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Nuclear Chemistry Unit 4 History Wilhelm Conrad Roentgen (1845-1923) Awarded a Nobel Prize in Physics in 1901 Discovered X-Rays - November 8, 1895 Saw a glowing fluorescent screen on a nearby table and determined that the fluorescence was caused by invisible rays that were able to go through opaque black paper Died of unrelated causes at 77, one of few scientists to always use a lead shield History Antoine Henri Becquerel (1852-1908) Nobel Prize in Physics, 1903 Discovered radioactivity Proved that uranium emitted radiation without an external source of energy such as the sun Discovered that radiation emitted by uranium shared certain characteristics with X rays but could be deflected by a magnetic field and therefore must consist of charged particles History Pierre Curie (1859-1906) Nobel Prize in Physics, 1903 Worked with his wife Marie to investigate the phenomenon of radioactivity in uranium ore Died in an accident crossing the street in a rainstorm History Marie Curie (1867-1934) Nobel Prize in Physics, 1903 Nobel Prize in Chemistry, 1911 Discovered the elements Polonium (Po) and Radium (Ra) First person to win two Nobel Prizes Died of overexposure to radiation Transmutation Involves reactions where the nucleus of the atom is changed When the atom’s nucleus is changed, radiation is emitted Transmutation: The process when the nucleus changes atomic number and a new element is formed Radioactive or not? Not all elements are radioactive. Isotopes (also called nuclides) that are not radioactive are called stable isotopes Unstable isotopes: Isotopes that undergo nuclear reactions and emit radioactivity All elements above atomic number 83 are radioactive! Also known as radioisotopes or radionuclides Unstable Nucleus In the nucleus, there is a lot of + charge, so there should be an electrostatic force pushing all of the protons apart This doesn’t happen – why? There is a second force that acts on the protons when they are very close together. It is called the strong force Strong Force There is a “magic number” of protons and neutrons that keep the nuclei stable When there are an even number of protons and an even number of neutrons, the nucleus is very stable Strong Force When there are an even number of protons and an odd number of neutrons, the nucleus is less stable Strong Force When there are an odd number of protons and an odd number of neutrons, the nucleus is very unstable Stability Factors The ratio of neutrons to protons also contributes to stability Maximum stability: Smaller atoms = Ratio of 1 proton for every 1 neutron (A 1:1 ratio.) Larger atoms = More neutrons than protons (the extra mass keeps the atom stable) Increasing Atomic Number It makes sense that as one adds more protons, it would take more neutrons to help hold the nucleus together Remember - all elements above Bismuth (83) are radioactive! If the atomic number is less than 83, radioactivity will be determined by the number of protons and neutrons Radioactive Decay Process where an unstable nucleus emits particles and/or electromagnetic radiation We say that the nucleus has spontaneously disintegrated to produce a new element Transmutation occurs naturally Artificial Transmutation Also known as induced radioactivity A nucleus alters its identity by reacting with or capturing a neutron or another nucleus We say that we have changed or transmuted the nucleus by bombarding it with other particles Alpha Decay Radioactive decay where an alpha particle is emitted Alpha is the weakest type of radiation, with the least penetrating power A sheet of paper can block alpha particles Alpha particles are Helium particles Alpha Decay, Continued There is always a conservation of mass and charge Mass number 238 = 4 + 234 Atomic number 92 = 2 + 90 Beta Decay Radioactive decay where a beta particle is emitted Beta particles have more penetrating power than alpha. It would take a thin sheet of aluminum or your hand to block beta particles Beta particles are high speed electrons Beta Decay, Continued There is always a conservation of mass and charge Mass number 14 = 14 + 0 Atomic number 6 = 7 + (-1) Gamma Decay Radioactive decay where gamma radiation is emitted Gamma radiation has the greatest penetrating power Gamma rays are high energy photons Mass = 0, Charge = 0 Other Radiation Neutron Proton Mass of 1, Charge is neutral Mass of 1, Charge is positive Positron (Opposite of Beta) Mass of 0, Charge is positive Nuclear Symbols Relative Strength of Radiation Radiation Review Separation of Particles Use an electric field to separate a mixture of alpha, beta, and gamma radiation Alpha is +, so they are attracted to the - plate Beta is -, so they are attracted to the + plate Gamma is neutral, so it passes straight through Half Life The amount of time needed for one half of the nuclei of a substance to decay Any substance that is radioactive will disappear over time as it changes into other substances Half Life Example A radioactive substance has a half-life of 20 minutes. If we begin with a 500 g sample, how much of the original sample remains after two hours? Solution The easiest way to attack these questions is to start with the original amount of the sample, then draw arrows representing each half-life Two hours is 120 minutes, so that’s six half-lives: 500 250 125 62.5 31.25 15.625 7.8125 At the end of the stated time period, 7.8 g remains Half Life Nuclear Reactions Cause transmutation of elements with the release of a large amount of energy These reactions are the source of electric energy at nuclear power plants as well as the energy from the Sun and stars This immense amount of energy comes from the conversion of matter to energy Nuclear Reactions The mass of a nucleus is not exactly equal to the sum of the masses of its nucleons This difference in mass means that some nuclear mass is converted to energy Nuclear Fission Nuclear Fission When a neutron strikes a Uranium-235 nucleus it can cause the nucleus to break apart into smaller nuclei The fission reaction produces smaller nuclei as well as loose neutrons The loose neutron can strike the smaller nuclei, causing that nuclei to divide This is known as a nuclear chain reaction Nuclear Fission Nuclear Fission Splitting a nucleus into smaller pieces increases overall stability Fission = division (of the nucleus) Nuclear fission is used as a source of electricity in nuclear power plants The most common fission reaction is the fission of Uranium-235 Nuclear Power When the chain reaction is controlled, the energy can be captured and converted into electricity Nuclear Power Advantages: Not as much fuel needed No pollutants or greenhouse gases released Disadvantages: Waste material is extremely radioactive, and stays that way for thousands of years They need fuel to operate Nuclear Fusion Nuclei of smaller atoms join together to form a larger atom Converts matter into large amounts of energy Nuclear Power Fission Animation Animations Fusion Animation Vision Learning