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Radioactivity Radioisotopes • The discovery of radioactivity by Becquerel and the Curies showed that one of Dalton’s ideas, that matter is indestructible and indivisible, is not always true. • Certain isotopes, because of their size and/or ratio of protons and neutrons are not stable. • Radioisotopes have unstable, high energy nuclei • Radioisotopes lose energy by emitting radiation and matter Radioactive Decay • Radioisotopes decay from one element to another until they are transformed into stable, non-radioactive isotopes. • For example, 238U decays 11 times, shedding mass and energy each time, eventually becoming 206Pb, a stable isotope. • Radioactive decay is spontaneous – it does not require an input of energy. • Another term for radioactive decay is transmutation. Three Types of Radiation • alpha radiation • beta radiation • gamma radiation α β- , β+ ϒ Alpha Radation • In alpha radiation, helium nuclei are ejected from the nucleus. • The process studied by Curie involved uranium-238, among other radioisotopes: 238 92 U Th He radioactive_ decay 234 90 4 2 In alpha decay, the atomic number decreases by 2, the mass number decreases by 4. 238 92 U Th He radioactive_ decay 234 90 4 2 • The helium nucleus is known as an alpha particle. This process is known as emission. In this case, an atom of uranium changes into an atom of thorium. • The general process of atoms changing into different elements is known as transmutation. • These alpha particles are less dangerous than other forms of radiation. • Their large mass insures that they do not penetrate very far into materials. • This is a good thing because they steal two electrons from other atoms they encounter. • Because of their positive charge, alpha particles are easily detected in electric and magnetic fields. Penetrating Power of Radiation Beta Radiation • Beta radiation is composed of high energy electrons, known as beta particles. A neutron breaks down to form a proton and an beta particle (): 1 0 n H e 1 1 o 1 Alpha Decay Equations Nuclide 215At 217Rn 218Fr 228U 236Pu 241Am 217Po +1 1p o 1n Decay Equation Beta Particles and Neutrinos • The v particle is a neutrino, a very tiny particle of negligible mass. Note that this equation can also be written 1 0 • Table O: n H 1 1 o 1 1 0 n H 1 1 o 1 • Neutrinos are virtually undetectable and are often left out of nuclear equations. • The proton remains in the nucleus and is not ejected. Thus in beta decay, the atomic number increases by one! The most famous example is carbon-14. 14 6 C N e 14 7 0 1 14 6 C N e 14 7 0 1 • Carbon-14 is radioactive. By decaying to nitrogen-14, it attains stability. • Beta particles are less massive, and therefore more penetrating. They represent a greater health risk than alpha particles. Beta Minus Decay Equations Nuclide 16C 19O 25Na 10Be 66Cu 71Zn 213Bi +1 1p o 1n Decay Equation Positron Emission • There is another interesting form of beta emission. • A proton will transform itself into a neutron, releasing a positron, a positively charged electron-like particle. • Positrons are a form of antimatter. 1 1 p n 1 0 0 1 Positron Emission • Artificially created radioactive nuclei of the lighter elements often undergo positron emission: 30 15 P Si 30 14 0 1 • Many artificial radionuclides are have too many protons, so a proton changes into a neutron. • In positron emission, the atomic number goes down by 1, and the mass number remains unchanged. Positron Emission Equations Nuclide 10C 14O 22Na 7Be 58Cu 63Zn 203Bi +1 1p o 1n Decay Equation Gamma Radiation • Gamma rays are high frequency radiation – they have no mass or charge. • Gamma emission does not change the atomic number or mass number of the atom, and often accompanies or emission Gamma Radiation Often Accompanies Other Forms of Decay 234 92 U Th He 230 90 4 2 Th Pa 234 90 234 91 0 1 • Gamma rays are extremely dangerous and will pass completely through the body, damaging cells as they go. • Several cm of lead is necessary to contain gamma rays. Which type of emission has the highest penetrating power? • • • • alpha beta positron gamma Natural Transmutation • Alpha decay: Mass number ↓4, atomic number ↓2 • Beta minus decay: mass number unchanged, atomic number↑1 • Beta plus decay: mass number unchanged, atomic number ↓1 Deflection by Magnetic/Electric Fields The diagram represents radiation passing through an electric field. • Which type of emanation is represented by the arrow labeled 2? • alpha particle • beta particle • positron • gamma radiation • Which type of radiation would be attracted to the positive electrode in an electric field? Which type of radiation has neither mass nor charge? • • • • gamma neutron alpha beta Which radioactive emanations have a charge of 2+? • • • • alpha particles beta particles gamma rays neutrons Famous Radioisotopes • The type of decay is indicicated by the decay mode. • Homework: Write decay equations for each nuclide on Table N Which radioisotope is a beta emitter? • • • • 90Sr 220Fr 37K 238U Which isotope will spontaneously decay and emit particles with a charge of +2? • • • • 53Fe 137Cs 198Au 220Fr Positrons are spontaneously emitted from the nuclei of • • • • potassium-37 radium-226 nitrogen-16 thorium-232 In the equation: • Which particle is represented by X? • The fossilized remains of a plant were found at a construction site. The fossilized remains contain the amount of carbon-14 that is present in a living plant. • Which answer choice correctly completes the nuclear equation for the decay of C-14?