CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry 3 Kinds of Radiation TypeSymbolCharge Penetrating Power Energy α Helium nuclei – 0.04 mm3-9 MeV β Electrons 0 – 4 mm1-3 MeV γ γ Gamma ray 01 – 20 cm0.1 – 10 MeV γ γ X-ray – 1 cm0.1 – 10 MeV

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry Although alpha particles cause more damage than X-rays or gamma radiation, they have very low penetrating power and cannot pass through skin. Consequently alpha particles are not harmful to humans or animals as long as they do not get into the body; if they do get into the body, they can be quite harmful. Beta particles are less damaging to tissue than alpha particles but penetrate farther and so are generally more harmful. Gamma rays, which can easily penetrate skin, are by far the most dangerous and harmful form of radiation. αβγαβγ

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry There are more than 300 naturally occurring isotopes. Of these 264 are stable, meaning that the nuclei of these isotopes are not radioactive (they do not give off radioactivity); the remainder are radioactive isotopes. Among the lighter elements, stable isotopes have approximately the same number of protons and neutrons; this is the case of 12 6 C, 16 8 O, and Ne. Among the heavier elements, stability requires more neutrons than protons; the most stable isotope of lead, for example, is lead-206, Pb. More than 1000 artificial isotopes have been made in the laboratory; all are radioactive. Where does radioactivity comes from?

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry Beta emission: Beta emission: A type of nuclear decay in which a neutron is converted to a proton and an electron, and the electron is emitted from the nucleus: Emission of a beta particle transforms the element into a new element with the same mass number but an atomic number one unit greater. Phosphorus-32, for example, is a beta emitter: Note in this and any nuclear decay equation that the sum of the mass numbers and atomic numbers are the same on each side of the equation.

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry Alpha emission: Alpha emission: A type of nuclear decay in which a helium nucleus is emitted from the nucleus. In alpha emission, the new element formed has an atomic number two units lower and a mass number four units lower than the original nucleus. gamma emission In pure gamma emission, there is no change in either the atomic number or the mass number of the element. A nucleus in a higher-energy (excited) state emits gamma radiation as it returns to its ground state (its most stable energy state.

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry Half-life of a radioisotope, t 1/2 : Half-life of a radioisotope, t 1/2 : The time it takes one half of a sample of a radioisotope to decay. Iodine-131, with a half-life of 8 days, decays by beta, gamma emission.

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry A single whole-body irradiation of 25 rem is noticeable in white blood cell count. A single dose of 100 rem causes typical symptoms of radiation sickness, which include nausea, vomiting, a decrease in white blood cell count, and loss of hair. A single dose of 400 rem causes death within one month in 50% of the exposed persons. A single dose of 600 rem is almost invariably lethal within a month. It is estimated that a single dose of 50,000 rem is needed to kill bacteria, and up to 10 6 rem is needed to inactivate viruses. Radiation Dosimetry

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry The transmutation of two hydrogen nuclei into a helium nucleus liberates energy in the form of photons. nuclear fusion. This process is called nuclear fusion. All transuranium elements (elements with atomic number greater than 92) are artificial and have been prepared by nuclear fusion. To prepare them, heavy nuclei are bombarded with lighter ones.

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry The fusion of deuterium and tritium gives off a very large amount of energy. What is source of the energy? The answer is that there is a decrease in mass, and the missing mass is converted to energy.

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry All transuranium elements (elements with atomic number greater than 92) are artificial and have been prepared by nuclear fusion. To prepare them, heavy nuclei are bombarded with lighter ones. Examples are: These transuranium elements are unstable and have very short half-lives; that of lawrencium-257, for example, is only 0.65 second.

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry Nuclear fission: Nuclear fission: The fragmentation of larger nuclei into smaller ones. When uranium-235 is bombarded with neutrons, it is broken into two smaller elements. More importantly, energy is released because the products have less mass than the starting materials. The mass decrease in fission is converted into energy. atomic energy. This form of energy is called atomic energy.

CMH 121 Luca Preziati Chapter 9: Nuclear Chemistry Today more than 15% of the electrical energy in the United States is supplied by nuclear power plants. Disposal of spent but still radioactive nuclear fuel materials is a major long-term problem. Spent fuel contains high-level fission products together with recoverable uranium and plutonium. In addition, there are radioactive wastes from nuclear weapons programs, research reactors, and so forth. Recently the government gave its final approval to store nuclear wastes at a site deep under Yucca Mountain in Nevada.