NUCLEAR ENERGY Nuclear chemistry is the study of nuclear reactions, with an emphasis on their uses in chemistry and their effects on biological systems Nuclear reaction, where matter changes originating in its nucleus When nuclei change spontaneously, emitting radiation, they are said to be radioactive Radioactivity We recall first, that two subatomic particles reside in the nucleus are protons and neutron We will refer these particles as nucleons Also that all atoms of a given element have the same protons; that determines the atomic number of an element However, atoms of a given element can have different number of neutrons and therefore different mass numbers Atoms with the same atomic numbers but different mass numbers are known as isotopes The different isotopes of an element is distinguished by their mass umbers Radioactivity Some nuclei are unstable; they emit particles and/or electromagnetic radiation spontaneously. The name for this phenomenon is radioactivity. All elements having an atomic number greater than 83 are radioactive. For example, the isotope of polonium, polonium-210 ( 210 84𝑃𝑜), decays spontaneously to ( 206 82𝑃𝑏) by emitting an ∝ particle. Radioactivity Indeed, the nuclear properties of an atom depend on the number of protons and neutrons in its nucleus. The term nuclide applies to a nucleus with a specified number of protons and neutrons Nuclei that are radioactive are called radionuclides, and atoms containing these nuclei are called radioisotopes Nuclear Reaction Most nuclei in nature are stable and remain intact indefinitely. Radionuclides, however, are unstable and spontaneously emit particles and electromagnetic radiation. Emission of radiation is one of the ways in which an unstable nucleus is transformed into a more stable one that has less energy. The emitted radiation is the carrier of the excess energy. Nuclear Reaction In balancing any nuclear equation, we observe the following rules: The total number of protons plus neutrons in the products and in the reactants must be the same (conservation of mass number). The total number of nuclear charges in the products and in the reactants must be the same (conservation of atomic number). Nuclear Reaction Example: 1. What product is formed when radium-226 undergoes alpha decay? 2. Which element undergoes alpha decay to form lead-208? Types of Radioactivity Alpha Decay Beta Decay alpha radiation consists of a stream of helium-4 nuclei known as alpha particles, which we denote as 42He or 42∝ Beta radiation consists of streams of beta (β) particles, which are high-speed electrons emitted by an unstable nucleus. Beta particles are represented in nuclear equations by −10𝑒 or sometimes by −10𝛽 Gamma Radiation Gamma (𝜸) radiation (or gamma rays) consists of high-energy photons (that is, electromagnetic radiation of very short wavelength). It changes neither the atomic number nor the mass number of a nucleus and is represented as either 00𝛾 or simply 𝛾 Types of Radioactive Decay Positron Emission 0 1𝑒, is a particle that has the same mass as an electron (thus, we use the letter e and superscript 0 for the mass) but the opposite charge (represented by the subscript). In general, positron emission has the effect of converting a proton to a neutron, thereby decreasing the atomic number of the nucleus by 1 Electron Capture is the capture by the nucleus of an electron from the electron cloud surrounding the nucleus electron is consumed rather than formed in the process, it is shown on the reactant side of the equation. Electron capture, like positron emission, has the effect of converting a proton to a neutron Types of Radioactive Decay Example: 1. Write nuclear equations for (a) mercury-201 undergoing electron capture; (b) thorium-231 decaying to protactinium-231. 2. Write a balanced nuclear equation for the reaction in which oxygen-15 undergoes positron emission. 3. Balance the following nuclear equations (that is, identify the product X): (a) 212 84Po → 208 82Pb + (b) 137 55Cs → 137 56Ba X +X Nuclear Transmutation Nuclear Transmutation Another type of radioactivity that results from the bombardment of nuclei by neutrons, protons, or other nuclei An example of a nuclear transmutation is the conversion of atmospheric 147N to 146C and 11H, which results when the nitrogen isotope captures a neutron (from the sun). In some cases, heavier elements are synthesized from lighter elements. This type of transmutation occurs naturally in outer space, but it can also be achieved artificially. Nuclear Transmutation Nuclear Transmutation Nuclear Stability The principal factor that determines whether a nucleus is stable is the neutron-to proton ratio (n/p). For stable atoms of elements having low atomic number, the n/p value is close to 1. As the atomic number increases, the neutron-to-proton ratios of the stable nuclei become greater than 1. This deviation at higher atomic numbers arises because a larger number of neutrons is needed to counteract the strong repulsion among the protons and stabilize the nucleus. Nuclear Stability The type of radioactive decay that a particular radionuclide undergoes depends largely on how its neutron-to-proton ratio compares with those of nearby nuclei that lie within the belt of stability. We can envision three general situations: 1. Nuclei above the belt of stability (high neutron-to-proton ratios). These neutron-rich nuclei can lower their ratio and thereby move toward the belt of stability by emitting a beta particle because beta emission decreases the number of neutrons and increases the number of protons Nuclear Stability 2. Nuclei below the belt of stability (low neutron-to-proton ratios). These proton-rich nuclei can increase their ratio and so move closer to the belt of stability by either positron emission or electron capture because both decays increase the number of neutrons and decrease the number of protons. Positron emission is more common among lighter nuclei. Electron capture becomes increasingly common as the nuclear charge increases. 3. Nuclei with atomic numbers ≥ 𝟖𝟒 . These heavy nuclei tend to undergo alpha emission, which decreases both the number of neutrons and the number of protons by two, moving the nucleus diagonally toward the belt of stability. Nuclear Stability The following rules are useful in predicting nuclear stability: Nuclei that contain 2, 8, 20, 50, 82, or 126 protons or neutrons are generally more stable than nuclei that do not possess these numbers. For example, there are ten stable isotopes of tin (Sn) with the atomic number 50 and only two stable isotopes of antimony (Sb) with the atomic number 51. Nuclei with even numbers of both protons and neutrons are generally more stable than those with odd numbers of these particles All isotopes of the elements with atomic numbers higher than 83 are radioactive. All isotopes of technetium (Tc, Z=43) and promethium (Pm, Z=61) are radioactive. Example: 1. Predict the mode of decay of (a) carbon-14, (b) xenon-118. 2. Predict the mode of decay of (a) plutonium-239, (b) indium-120. Radioactive Decay Radioactive decay series is a sequence of nuclear reactions that ultimately result in the formation of a stable isotope Half life is the time required for half of any given quantity of a substance to react Radiometric Dating Because the half-life of any particular nuclide is constant, the half-life can serve as a nuclear clock to determine the age of objects. The method of dating objects based on their isotopes and isotope abundances is called radiometric dating. When carbon-14 is used in radiometric dating, the technique is known as radiocarbon dating. The procedure is based on the formation of carbon-14 as neutrons created by cosmic rays in the upper atmosphere convert nitrogen-14 into carbon-14 Kinetics of Radioactive Decay As noted earlier, radioactive decay is a first-order kinetic process. Its rate, therefore, is proportional to the number of radioactive nuclei N in a sample: Rate = kN The first-order rate constant, k, is called the decay constant. The rate at which a sample decays is called its activity, and it is often expressed as number of disintegrations per unit time. The becquerel (Bq) is the SI unit for expressing activity. A becquerel is defined as one nuclear disintegration per second. An older, but still widely used, unit of activity is the curie (Ci), defined as disintegrations 3.7x 1010 per second, which is the rate of decay of 1 g of radium. Kinetics of Radioactive Decay Kinetics of Radioactive Decay Example: 1. A rock contains 0.257 mg of lead-206 for every milligram of uranium-238. The half-life for the decay of uranium-238 to lead-206 is 4.5x 109 yr. How old is the rock? 2. A wooden object from an archaeological site is subjected to radiocarbon dating. The activity due to 14C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5715 yr. What is the age of the archaeological sample? Energy Changes in Nuclear Reactions Einstein’s celebrated equation relating mass and energy: 𝐸 = 𝑚𝑐 2 E stands for energy m for mass, and c for the speed of light, C=2.9979x108 m/s Energy Changes in Nuclear Reactions This equation states that the mass and energy of an object are proportional. If a system loses mass, it loses energy (exothermic); if it gains mass, it gains energy (endothermic). Because the proportionality constant in the equation, c2, is such a large number, even small changes in mass are accompanied by large changes in energy. The mass changes and the associated energy changes in nuclear reactions are much greater than those in chemical reactions. Nuclear Binding Energy Nuclear binding energy The energy required to separate a nucleus into its individual nucleons This quantity represents the conversion of mass to energy that occurs during an exothermic nuclear reaction. The concept of nuclear binding energy evolved from studies of nuclear properties showing that the masses of nuclei are always less than the sum of the masses of the nucleons, which is a general term for the protons and neutrons in a nucleus. Mass Defect The mass difference between a nucleus and its constituent nucleons Relativity theory tells us that the loss in mass shows up as energy (heat) given off to the surroundings Nuclear Power: Fission Nuclear Fission Heavy nuclei gain stability and therefore give off energy if they are fragmented into two midsized nuclei is the process in which a heavy nucleus (mass number > 200) divides to form smaller nuclei of intermediate mass and one or more neutrons Because the heavy nucleus is less stable than its products (see Figure 23.2), this process releases a large amount of energy. Nuclear Power: Fission Nuclear chain reaction a self-sustaining sequence of nuclear fission reactions The number of fissions and the energy released quickly escalate, and if the process is unchecked, the result is a violent explosion. For a fission chain reaction to occur, the sample of fissionable material must have a certain minimum mass. Otherwise, neutrons escape from the sample before they have the opportunity to strike other nuclei and cause additional fission. Nuclear Power: Fission Critical mass The amount of fissionable material large enough to maintain a chain reaction with a constant rate of fission When a critical mass of material is present, one neutron on average from each fission is subsequently effective in producing another fission and the fission continues at a constant, controllable rate. The critical mass of uranium-235 is about 50 kg for a bare sphere of the metal. Supercritical mass If more than a critical mass of fissionable material is present, very few neutrons escape. The chain reaction thus multiplies the number of fissions, which can lead to a nuclear explosion. A mass in excess of a critical mass Nuclear Power: Nuclear reactors Nuclear power plants use nuclear fission to generate energy. The core of a typical nuclear reactor consists of four principal components: fuel elements, control rods, a moderator, and a primary coolant The fuel is a fissionable substance, such as uranium-235. The fuel elements contain enriched uranium in the form of UO2 pellets encased in zirconium or stainless steel tubes. The control rods are composed of materials that absorb neutrons, such as cadmium or boron. These rods regulate the flux of neutrons to keep the reaction chain self-sustaining and also prevent the reactor core from overheating Types of Nuclear Reactors Light Water Reactors Nuclear reactors that use light water (H O) as a moderator are called light water reactors because 1H is the lightest isotope of the element hydrogen. 2 Heavy Water Reactors Another type of nuclear reactor uses D O, or heavy water, as the moderator, rather than H2O. Deuterium absorbs neutrons much less efficiently than does ordinary hydrogen. The main advantage of a heavy water reactor is that it eliminates the need for building expensive uranium enrichment facilities. However, D2O must be prepared by either fractional distillation or electrolysis of ordinary water, which can be very expensive considering the amount of water used in a nuclear reactor. 2 Types of Nuclear Reactors Breeder Reactors A breeder reactor uses uranium fuel, but unlike a conventional nuclear reactor, it produces more fissionable materials than it uses. In a typical breeder reactor, nuclear fuel containing uranium-235 or plutonium- 239 is mixed with uranium-238 so that breeding takes place within the core. For every uranium-235 (or plutonium-239) nucleus undergoing fission, more than one neutron is captured by uranium-238 to generate plutonium-239. Nuclear Power: Fusion In contrast to the nuclear fission process, nuclear fusion, the combining of small nuclei into larger ones, is largely exempt from the waste disposal problem. Biological Effect of Radiation When matter absorbs radiation, the radiation energy can cause atoms in the matter to be either excited or ionized. In general, radiation that causes ionization, called ionizing radiation, is far more harmful to biological systems than radiation that does not cause ionization. The latter, called nonionizing radiation, is generally of lower energy, such as radiofrequency electromagnetic radiation or slow-moving neutrons When living tissue is irradiated, water molecules absorb most of the energy of the radiation. Thus, it is common to define ionizing radiation as radiation that can ionize water, a process requiring a minimum energy of 1216 kJ>mol. Alpha, beta, and gamma rays (as well as X-rays and higher-energy ultraviolet radiation) possess energies in excess of this quantity and are therefore forms of ionizing radiation. The damage produced by radiation depends on the activity and energy of the radiation, the length of exposure, and whether the source is inside or outside the body Gamma rays are particularly harmful outside the body because they penetrate human tissue very effectively, just as X-rays do. Consequently, their damage is not limited to the skin. In contrast, most alpha rays are stopped by skin, and beta rays are able to penetrate only about 1 cm beyond the skin surface Neither alpha rays nor beta rays are as dangerous as gamma rays, therefore, unless the radiation source somehow enters the body. Within the body, alpha rays are particularly dangerous because they transfer their energy efficiently to the surrounding tissue, causing considerable damage. In general, the tissues damaged most by radiation are those that reproduce rapidly, such as bone marrow, blood-forming tissues, and lymph nodes. The principal effect of extended exposure to low doses of radiation is to cause cancer. Cancer is caused by damage to the growth-regulation mechanism of cells, inducing the cells to reproduce uncontrollably. Leukemia, which is characterized by excessive growth of white blood cells, is probably the major type of radiation-caused cancer.
