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Nuclear Chemistry. History of Nuclear Chemistry. - PowerPoint PPT Presentation


  • Nuclear Chemistry

  • History of Nuclear ChemistryHenri Becquerel in 1896 he found that uranium ore gave off invisible radiation. This was an accidental discovery using photographic film wrapped in light proof paper with a piece of fluorescent uranium salt. The film showed an image proving that something given off by the salt, rather than light, caused the image. Marie Curie and her husband, in 1898, discovered the new radioactive elements they named polonium and radium.

  • Becquerel and the Curies were awarded the 1903 Physics Nobel Prize Curie died as a result of complications due to exposure to radiation

  • Nuclear ChemistryUnderstanding Radioactivity and Radioactive DecayFiguring Out Half-LivesTracing the Effects of RadiationThe Basics of Nuclear FissionLooking at Nuclear Fusion

  • It all starts with the AtomNuclear Chemistry refers to changes that occur in the dense central core of the atom containing protons (+) and neutrons (0)Atoms of the same element with different numbers of neutrons are called isotopes. Many elements have several isotopic forms

  • Nuclide name given to the unstable nucleus of a radioactive atomHow many protons and neutrons are found in an atom of uranium-239?UAtomic number = protons There are 92 protons Mass number = sum of protons and neutrons239 92 = 147There are 147 neutrons

  • Radioactivity and Radioactive Decay Radioactivity is the spontaneous decay of an unstable nucleusAn unstable nucleus may break apart into two or more other particles with the release of energyTransmutation is the change which occurs when one element is changed into another as a result of an alteration in the nucleus (number of protons changes)

  • You can predict particles of radioactive decay by balancing nuclear reactionsReactions are represented as follows:Reactants ProductsReactants are substances you start withProducts are substances being formedThe reaction arrow indicates that a reaction has taken place

  • 35Cl + 1n X? + 1H

    170For a nuclear reaction to be balanced, the sum of all the atomic numbers on the left side of the reaction arrow must equal the sum of all the atomic numbers on the right side of the arrow. The same is true for the mass numbers. In the above example, an isotope of Chlorine (Cl-35) is bombarded by a neutron. You observe that an isotope of hydrogen (H-1) is created along with another isotope that you need to identify.1

  • To figure out the unknown isotope (X) you need to balance the equation. The sum of the atomic numbers on the left is 17 (17 + 0), so you want the sum of atomic numbers on the right to be 17 also. You already have an atomic number of 1 on the right, so the atomic number of the unknown is 17-1 = 16.35Cl + 1n ?X + 1H 17 0 16 135Cl + 1n X? + 1H 170116

  • Now look at the mass numbers in the equation. The sum of the mass numbers on the left is 36 (35 + 1), and you want the sum of the mass numbers on the right to equal 36, too. Right now there is a mass number of 1 on the right; 36 1 = 35, so thats the mass number of the unknown isotope. You can now determine that the unknown isotope is sulfur (atomic #16), and the balanced equation looks like this:35Cl + 1n 35S + 1H 17 0 16 1

    35Cl + 1n ?X + 1H 170116

  • TransmutationThis example represents a nuclear transmutation, the conversion of one element into another. Nuclear transmutation is a process human beings control. S-35 is an isotope that doesnt exist in nature it is a manmade isotope. Alchemy age old attempt to turn common metals into gold

  • Natural Radioactive DecayCertain isotopes are unstable. Their nucleus breaks apart, undergoing nuclear decay. The nucleus has all the positively charged protons tightly packed together in an extremely small space with all the protons repelling each other. All elements with 84 or more protons are unstable.Other isotopes with fewer than 84 protons can be radioactive if the neutron / proton ratio is too high. If the isotope is neutron rich it is unstable. Thats why some isotopes of an element are stable and others are radioactive.

  • Naturally occurring radioactive isotopes can decay in the following ways:Alpha particle emission - Beta particle emission - Gamma radiation emission -

    Less common types of radioactive decay:Positron emissionElectron capture

  • Alpha EmissionAn alpha particle is the positive nucleus of a He atom, and is represented as 4He. It is actually a He2+ ion Helium ion with no electrons.Normally an alpha particle given off picks up two electrons quickly and becomes a neutral helium atom. (electrons are easy to pick up or lose)Radon-222 is an example of an isotope that undergoes alpha emission as follows:222Rn 218Po + 4He286842alpha particle

  • Beta EmissionIsotopes with high neutron / proton ratio undergo beta emission.In beta emission, a neutron decays into a proton and an electron.The atomic number increases by 1, and a beta particle is given off.Iodine-131 which is used in the detection and treatment of thyroid cancer, is a beta particle emitter:131I 131Xe + 0e53 54-1beta particle

  • Gamma EmissionAlpha and beta particles have the characteristics of matter. They have definite masses and volumes. Because there is no mass change associated with gamma emission, it is referred to as gamma radiation emission, and is similar to x-rays (high energy, short wavelength electromagnetic radiation).Gamma radiation emission often accompanies both alpha and beta emission, but is not usually shown in a balanced nuclear reaction.Some isotopes give off large amounts of gamma radiation such as Cobalt-60, which is used in the radiation treatment of cancer. Gamma rays are focused on the tumor, destroying cells.

  • A problem with gamma radiation:

  • Positron EmissionPositron emission occurs with some manmade radioactive isotopes.A positron is an electron with a positive charge.When a proton decays a neutron and a positron are formed and the positron is emitted from the nucleus.Potassium-40 is an example of a positron emitter:40K 40Ar + 0e1918 +1positron

  • AntimatterDo you watch Star Trek?A positron is a tiny bit of antimatter.When a positron comes in contact with an electron, both particles are destroyed with the release of energy.Fortunately, not many positrons are produced, so we dont have to go around dodging explosions.

  • Electron CaptureElectron capture is a rare type of nuclear decay in which an electron from the innermost energy level (1s) is captured by the nucleus. This electron combines with a proton becoming a neutron.The following equation shows the electron capture of Polonium-204:204Po + 0e 204Bi + x-rays84-183With the 1s orbital vacant, electrons drop down releasing energy x-rays

  • Half-Lives and Radioactive DatingIt takes a certain amount of time for half the atoms in a sample of radioactive material to decay. It then takes the same amount of time for half the remaining radioactive atoms to decay, and the same amount of time for half of those remaining radioactive atoms to decay, and so onThe amount of time it takes for one half of a sample to decay is called the half life of the isotope and is given the symbol: t The decay of a radioactive isotope is exponential

  • Radioactive Decay

  • Carbon-14 Radioactive Decay

  • Half-Lives of some Radioactive Isotopes

    RadioisotopeRadiation EmittedHalf - LifeKr-94Beta1.4 secondsRn-222Alpha3.8 daysI-131Beta8 daysCo-60Gamma5.2 yearsH-3Beta12.3 yearsC-14Beta5,730 yearsU-235Alpha4.5 billion yearsRe-187Beta70 billion years

  • Radioactive DatingCarbon-14 is absorbed by plants from carbon dioxide in the atmosphere and enters the food chain.As long as an organism is alive, the amount of C-14 remains constant.C-14 begins to decrease at a predictable rate when an organism dies. (t of C-14 is 5,730 years) Radioactive dating using C-14 has been used to determine the age of skeletons found at archeological sites. For nonliving substances, scientists use other isotopes, such as potassium-40.

  • Safe HandlingKnowing about half-lives allows scientists to know when a sample of a radioactive material is safe to handle. The rule is that a sample is safe when its radioactivity has dropped below detection limits. This usually occurs at 10 half lives.If radioactive iodine-131 (t = 8 days) is injected into the body to treat thyroid cancer, it will be gone in 10 half lives or 80 days.

  • Measuring RadiationCurie (Ci) used to measure radioactivity1 Ci = 3.7 X 1010 disintegrations per secondRadiation Absorbed Dose (rad) the amount of energy absorbed per gramRadiation Equivalent for Man (rem) used to estimate radiation effects on the body

  • Detecting RadiationGeiger Counter gas filled metal tube that when exposed to radiation causes a current to flow creating an audible click from a built in speaker.Scintillation Counter uses a specially coated phosphorous surface to detect radiation. Radiation striking the surface causes bright flashes of light, or scintillations.Film badge worn by people who work near radiation sources. It is several layers of photographic film covered with black, light proof paper. Radiation exposure will cause a darkening of the film.