Chemistry - Unit 13

 Discovery of Radioactivity  In 1895 Wilhelm Roentgen found that invisible rays were emitted when electrons bombarded the surface of certain materials  X-rays  Henri Becquerel accidentally discovered that phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates

 Marie Curie and her husband Pierre took Becquerel’s mineral sample, isolated the components emitting the rays, and concluded the darkening was due to rays emitted specifically from the uranium atoms present  Radioactivity - process by which materials give off such rays  Radiation – rays and particles emitted by a radioactive source  The Curies identified 2 new elements – polonium and radium

 Types of Radiation  Alpha (α) – same composition as a helium nucleus (2 protons and 2 neutrons)  Charge is +2  Symbol is 4 2 He  When a radioactive nucleus emits α, the product nucleus has an atomic number that is lower by 2 and a mass number that is lower by 4  Can be stopped by a piece of paper

 Beta (β) – very fast electron that has been emitted from a neutron of an unstable nucleus  Charge is -1  Symbol is 0 -1 β  When β is emitted, the mass number of the product is the same, but its atomic number has increased by 1  More penetrating than α; thin metal foil is required to stop them  Gamma (γ) – high-energy electromagnetic radiation  Symbol is 0 0 γ  Almost always accompany α and β radiation

 X-rays – high-energy electromagnetic radiation emitted from certain materials that are in an excited electron state

 Nuclear Stability  Nucleons – protons and neutrons  Strong nuclear force – overcomes the electrostatic repulsion between protons, allowing the nucleus to be densely packed with protons  Force that acts only on subatomic particles that are extremely close together  The stability of a nucleus is correlated w/ its neutron-to-proton ratio  Atoms w/ low atomic numbers desire a ratio of 1:1

 The number of neutrons required for a stable nucleus increases as the number of protons increases  Band of stability – area which all stable nuclei are found (p. 811)  Types of Radioactive Decay – the type of radioactive decay a radioisotope undergoes depends on the reason for its stability  Atoms above the band of stability generally have too many neutrons  Atoms below the band of stability generally have too many protons

 Beta decay – increases stability of a neutron-rich atom by lowering its neutron-to-proton ratio  Alpha decay – all nuclei w/ more than 83 protons are radioactive and decay spontaneously  Nuclei are heavy and often decay by emitting α particles  Positron emission – involves the emission of a positron from a nucleus  Positron – particle w/ the same mass as an electron but opposite charge

 Electron capture – nucleus of an atom draws in a surrounding electron; this electron combines w/ a proton to form a neutron  Table 25-3 (p. 812)

 Transmutation – conversion of an atom of one element to an atom of another element  Induced transmutation – process that involves striking nuclei w/ high-velocity charged particles  Uses very strong electrostatic and magnetic fields (atom smashers)  Transuranium elements – elements w/ atomic numbers 93 and greater  All are man-made and radioactive

 Radioactive decay rates  Half-life – time required for ½ of a radioisotope’s nuclei to decay into its products Amount remaining = (initial amount)(1/2) n Amount remaining = (initial amount )(1/2) n/T  Radiochemical dating – process of determining the age of an object by measuring the amount of a certain radioisotope remaining in that object  Carbon dating is commonly used to measure the age of artifacts that were once part of a living organism

 Exothermic reactions can be used to generate electricity  Fossil fuels  Nuclear  Nuclear Reactions and Energy  Energy and mass can be converted into each other E = mc 2 E = change in energy; m= change in mass; c = speed of light  Loss or gain in mass accompanies any reaction that produces or consumes energy

 Mass defect - mss of the nucleus is always less than the sum of the masses of the individual protons and neutrons that comprise it  Nuclear Fission – splitting of a nucleus into fragments  Nuclear power  Chain reaction – self-sustaining process in which one reaction initiates the next  Releases neutrons  Critical mass – sample that is massive enough to sustain a chain reaction  Can lead to a violent explosion

 Nuclear Reactors  Common fuel is uranium rods  Additional rods composed of cadmium or boron control the fission process by absorbing neutrons  Sensitive to control  Three-Mile Island, Chernobyl  A coolant (usually water) is used to cool the fission reactions  Approx. 20 ½-lives are required for the radioactivity of nuclear waste material to reach safe levels

 Nuclear Fusion – combining of atomic nuclei  Products are generally not radioactive  Requires extremely high energies  Confining the reaction is a big challenge