* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Document related concepts
Radioactivity Dalton’s Atomic Theory (1805) • All elements are composed of atoms, which are indestructible and indivisible. • All atoms of the same element are exactly alike – they have the same mass. • Atoms of different elements are different – they have different masses. • Compounds are formed by the joining of atoms of two or more elements. In any compound, the atoms of the elements are joined in a definite whole number ratio (1:1, 1:2, 3:2, etc) The “Smoking Gun:” Becquerel’s Photographic Plates (1896) 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. U-238 Decay Series 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. • 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. Charged Particles are deflected in electric/magnetic fields • Because of their positive charge, alpha particles are easily detected in electric and magnetic fields. Penetrating Power of Radiation Alpha Decay Equations Nuclide 215At 217Rn 218Fr 228U 236Pu 241Am 217Po +1 1p 1 o n Decay Equation Beta Decay • Protons and neutrons have energy states, just like electrons • Like electrons, nucleons tend to be more stable in pairs • The ratio of protons and neutrons is also important • If a nucleus has too may neutrons, a neutron can eject an electron, becoming a proton. 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 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 92 234 90 U Th He 230 90 4 2 Th Pa 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 The Zone of Stability For a given element, there is an optimal ratio of protons to neutrons ~1:1 ratios common with lighter elements Heavier elements: 1: 1.5 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 indicated 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?