Introduction to Atomic Reactions
The second of three pages designed to give the reader a background in the concepts of Radiometric Dating Techniques.
If you are having problems understanding terms such as half-life, Isotopes, Nuclides, nucleon, mass defect, Nuclear Binding Energy, and various Atomic Symbols See the Atomic Structure Page.
Nuclear Reactions are regulated by the Nuclear Binding Energy.
There is a direct correlation between the average nuclear binding energy and nuclide stability. The higher the average nuclear binding energy is calculated to be, the more stable the nucleus of the nuclide. Looking at the graphic below, we can see that the binding energy per nucleon is highest in the iron-56 range, so the intermediate mass numbered nuclides have a more tightly bound nucleons than all the rest of the nuclides. They are the most stable of all the nuclides. For both the smaller and more massive nuclides, the stability is less. This leads to some interesting reactions.
In Chemistry, molecules that are unstable tend to react with other molecules. When this happens, lots of energy is released, so the reaction is called an exothermic reaction. The same thing is true for nuclear reactions. Reactions that would tend to change a nuclide from an unstable condition to a more stable condition would also be exothermic.
We can see from this chart, to the left or above, that there are two kinds of unstable nuclides; Very small and very large nuclides. There are two types of reactions that take advantage of these two kinds of unstable nuclides. Both reactions take unstable nuclides and form the intermediate mass numbered nuclides which are more stable.
Fission, the first reaction, involves the splitting of the very large nuclides to produce a smaller more stable nuclides. This is a highly exothermic reaction and it is responsible for the energy that is produced in an Atom Bomb.
Fusion, the second reaction, involves the coming together of two unstable small nuclides to form a larger more stable nuclide. This is also a highly exothermic reaction, producing much more energy then even Fission. The Fusion reaction is responsible for the energy produced in the Hydrogen Bomb.
So, Both Fission and Fusion take advantage of instability resulting in an exothermic reaction. The same can be said for other types of nuclear reactions such as: Alpha, Beta, Gamma and Positron decay.
In the graphic to the left or above, we see that there are two stable isotopes of Carbon, C 12 and C 13. All the other isotopes are radioactive in that they decay. The ratio of protons and neutrons must be approximately 1:1 for a nuclide to be stable. (Below we will see that as we look at larger nuclides, the ratio slowly changes to 1:1.5.) Looking at the half-lives we can see that the further away from the proper ratio of protons and neutrons the more unstable it is. They have shorter and shorter half-lives so they disappear faster.
When we looked at the fission and Fusion reactions we saw that the products were more stable thus allowing an exothermic nuclear reaction to occur. The same is true for other nuclear reactions. Looking at Carbon 14, 15, and 16; We can see that there are too many neutrons. What the beta decay process does is to effectively turn a neutron into a proton. So what is happening, is that the nuclide turns into another nuclide that has a better ratio of neutrons and protons. It is getting closer to the normal ratio, thus creating a more stable nuclide in an exothermic reaction.
A positron decay is the opposite of a Beta decay. Looking at the following examples: Carbon 11, 10, and 9 have too few neutrons, so the nuclide undergoes a positron decay which effectively turns a proton into a neutron. Again a more stable nuclide is produced because it has a more normal ratio of protons to neutrons. More will be said when we look at these processes. But first let's look at the different nuclear reactions.
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Types of Radioactive Decay
The three most common types of radioactive decay which are naturally occurring, are alpha, beta, and gamma radiation. They were found when it was discovered that they responded differently in an electrical field. As can be seen in the graphic, to the left or above, this experiment is very simple. We have a radioactive sample that emits all three types of radiation. The sample is surrounded by a lead block except for a hole that leads to the experiment. A lead plate with a hole serves as a focuser to let only a small beam through. As the rays go through the charged field, some of the particles start bending, going in different directions.
When the beta rays go through a charged field they bend toward the (+) positive charged plate. So beta rays are (-) negatively charged. It is now known that beta particles have a charge of -1.
Alpha rays bend toward the (-) negative charged plate, so alpha rays are (+) positively charged. It is now known that alpha particles have a charge of +2. Gamma rays are not affected by the charged field at all, so they are not charged at all.
In addition to charge, alpha, beta and gamma radiation differ in the degree that they go through matter.
A simple sheet of paper, clothing, or even skin will stop alpha rays. It is fortunate that alpha radiation is relatively harmless when coming from external sources, outside of the body, because alpha radiation has the greatest capacity to ionize matter and cause damage to the body. They have 10 times the ionizing power of x rays, gamma rays, and beta rays. So Alpha radiation is the most deadly when ingested into the body because when the alpha emitting particles are in the actual tissues of the body, there is no dead skin (the epidermal layers of the skin) to stop the alpha rays before it reaches sensitive tissue. Every tissue that the Alpha particle can run into is living tissue!
Beta rays are able to penetrate greater thicknesses of matter so that it takes a thin sheet of metal to stop beta rays.
Gamma rays have the greatest penetrative abilities. It takes several inches of lead or a few feet of concrete to stop gamma rays. Both gamma radiation and x rays are hazardous even when external to the body because they penetrate so well.
Let's explore each one of these radiations types. First we will look at the naturally occurring types; alpha, beta, and gamma. Then we will look at some of the varieties found in the laboratory; positron emission, and electron capture. Look at the graphic below to view a summary of their properties.
Alpha particles are made of a particle that is identical to the nuclei of a helium 4 atom, having two protons and two neutrons, except that the alpha particle is moving fast, 1/10 the speed of light. Look at the chart below (below the Electromagnetic Spectrum graphic) for the summary information on the alpha and other radiation types.
Beta particles are made of a particle that is identical to electrons, except that beta particles move fast, much faster than alpha particles. They go 9/10 the speed of light. Thus they have a much higher penetrating power than alpha particles.
Gamma rays are an extremely high energy penetrating form of radiation. Actually they are a form of electromagnetic radiation. They are composed of bundles of energy called photons in much the same way as light is composed of photons, except that the frequency of gamma rays are very much higher than light, so the energy is much higher than visible light and much more dangerous. Gamma rays are the fastest of the types of radiation, going at the speed of light.
An unstable nuclide will emit beta rays when the ratio of neutrons to protons is too high. These are the heavier nuclides (look at the graphic above on this page that has the isotopes of Carbon). If an unstable nuclide has a ratio of neutrons to protons that is too low, making it a lighter isotope, it will emit positron rays. A positron is a positively charged particle that has the same mass as a beta particle or electron. There are no naturally occurring nuclides that produce positrons, only artificially produced nuclides exist.
Electron capture achieves the same effect as a positron emission. Rather then emitting something, an electron is captured. Usually the electron comes from some inner electron shell.
Protons and Neutrons
In nuclear lab experiments, neutrons are used from a neutron howitzer or neutron generator to make new nuclides. Since neutrons are not charged, they are not repelled by the electrons nor the protons. So they do not need to be made to go at extremely high speeds to cause a nuclear reaction.
Protons, on the other hand, are charged, so they need to be made to ram into atoms at very high speeds. So a particle accelerator (an atom smasher) which is often called names such as cyclotron and synchrotron, uses the charge in the projectile itself to accelerate the particle. In addition to protons, any charged particle can be used.
There are a few things we need to look at when you want to understand a nuclear reaction. Look at the graphic below and see the nuclear reaction examples.
The mass numbers (number of protons + neutrons)
The mass numbers on the left side of the equation equals the mass numbers on the right side of the equation. Look at the alpha decay example. U has a mass of 238. On the other side we see that the mass numbers for Th and He (the alpha particle) 234 and 4 add up to 238. This is true for all the other examples as well: 131 = 131 + 0; 234 = 230 + 0; and 11 = 11 + 0.
The atomic numbers (number of protons)
The atomic numbers on the left side of the equation also equals the atomic numbers on the right side of the equation. Look at the alpha decay example. U has an atomic number of 92. On the other side we see that the atomic numbers for Th and He (the alpha particle) 90 and 2 add up to 92. This is true for all the other examples as well: 53 = 54 + -1; 90 = 90 + 0; and 6 = 5 + 1.
An Alpha Decay Reaction
In the example graphic to the left or above, you can see that U 238 has turned into Th 234. An alpha particle has two protons and two neutrons just like a He 4 atom So one of the symbols for an alpha particle is He 4. We can see that the alpha particle took 4 from the mass number and 2 from the atomic number. Thus a Th atom is produced. (Remember that the number of protons or the Atomic number determines what element a nuclide is.)
A Beta Decay Reaction
In the example graphic to the left or above, you can see that I 131 has turned into Xe 131. The mass number did not change because all that is lost is an electron which has very little mass. What happens is that a neutron is effectively turned into a proton. That is why the mass does not change but the atomic number does change. Now there is one more proton and 1 less neutron.
A Gamma Decay Reaction
In the example graphic to the left or above, you can see that Th 234 does not turn into a different element. Nothing seems to happen except that we start with Th*. That means there is extra energy, that needs to be released. So a gamma ray is released to lower the energy state of Th. Other wise, nothing else happens.
A Positron Decay Reaction
In the example graphic to the left or above, you can see that C 11 has turned into B 11. The mass number did not change because all that is lost is a positron (which is a positively charged electron) which has very little mass. What happens is that a proton is effectively turned into a neutron. (This is the opposite of what happened in a beta decay reaction.) That is why the mass does not change but the atomic number does change. Now there is one less proton and 1 more neutron.
Using the Nuclide Chart to Visually Predict the Nuclide Produced by alpha, beta, gamma, and positron decay.
Since a radioactive decay reaction is predictable, we can use a nuclide chart to visually predict what the product nuclide should be.
In a beta decay reaction a neutron is effectively converted into a proton. So the product should have one more proton, and one fewer neutron. On the chart, that is the same as going up to the left.
In a positron decay reaction, the opposite happens. A proton is effectively converted into a neutron. So the product should have one more neutron and one few proton. On the chart, that is the same as going down toward the right.
In an alpha decay reaction, two protons and two neutrons are taken away from the original nuclide. So, on the chart, that is the same as going down toward the left two places.
In a gamma decay reaction, no change occurs. Only energy is released, so the chart does not indicate any change.
Lets look at an example. On the carbon 14 dating page, we looked at all of the Carbon isotopes. The carbon atoms that are too heavy, tend to break down by beta decay. On the chart we see the arrows going from C14, C15, and C16 to N14, N15, and N16 respectively. There were too many neutrons, so a neutron is effectively converted into a proton. So we get Nitrogen out of Carbon.
The carbon atoms that are too light, tend to break down by positron decay. On the chart we see the arrows going from C9, C10, and C11 to B9, B10, B11 respectively. There were too many protons, so a proton is effectively converted into a neutron. So we get Boron out of Carbon.
Now here is a very interesting chain reaction. U238 (Uranium 238) is unstable by alpha decay so it looses two protons and two neutrons. We can see the arrow going down to the left for two spaces. That is an alpha decay. We can see all the other alpha decays on the chart because they all go in the same direction.
Th 234 (Thorium 234), however, is also unstable. It breaks down by beta decay, and you can see that there is a chain reaction that goes for 14 reactions. We can see that it is a chain of both alpha and beta decays until we arrive at Pb 206 (Lead 206) which is the first stable nuclide. In addition, there are some nuclides that can bread down by either alpha or beta decay. They are Po 218 (Polonium 218), Bi 214 (Bismuth 214), and Bi 210 (Bismuth 210).
In exploring this world of atoms, you might want to take a good look at the nuclide chart available on the internet from Japan.
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