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© Copyright 1998, Jim Loy
This subject sounds pretty tough, but the basic ideas are fairly simple.
An atom is mainly made up of a tiny nucleus (made of protons and neutrons), and a cloud of electrons around the nucleus. A proton has a positive electrical charge, an electron has an equal negative charge, a neutron is neutral. Protons and neutrons have almost exactly the same mass (weight). An atom generally has an equal number of protons and electrons, and is neutral in charge because of this. The electrons determine the chemical properties and color of the atom. The nucleus determines the mass of the atom, and whether or not it is radioactive.
Let's take a normal nucleus, carbon-12. This nucleus is made up of 6 protons and 6 neutrons. Together they give this nucleus a mass of 12. Carbon-12 is abbreviated as 6C12. Another kind of carbon nucleus is carbon-14 (6C14). This one has 6 protons and 8 neutrons. Since an atom normally has an equal number of protons and electrons, these two nuclei have the same number of electrons, and have the same chemical properties (they both act like carbon). Well, 6C14 has too many neutrons. This nucleus tends to lose tiny pieces, which fly off as radiation. 6C14 is radioactive, it decays.
There are three main ways that a nucleus can decay. It can lose an alpha particle (a helium nucleus) 2HE4, it loses two protons and two neutrons. Or it can lose a negative beta particle, which is the same as an electron, which converts a neutron into a proton. Or it can lose a positive beta particle, which is also called a positron, and is the same as a positive electron, and converts a proton into a neutron. There are other forms of radioactive decay. But, we will ignore them, in this article.
All three of these forms of decay result in a different kind of atom (a different element). Alpha decay results in the loss of two protons. Carbon would become beryllium, the atom with 4 protons. Negative beta decay changes a neutron into a proton. Carbon would become Nitrogen, the atom with 7 protons. And positron decay changes a proton into a neutron. Carbon would become boron, the atom with 5 protons.
As I said above, 6C14 has too many neutrons. Can you guess what kind of decay normally happens to a 6C14 nucleus? If you said "negative beta decay," you were right. And it becomes 7N14, one of the forms of nitrogen. This nitrogen does not have too many neutrons or protons, it is just right (Mamma Bear). So 7N14 is not radioactive, it does not decay, it is stable.
When a nucleus has too many neutrons, it tends to beta decay. When it has too many protons, it tends to positron decay. And when it is just too big, it tends to alpha decay.
Let's take a look at Uranium-238 (92U238), which is found in volcanic rock (such as granite). This nucleus is almost stable. It may sit there for billions of years and never decay. But it is a little too big, and will eventually alpha decay. As we saw above, alpha decay means losing two protons and two neutrons (as an alpha particle, which is a helium nucleus). Well, losing two protons and two neutrons makes 92U238 become 90Th234 (a variety of thorium).
We're just shuffling numbers here. We don't have to understand much about this.
90Th234 has too many neutrons, so it negative beta decays, becoming 91Pa234 (protactinium). This too has too many neutrons, and beta decays to become 92U234. This is too big and alpha decays to become 90Th230. This too is too big and alpha decays to become 88Ra226 (radium). This is the world's main source of radium. 88Ra226 is too big, and alpha decays to become 86Rn222 (radon). Radon is a gas, and this process is the source of radon in people's homes.
86Rn222 is too big and alpha decays to 84Po218 (polonium). This alpha decays to 82Pb214 (Lead). This beta decays to 83Bi214 (bismuth). This beta decays to 84Po214. This alpha decays to 82Pb210. To 83Bi210 to 84Po210. And finally, this alpha decays to 82Pb206. This third variety of lead is completely stable, and is the end of the series. Lead, found in nature, contains a mixture of all of these other elements.
It's not quite that straightforward. There are a few alternative branches to the above series. For example, rarely a 84Po218 nucleus will beta decay, instead of alpha decay.
That's a long series. But, since we just described it, instead of explaining it, it is really fairly simple. But, nuclear physicists don't explain it either. Mainly, they just describe it (in more detail than I did above).
And there are other decay series. 90Th232 ends up as 82Pb208. 92U235 ends up as 82Pb207. And 96Cm245 ends up as 83Bi209
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