Learning Objectives

Nuclear Chemistry

The nuclei of most naturally occurring elements are very stable in spite of the fact that the protons in the nucleus repel each other because of electrical repulsion. Some nuclei with certain ratios of neutrons to protons and nuclei with large numbers of neutrons and protons are not stable. These nuclei undergo radioactive decay in order to achieve a more stable nuclear structure.  These unstable elements are called radioisotopes.

Nuclear reactions involve changes in the composition of the nucleus of an atom. Radioactivity is the term for this process in which the nuclei of certain heavy atoms disintegrate and emit radiation. The elements formed as the products of nuclear reactions will be different then the starting element.

Three types of radiation are given off by naturally occurring radioactive compounds. These types of radiation were given the names of the first three letters of the Greek alphabet...alpha, beta and gamma.

The nucleus of an atom is changed when it emits either an alpha or a beta particle.  The atomic number is altered in both cases...remember that it is the atomic number that determines the identity of an element.

When an element emits a beta particle, it is converted into an element having an atomic number one greater than the original and an atomic mass that is unchanged. What element is formed when tritium, hydrogen-3, loses a beta particle?

When an element emits an alpha particle, it is converted into an element having an atomic number two less than the original and an atomic mass four less than the original.  The classic example of radioactive decay is the transmutation of uranium into lead.

Half-Life of Nuclear Decay Reactions

Each of the nuclear reactions in the transmutation of uranium takes time to occur. This time is reported as a half-life. Half-life is defined as the time required for the level of radioactivity to fall to one-half of its initial value.

The half-life for the beta decay of the radioactive isotope of phosphorus-32 is 14 days. Let's see how this works for a given amount of phosphorus.

The half-lives for the various steps in the transmutation of uranium-238 to lead-206 are incredibly long.

Health Risks of Radiation

Radiation induced biological damage depends, in part, on the total amount of radiation energy received by the organism. This is measured in rads which is an abbreviation for radiation absorbed dose. A rad is defined as the absorption of 0.01 joule of radiant energy per kilogram of tissue. 

Not all radiation causes the same damage to tissue. Very harmful radiation, such as alpha particles are given a weighting factor of 10.  Gamma rays and beta particles are given a weighting factor of 1.  When these factors are multiplied times the number of rads, a new measure of dosage is obtained. This unit is called rem for roentgen equivalent man. The following table gives some of the physiological effects for a single dose of radiation measured in rems.

One of the most common dangers of radiation involves the valence electrons of molecules found in living organisms. The energy transfer from the radiation to the molecule is sufficient to remove a valence electron and form a free electron and a radical cation. The radical cation can then break down to a new cation and a radical. These radicals are highly reactive entities which react readily with other compounds. An example of this process is shown by the following reaction of water with radiation. The electron that is lost can interact and ionize additional water molecules.

Nuclear Power Plants...Nuclear Fission

Most electricity is generated by boiling water and letting the steam drive a turbine which rotates a large coil of wire within a magnetic field.  Electricity is generated within the coil. Nuclear power generation operates on the same principle but the heat needed to boil the water comes from a nuclear reactor.

If an isotope of uranium, uranium-235, is bombarded with a neutron, it can form two lighter elements, krypton and barium.  This bombardment also produces three neutrons as well as a considerable amount of energy. These 3 newly produced neutrons can bombard three more uranium-235 nuclei which in turn now produce nine neutrons.  This is the start of a chain reaction that will continue unchecked and will escalate until all of the uranium is gone. The original nucleus of uranium was effectively split in half in this process which is called nuclear fission.

If the uranium is at a certain critical mass, an atomic explosion is the result.

If the uranium is at a sub-critical mass, the reaction can be a controllable source of energy.  

Nuclear Fusion

Nuclear fusion is the combination of two or more light nuclei to yield a heavier nucleus. The development of nuclear fusion was different than that of nuclear fission.  In the case of nuclear fusion, the hydrogen bomb was developed first and we still are yet to develop a fusion reactor.

The reaction that is most likely to be used to fuel the first fusion reactors will be the fusion of deuterium and tritium to form helium and a neutron.

The dynamics of this reaction involves fusing two positively charged nuclei together and somehow overcoming the strong repulsion forces between like charges. The particles must be given enough thermal energy so that they will fuse when they collide. This temperature required for this reaction is about 10⁸ degrees centigrade. The only known source for this amount of thermal energy is an atomic bomb.  Thus a small atomic bomb is used to start the fusion reaction in a hydrogen bomb.

There are many problems that have to be solved in order for nuclear fusion reactors to be feasible. The first problem is that, at 10⁸ degrees centigrade, all substances are in the gas phase. The only viable proposal is to contain the gases in a magnetic field. The second problem is starting the reaction in a controllable fashion. One proposal is to impact the fuel with lasers. There is then the problem of control of the reaction once it is started. Ultimately the most promising answer for nuclear power will be the development of nuclear fusion power plants.

Nuclear Medicine

Radioisotopes find considerable use in the medical field both as diagnostic probes and in the treatment of cancer. These radioisotopes are beta emitters.

Iodine-131 is used to diagnose the activity of the thyroid gland and in the treatment of thyroid cancer. Cobalt-60, phosphorus-32, gallium-67 and cesium-137 are used in the radiation treatment of many cancers. Sodium-24 is used to detect constrictions and obstructions in the circulatory system.

If a compound that contains a beta emitter is injected into a tumor, the beta particles will destroy the tumor and, since the beta particles will only travel a short distance, very little healthy tissue will be destroyed.

Carbon Dating

Carbon-14 is a beta emitter that decays to nitrogen-14 with a half-life of 5,730 years. This isotope of carbon is often used to date the remains of anything that is made of carbon. There is a naturally occurring ratio of one carbon-14 atom to one trillion carbon-12 atoms. All life is based on carbon compounds and as an organism grows it incorporates carbon-14 continuously in this ratio. When the organism dies, the uptake of carbon ceases and no additional carbon-14 will be added. The concentration of the carbon-14 will decrease steadily with time by decaying to nitrogen-14. If the carbon-14 to carbon-12 ratio in a sample is measured, the age of the sample can be estimated with reasonable accuracy. The results of this technique agree to within 10% of historical records. What famous artifact has given rise to much controversy when subjected to carbon dating?

Problems

1. Which nuclear emission gives off a helium nucleus?

2. Which nuclear emission gives off an electron?

3. What element is formed when phosphorus-32 decays by beta emission?

4. What element is formed when iodine-131 decays by beta emission?

5. What element will yield calcium-43 after beta emission?

6. What element will yield radium-218 after alpha emission?

7. A hospital needs 0.1 gram of iodine-131 for a thyroid therapy.  It takes 16 days to receive the shipment.  How many grams of iodine-131 should they order if the half life for iodine-131 is 8 days?

8. The half-life of carbon-14 is 5,730 years.  How old would an artifact be if it had a carbon-14 percent that was 25%.

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Copyright © January 2001 by Richard C. Banks...all rights reserved.