uranium nucleus, it is uncertain when it will decay, that is, it could decay after 1 second or after 100 million years, but if enough uranium nuclei are observed, their decay possesses an obvious pattern.

This is where the concept of "half-life" comes into play, which can be understood simply as the time it takes for half of the nuclei of a large pile of radioactive elements to decay.

For example, if we assume that the half-life of a radioactive element is 1 second, then when we observe 200 million of these nuclei, it takes only 1 second for 100 million of them to decay, and in the next second, 50 million of the remaining 100 million nuclei that have not decayed before will decay, and over time, these nuclei will continue to "halve" until their number is halved" until their number is so small that they are no longer statistically significant.

Studies have shown that the only three naturally occurring isotopes of uranium on Earth are uranium-238, uranium-235, and uranium-234, all of which decay in "alpha decay", with uranium-238 having the longest half-life of about 4.468 billion years, which means that after 4.55 billion years, uranium-238 on Earth has only decayed by about half. -238 has only decayed by about half.

Uranium-235 and U-234 have relatively shorter half-lives of about 704 million years and 245,500 years, respectively, but in 4.55 billion years, they have not "halved" enough to be lost to nature, except that their relative abundance is lower than that of U-238, and measurements Measurements show that the relative abundance of U-238, U-235, and U-234 on Earth is 99.2742%, 0.7204%, and 0.0054%, respectively.

A summary is that although uranium keeps decaying, we can still find traces of all three isotopes on Earth after 4.55 billion years because they have relatively long half-lives.

It's worth mentioning that all the uranium elements in the universe were created in high-energy events like supernova explosions and neutron star collisions, and it took a long time before a tiny fraction of them came to Earth, so the uranium on Earth is older than we think, and they decayed for a long time before coming to Earth.

Well, that's all we have for today, welcome to follow us and we'll see you next time.


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