NUCLEAR CHEMISTRY The Isotopic Symbol Remember that the nucleus is comprised of the two nucleons, protons(p) and neutrons(n). The number of protons is the atomic number (Z). The number of protons and neutrons together is effectively the mass of the atom- mass number (A). Isotopes Isotopes are atoms of the same element that have different masses due to different numbers of neutrons in those atoms. There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238 Uranium has 92 protons. How many neutrons do each of the above isotopes have? Radioactivity It is not uncommon for some nuclides of an element to be unstable, or radioactive. We refer to these as radioisotopes. When they decay, they produce radiation and another element called a daughter product. There are several ways radioisotopes can decay into a different nuclide. These are called nuclear transformations. The 3 main types are: Alpha radiation (helium nucleus) Beta radiation (electron) Gamma radiation (electromagnetic energy) Types of Radioactive Decay Alpha Decay Loss of an -particle (a helium nucleus) 4 2 238 92 U He 234 90 4 2 Th+ He Types of Radioactive Decay Beta Decay Loss of a -particle (a high energy electron) 0 −1 131 53 I 0 or −1 131 54 e Xe + 0 −1 e Types of Radioactive Decay Gamma Emission Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle) 0 0 Notice that there is no mass indicated in the nuclear symbol for this type of radiation For more info go to this website Penetrating ability Question Write the nuclear equation for the alpha decay of radium-226 Write the nuclear equation for the beta decay of uranium-239 Neutron-Proton Ratios Like charges repel - Any element with more than one proton (i.e., anything but hydrogen) will have electrostatic repulsions between the protons in the nucleus. A strong nuclear force helps keep the nucleus from flying apart. Neutrons play a key role stabilizing the nucleus. Therefore, the ratio of neutrons to protons (n/Z) is an important factor for stability. Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called Belt of stability. Neutron-Proton Ratios For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1. Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus. Notice the n/Z ratio has increased in the “Belt of stability” Beta Emitters Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles. Because a n0 has been converted to a p+, n/Z has decreased into the “belt of stability” Positron or Electron capture Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture. Because the number of protons has been reduced, the n/Z has increased. Alpha Emitters There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission. Radioactive Decay Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead). This shows the decay of U-238. What are the types of decay shown? Half-life Half- life is the time it takes for half of the atoms in a sample to decay Each isotope has a particular half-life associated with it. Some common ones are to the right. Half-life Decay of 20.0 mg of 15O. What remains after 3 half-lives? After 5 half-lives? Nuclear Transformations Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide. Transuranic (Z>92)elements are made this way. These particle accelerators are enormous, having circular tracks with radii that are miles long. Measuring Radioactivity One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. The ionizing radiation creates ions, which conduct a current that is detected by the instrument. Energy in Nuclear Reactions There is a tremendous amount of energy stored in nuclei. Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy. In chemical reactions the amount of mass converted to energy is minimal. However, these energies are many thousands of times greater in nuclear reactions. Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is 0.0046 g. The change in energy, E, is then E = (m) c2 E = (4.6 10−6 kg)(3.00 108 m/s)2 E = 4.1 1011 J Uses of Radioactivity There are a number of uses for radioactivity. They include: Carbon dating – determine age of dead material (C-14) Medical uses – radiation therapy, nuclear imaging techniques (Tc-99), sterilising equipment Industrial uses – fire detectors (Am-241), thickness gauges, detecting cracks in pipes Food irradiation – to kill bacteria and fungi (Co-60 or Cs-137) Nuclear Fission Fission is the splitting of a radionuclide releasing energy Nuclear fission is the type of reaction carried out in nuclear reactors. Nuclear Fission Bombardment of the radioactive nuclide with a neutron starts the process. Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. This process continues in what we call a nuclear chain reaction. Nuclear Fission If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out. Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained. This is known as the Critical Mass. Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator. The process is similar to a coal fired power plant, but Uranium has a greater energy content than coal. A 1000MW power plant will use 8,500,00kg of coal or 74 kg of uranium Nuclear Reactors –Are they safe? The reaction is kept in check by the use of control rods. These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass which can generate too much heat and cause a meltdown. Nuclear Fission and Power Currently over 100 nuclear power plants in the U.S. and over 400 worldwide. Approximately 17% of the world’s energy comes from nuclear. Nuclear Fusion Fusion small nuclei combine 2H 1 + 2H 1 4He 2 + 1n + Energy 0 Occurs in the sun and other stars Nuclear Fusion Fusion (combining nuclei together) would be a far superior method of generating power. The good news is that the products of the reaction are not radioactive. The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins. Tokamak apparati like the one shown at the right show promise for carrying out these reactions. They use magnetic fields to heat the material.