Chapter 17 Radioactivity and Nuclear Chemistry Problems 1 20 24 30 32 46 49 70 74 88 99 101 17 2 THE DISCOVERY OF RADIOACTIVITY In 1896 a French physicist named Henri Becquerel discovered that uranium containing crystals emitted rays that could expose and fog photographic plates He found that these rays originated from changes within the atomic nuclei of the U atoms He proposed that the uranium atoms were unstable They emitted particles and or energy to become more stable These emissions he called uranic rays One of the first women in France to do graduate work Marie Sklodowska Curie carried out her doctoral work to determine if other substances emitted uranic rays She discovered two elements which she later named polonium after her native Poland and radium which emitted high levels of radioactivity Maric Curie changed the name of uranic rays to radioactivity or radioactive decay She shared the Nobel prize in physics with Becquerel and her husband Pierre Curie for discovering radioactivity Except for 11H every atom has protons and neutrons in its nucleus Elements may exist as different isotopes e g the three isotopes of hydrogen are 11H hydrogen 21H deuterium and 31H tritium which differ only in the number of neutrons indicated by the mass number atomic number Some isotopes are stable e g 11H and 21H Some isotopes are unstable and radioactive e g 31H The unstable radioactive isotopes are called radionuclides Atomic Notation or Nuclear Symbol shorthand for keeping track of protons and neutrons in the nucleus of protons of neutrons mass number A of protons atomic number Z Atomic notation of common particles CHEM 121 Chapter 17 electron E element symbol e 0 1 page 1 of 12 proton 11p neutron 01n 17 3 TYPES OF RADIOACTIVITY ALPHA BETA AND GAMMA DECAY alpha particle a helium nucleus 42 or 42He without electrons positively charged largest particle emitted by radioactive nuclei has the highest charge beta particle a beta particle 10 or 0 1 e is an electron emitted from an atomic nucleus positron the antiparticle of an electron beta particle 0 1 or 01 e The same size as an electron but with a positive charge gamma rays high energy rays like X rays The Electromagnetic Spectrum shows the different forms of electromagnetic radiation with cosmic and gamma rays having the highest frequency and the highest energy making them potentially the most dangerous to humans Different Types of Radioactivity Alpha emission a helium nucleus 42 or 238 92U 238 92 CHEM 121 Chapter 17 U 24 4 2He page 2 of 12 is emitted 234 90Th 234 90 Th 4 2 0 1 Beta emission a beta particle or electron 234 90Th 0 1 or 0 0 e is emitted 234 91Pa Gamma emission high energy rays like X rays 99m 43Tc 0 1 are emitted 0 0 99 43Tc Positron emission a positron the antiparticle of an electron 39 K 19 0e 1 0 1 or 01 e is emitted 39 Ar 18 WRITING NUCLEAR EQUATIONS nuclide a specific atom with a given number of protons and neutrons We use the term parent daughter nuclides to describe a parent nuclide decaying to produce a daughter nuclide Balancing Nuclear Equations Differ from general chemical equations in that mass numbers protons neutrons and atomic numbers are balanced not the elements or atoms present parent 238 92U For example 4 2 daughter 234 90Th where the mass numbers are equal to 238 and the atomic numbers are equal to 92 Ex 1 Complete the following nuclear equations by identifying the unknown a 20 8O b 26 12Mg c 9 4Be 20 9F 1 1p 24 4 2 12 6C Ex 2 Write complete nuclear equations for the following processes a Iron 59 decays by beta emission CHEM 121 Chapter 17 page 3 of 12 17 5 b Ra 226 decays by alpha emission c Mn 50 decays by positron emission d Cs 118 is produced when a radionuclide decays by beta emission NATURAL RADIOACTIVITY AND HALF LIFE When a radioactive sample decays it emits particles and or energy and continues to decay When Becquerel studied radioactivity he learned that unlike chemical reactions the rate of the radioactive decay cannot be changed by changing temperature pressure or any other factors Thus a radioactive sample will always continue to decay The amount of radiation given off is called the activity of the sample and can be measured using a Geiger counter This instrument developed by Hans Geiger measures emissions from a radioactive sample as clicks When more of the radioactive sample is present the activity is higher As the sample decays less of the radioactive sample remains so the activity decreases CHEM 121 Chapter 17 page 4 of 12 Half Life t1 2 the amount of time required for the amount or activity of a radioactive sample to decrease by half Note with each half life passing only half of the previous amount of sample is left Example Rn 222 a radioactive isotope is one of the resulting daughter products in the U 238 decay series The daughter product when Rn 222 decays is Po 218 a If 96 0 mg of Rn 222 t1 2 4 days is inhaled but not exhaled or otherwise eliminated what mass of it remains in the lungs after 12 days 222 Rn 86 24 218 Po 84 b Po 218 decays to form lead 214 If 84 0 mg of Po 218 t1 2 3 1 minutes formed in the lungs from the decay of Rn 222 what mass of it remains in the lungs after 9 3 minutes 218 Po 84 24 214 Pb 82 c Lead 214 undergoes beta decay to form Bi 214 If 10 2 mg of lead 214 t1 2 27 minutes formed in the lungs from the decay of Po 218 what mass remains after 54 minutes 214 Pb 82 CHEM 121 Chapter 17 0 1e page 5 of 12 214 Bi 83 17 10 THE EFFECTS OF RADIATION ON LIFE Penetrating Effects of Radiation Particles with the same energy but different masses can penetrate to different degrees Alpha particles are massive with a high charge They collide with other molecules and quickly lose energy They cannot penetrate a few sheets of paper or outer cells of skin But exposure to alpha particles can cause severe burns Beta particles penetrate more but can be stopped with a sheet of aluminum or plastic Gamma and X rays can penetrate even more but are stopped with a thick lead shield can penetrate much deeper and are very dangerous to living organisms Neutrons can penetrate even more but can be stopped by a thick wall of concrete ACUTE RADIATION DAMAGE AND GENETIC DEFECTS Chemical Effects of Radiation Why is radiation so dangerous Acute radiation damage results from exposure to large amounts of radiation in a short period of time which kills a large numbers of cells Weakened immune systems High energy or ionizing radiation can take electrons from stable compounds in living organisms leaving