Chapter 20. Nuclear Chemistry. In chemical reaction, only the outer electrons of the atoms are disturbed and the nuclei of the atoms are not affected.

Transcription

1 Chapter 2 Nuclear Chemistry In chemical reaction, only the outer electrons of the atoms are disturbed and the nuclei of the atoms are not affected. Two types of nuclear reactions:. Radioactive decay - the process in which a nucleus spontaneously disintegrates, giving off radiation. 2. Nuclear Bombardment reaction - a nuclear reaction in which a nucleus is bombarded, or struck, by another nucleus or by a nuclear particle. Radioactivity Writing an equation for the nuclear reaction corresponding to the decay of uranium-238 just as you would an equation for a chemical reaction.

2 Alpha rays consist of a helium -4 nuclei as seen in the uranium example. Beta rays consist of high speed electrons. Gamma rays are similar to x rays except they are higher in energy. Proton Neutron Electron Positron Gamma photon - H or n e or e or γ P - β β Nucleon is a collective name for two particles: a proton and a neutron The nuclear force is a strong force of attraction between nucleons that act only at a very short distances ( about -5 m). The shell model of the nucleus is a model in which protons and neutrons exist in levels, or shells, analogous to the shell structure that exists for electrons in an atom. Certain numbers of protons or neutrons appear to be more stable than others, the numbers of interest are called magic numbers - the number of nuclear particles in a completed shell of protons and neutrons For protons the magic numbers are: 2, 8, 2, 28, 5 and 82. For neutrons they are: 2, 8, 2, 28, 5, 82 and 26. Types of Radioactive Emission. Alpha emission

3 2. Beta emission 3. Positron emission 4. Electron capture 5. Gamma emission

4 6. Spontaneous Fission All nuclei with an atomic number greater than Z= 83 are radioactive. Many decay by alpha emission. Alpha particles are, or He nuclei are especially stable. Naturally radioactive elements, such as uranium-238, give a radioactive decay series until it reaches a stable nucleus, which is an isotope of lead. Transmutation is the change of one element to another by bombarding the nucleus of the element with nuclear particles or nuclei. Transuranium elements are elements with atomic numbers greater than that of uranium. (Z = 92), the naturally occurring element of greatest Z. Transuranium elements have a number of commercial uses. Example: Plutonium-238 and Americium-24

5 Biological Effects and Radiation Dosage To monitor the effect of nuclear radiations on biological tissue, it is necessary to have a measure of radiation dosage. The rad - radiation absorbed dose, is the dosage of radiation that deposits -2 J of energy per kilogram of tissue. The biological effect of radiation depends on not only the energy deposited, but the type of radiation. Neutrons are more destructive than gamma rays of the same dosage measured in rads. Rem - unit of radiation dosage used to regulate various kinds of radiation in terms of biological destruction. Rem = rad RBE ( relative biological effectiveness) Rate of radioactive decay is the number of nuclei disintegrating per unit time Rate = k N t Where N t = number of radioactive nuclei at time t, and k = the radioactive decay constant. Example: A. mg sample of technetium-99 has an activity of.7-5 Ci, decaying by beta emission. What is the decay constant for Tc?

6 The half life of a radioactive nucleus is the time it takes for one half of the nuclei in a sample to decay. The half life is independent of the amount of the sample. =. Example: The decay constant, k, for the beta decay of Tc was found to be. -3 /s. What is the half life of this isotope in years? Radioactive Dating - because the rate of radioactive decay of a nuclide is constant, this can serve as a clock for dating very old rocks and human implements. Once you know the decay constant for a radioactive isotope, you can calculate the fraction of the radioactive nuclei that remains after a given period of time by the following equation Carbon -4 Dating Method Example ln = -kt A piece of charcoal from a tree killed by the volcanic eruption that formed the crater in Crater Lake, Oregon, gave 7. disintegrations of carbon-4 nuclei per minute per gram of total carbon. Present day carbon gives 5.3 disintegrations per minute per gram carbon. Determine the date of the volcanic eruption. Recall that the half life of carbon-4 is 573 y.

7 Nuclear Power Plants and Nuclear Fission and Fusion Nuclear reactions, like chemical reactions, involve changes in energy. The changes in nuclear reactions are enormous by comparison with those in chemical reactions. Nuclear fission and fusion are reactions in which nuclei attain sizes closer to an intermediate range (approx mass number s around 5). Nuclear fission of uranium- 235 is employed in nuclear power plants to generate electricity. Fission of uranium nuclei produce approximately 3 different elements of intermediate mass. When the uranium-235 nuclei splits, approx 2 or 3 neutrons are released. If the neutrons from each nuclear fission are absorbed by other uranium-235 nuclei, these nuclei split and release even more neutrons. In this way, a chain reaction can occur. Critical mass is the smallest mass of fissionable material in which a chain reaction can be sustained. A Nuclear fission reactor is a device that permits a controlled chain reaction of nuclear fissions. A power plant with a nuclear reactor is used to produce heat, which is then used to produce steam to drive an electric generator. Fuel rods are the cylinders that contain fissionable material. Control rods are cylinders composed of substances that absorb neutrons, such as boron and cadmium, and can therefore slow the chain reaction.

8 Light water nuclear reactor consists of fuel rods with interspersed control rods.