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N UCLEAR C HEMISTRY Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. C HEMICAL VS N UCLEAR R EACTIONS N UCLEAR S YMBOLS Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Mass Number Atomic Number A ZX Element Symbol D ECAY PARTICLES Alpha(α) Particle 2 protons and 2 neutrons Helium nucleus 4He 2 or 42a Gamma(γ) Particle Energy that accompanies a decay reaction 0 0 γ D ECAY PARTICLES Beta(β) Particles electron 0e or -10b 0e or +10b -1 Positron +1 O THER PARTICLES Proton 1p 1 or Neutron 1n 0 1H 1 B ALANCING N UCLEAR E QUATIONS 1. Conserve mass number (A). 2. Conserve atomic number (Z) or nuclear charge. P RACTICE P ROBLEM 212Po decays by alpha emission. 212Po + 84 212Po 84 4He 2 + 208Pb 82 In radioactive decay steps, the beginning radioactive isotope is called the parent and the product is called the daughter. D ECAY E XAMPLES Beta decay 14C 6 14N 7 +-10b + n 11B 5 ++10b + n Positron decay 11C 6 Electron capture decay 37 18 Ar + -10e 37Cl 17 +n Alpha decay 212Po 84 4He 2 + 208 82Pb N UCLEAR T RANSMUTATION Bombarding an element with particles to produce new elements 14N 7 27 13 Al + 24a + 24a 17O 8 + 11p 30P 15 + 01n Cyclotron Particle Accelerator 14N 7 + 11p 11C 6 + 42a U RANIUM D ECAY S ERIES n/p too large beta decay X Y n/p too small positron decay or electron capture N UCLEAR S TABILITY • Certain numbers of neutrons and protons are extra stable • n or p = 2, 8, 20, 50, 82 and 126 • Like extra stable numbers of electrons in noble gases (e- = 2, 10, 18, 36, 54 and 86) • Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons • All isotopes of the elements with atomic numbers higher than 83 are radioactive • All isotopes of Tc and Pm are radioactive H ALF L IFE H ALF -L IFE Time required for half a sample to decay The stability of the isotope is what determines the rate of decay. Less Stable = Faster Decay H ALF -L IFE After each half-life, half of the sample decays. Start = 100% 40 blue particles are present 1 half-life = 50% 20 blue remain 2 half-lives = 25% 10 blue remain 3 half-lives = 12.5% 5 blue remain 4 half-lives = 6.25% 2.5 blue remain Amount never becomes zero!! H ALF -L IFE After 10 half-lives sample considered nonradioactive because it approaches the level of background radiation. Because the amount never reaches zero, radioactive waste disposal and storage causes problems. Would you want radioactive waste stored in your community? How can we get rid of nuclear radioactive waste? H ALF L IFE P RACTICE Example 1: The half-life of mercury-195 is 31 hours. If you start with a sample of 5.00 g, how much of it will still be left after 93 hours? H ALF L IFE P RACTICE C ONT Example 2: How many half lives have passed if there is only 1.875 g left of a 30 g sample? If the half life for this sample is 1 hour, how many total hours have gone by? N UCLEAR S TABILITY N UCLEAR B INDING E NERGY Nuclear binding energy (BE): energy required to break up a nucleus into its component protons and neutrons. Einstein’s energy mass relationship: E = mc2 Links energy and mass calculations N UCLEAR BINDING ENERGY PER NUCLEON VS M ASS NUMBER nuclear binding energy nucleon nuclear stability N UCLEAR F USION Combining of small nuclei to form larger ones Occurs in stars at high temperatures Called a thermonuclear reaction N UCLEAR F ISSION Breaking down of a large massive atom into smaller ones Nuclear chain reaction: self-sustaining sequence of nuclear fission reactions. Ping Pong Video critical mass: minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction T WO T YPES Non-critical “controlled” OF F ISSION Super Critical C ONTROLLED N UCLEAR F ISSION Schematic diagram of a nuclear fission reactor G EIGER -M ÜLLER C OUNTER Used to measure level of Radioactivity