Compare qualitatively the ionizing and penetration power of alpha particles \(\left( \alpha \right)\), beta particles \(\left( \beta \right)\), and gamma rays \(\left( \gamma \right)\). Express the changes in the atomic number and mass number of a radioactive nuclei when an alpha, beta, or gamma particle is emitted. Write nuclear equations for alpha and beta decay reactions.

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Many nuclei are radioactive; that is, they decompose by emitting particles and in doing so, become a different nucleus. In our studies up to this point, atoms of one element were unable to change into different elements. That is because in all other types of changes discussed,only the electrons were changing. In these changes, the nucleus, which contains the protons thatdictate which element an atom is, is changing. All nuclei with 84 or more protons are radioactive, and elements with less than 84 protons have both stable and unstable isotopes. All of these elements can go through nuclear changes and turn into different elements.

In natural radioactive decay, three common emissions occur. When these emissions were originally observed, scientists were unable to identify them as some already known particles and so named them:

alpha particles (\(\alpha \)) beta particles\(\left( \beta \right)\) gamma rays \(\left( \gamma \right)\)

These particles were named using the first three letters of the Greek alphabet. Some later time, alpha particles were identified as helium-4 nuclei, beta particles were identified as electrons, and gamma rays as a form of electromagnetic radiationlike x-rays, except much higher in energy and even more dangerous to living systems.


The Ionizing and Penetration Power of Radiation

With all the radiation from natural and man-made sources, we should quite reasonably be concerned about how all the radiation might affect our health. The damage to living systems is done by radioactive emissions when the particles or rays strike tissue, cells, or molecules and alter them. These interactions can alter molecular structure and function; cells no longer carry out their proper function and molecules, such as DNA, no longer carry the appropriate information. Large amounts of radiation are very dangerous, even deadly. In most cases, radiation will damage a single (or very small number) of cells by breaking the cell wall or otherwise preventing a cell from reproducing.

The ability of radiation to damage molecules is analyzed in terms of what is called ionizing power. When a radiation particle interacts with atoms, the interaction can cause the atom to lose electrons and thus become ionized. The greater the likelihood that damage will occur by an interaction is the ionizing power of the radiation.

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Much of the threat from radiation is involved with the ease or difficulty of protecting oneself from the particles. How thick of a wall do you need to hide behind to be safe? The ability of each type of radiation to pass through matter is expressed in terms of penetration power. The more material the radiation can pass through, the greater the penetration power and the more dangerous it is. In general, the greater mass present, the greater the ionizing power, and the lower the penetration power.

Comparing only the three common types of ionizing radiation, alpha particles have the greatest mass. Alpha particles have approximately four times the mass of a proton or neutron and approximately 8,000 times the mass of a beta particle. Because of the large mass of the alpha particle, it has the highest ionizing power and the greatest ability to damage tissue. That same large size of alpha particles, however, makes them less able to penetrate matter. They collide with molecules very quickly when striking matter, add two electrons, and become a harmless helium atom. Alpha particles have the least penetration power and can be stopped by a thick sheet of paper or even a layer of clothes. They are also stopped by the outer layer of dead skin on people. This may seem to remove the threat from alpha particles, but it is only from external sources. In a nuclear explosion or some sort of nuclear accident, where radioactive emitters are spread around in the environment, the emitters can be inhaled or taken in with food or water and once the alpha emitter is inside you, you have no protection at all.

Beta particles are much smaller than alpha particles and therefore, have much less ionizing power (less ability to damage tissue), but their small size gives them much greater penetration power. Most resources say that beta particles can be stopped by a one-quarter inch thick sheet of aluminum. Once again, however, the greatest danger occurs when the beta emitting source gets inside of you.

Gamma rays are not particles, but a high energy form of electromagnetic radiation (like x-rays, except more powerful). Gamma rays are energy that has no mass or charge. Gamma rays have tremendous penetration power and require several inches of dense material (like lead) to shield them. Gamma rays may pass all the way through a human body without striking anything. They are considered to have the least ionizing power and the greatest penetration power.

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Figure 17.3.2: Three most common modes of nuclear decay.


"Nuclear Accounting"

When writing nuclear equations, there are some general rules that will help you:

The sum of the mass numbers (top numbers) on the reactant side equal the sum of the mass numbers on the product side. The atomic numbers (bottom numbers) on the two sides of the reaction will also be equal.

In the alpha decay of \(\ce{^{238}U}\) (Equation \(\ref{alpha1}\)), both atomic and mass numbers are conserved:

mass number: \(238 = 4 + 234\) atomic number: \(92 = 2 + 90\)

Confirm that this equation is correctly balanced by adding up the reactants" and products" atomic and mass numbers. Also, note that because this was an alpha reaction, one of the products is the alpha particle, \(\ce{_2^4He}\).

Note that both the mass numbers and the atomic numbers add up properly for the beta decay of thorium-234 (Equation \(\ref{beta2}\)):

mass number: \(234 = 0 + 234\) atomic number: \(90 = -1 + 91\)

The mass numbers of the original nucleus and the new nucleus are the same because a neutron has been lost, but a proton has been gained, and so the sum of protons plus neutrons remains the same. The atomic number in the process has been increased by one since the new nucleus has one more proton than the original nucleus. In this beta decay, a thorium-234 nucleus has one more proton than the original nucleus. In this beta decay, a thorium-234 nucleus has become a protactinium-234 nucleus. Protactinium-234 is also a beta emitter and produces uranium-234.

\<\ce{_{91}^{234}Pa} \rightarrow \ce{_{-1}^0e} + \ce{_{92}^{234}U} \label{nuke1}\>

Once again, the atomic number increases by one and the mass number remains the same; this confirms that the equation is correctly balanced.



Example \(\PageIndex{2}\)

Write each of the following nuclear reactions.

a) Carbon-14, used in carbon dating, decays by beta emission.

b) Uranium-238 decays by alpha emission.

Solution

a) Beta particles have the symbol \(\ce{_{-1}^0e}\). Emitting a beta particle causes the atomic number to increase by 1 and the mass number to not change. We get atomic numbers and symbols for elements using our periodic table. We are left with the following reaction:

\<\ce{_6^{14}C} \rightarrow \ce{_{-1}^0e} + \ce{_7^{14}N}\>

b) Alpha particles have the symbol \(\ce{_2^4He}\). Emitting an alpha particle causes the atomic number to decrease by 2 and the mass number to decrease by 4. We are left with:

\<\ce{_{92}^{238}U} \rightarrow \ce{_2^4He} + \ce{_{90}^{234}Th}\>



Decay Series

The decay of a radioactive nucleus is a move toward becoming stable. Often, a radioactive nucleus cannot reach a stable state through a single decay. In such cases, a series of decays will occur until a stable nucleus is formed. The decay of \(\ce{U}\)-238 is an example of this. The \(\ce{U}\)-238 decay series starts with \(\ce{U}\)-238 and goes through fourteen separate decays to finally reach a stable nucleus, \(\ce{Pb}\)-206 (Figure 17.3.3). There are similar decay series for \(\ce{U}\)-235 and \(\ce{Th}\)-232. The \(\ce{U}\)-235 series ends with \(\ce{Pb}\)-207 and the \(\ce{Th}\)-232 series ends with \(\ce{Pb}\)-208.

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Figure 17.3.3: Uranium-238 decay chain. (CC-BY-3.0 Tosaka)

Several of the radioactive nuclei that are found in nature are present there because they are produced in one of the radioactive decay series. For example, there may have been radon on the earth at the time of its formation, but that original radon would have all decayed by this time. The radon that is present now is present because it was formed in a decay series (mostly by U-238).

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Summary

A nuclear reaction is one that changes the structure of the nucleus of an atom. The atomic numbers and mass numbers in a nuclear equation must be balanced. Protons and neutrons are made up of quarks. The two most common modes of natural radioactivity are alpha decay and beta decay. Most nuclear reactions emit energy in the form of gamma rays.