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Science 10 – Notes Radioactivity Radioactivity and Its History • Radiation is everywhere, but can be difficult to detect. o Discovery of cathode ray tube. o Roentgen named X-rays with an “X” 100 years ago because they were previously unknown. o Becquerel placed uranium on photographic plate and saw that it exposed the plate . o Marie Curie and her husband Pierre named this energy radioactivity. Radioactivity and the Nucleus Rutherford Experiment (Gold foil experiment) o He discovered that the nucleus of an atom is both Small Dense Isotopes are different atoms of the same element, with the difference between the two atoms being the number of neutrons in the nucleus. o Eg. Carbon-12, Carbon-13, Carbon-14 o By having different numbers of neutrons, isotopes have different mass numbers. Isotopes of an element have the same symbol and same atomic number Atomic mass = proportional average of the mass numbers for all isotopes of an element. 19.9% of boron atoms have 5 neutrons, 80.1% have 6 neutrons 19.9% have a mass number of 10, and 80.1% have a mass number of 11 (.199 * 10) + (.801*11) = 10.8 = atomic mass of boron Representing Isotopes Isotopes are written using the following rule o Chemical symbol + atomic number + mass number. o Potassium has three isotopes, 39 40 41 19 19 19 Potassium is found in nature in a certain ratio of isotopes o 93.2% is potassium-39, 1.0% is potassium-40, and 6.7% is potassium-41 o Atomic mass = (.932 x 39) + (.001 x 40) + (.067 x 41) = 39.1 K, K, K Read pgs 275-282 Do 6, 10 pg 279 and do 3,9,11,13 pg 283 Day 2 Radioactive Decay: pgs 284-289 o Unlike all previously discovered chemical reactions, radioactivity sometimes resulted in the formation of completely new atoms. o o When these nuclei lose energy and break apart, decay occurs. Radioactive atoms release energy until they become stable, often as different atoms. These are called radioisotopes Three Types of Radiation o Positive alpha particles were attracted to the negative plate. o Negative beta particles were attracted the the positive plate o Neutral gamma particles did not move towards any plate. Alpha Radiation o Alpha radiation is a stream of alpha particles, . o Composed of 2 protons and 2 neutrons (no electrons) o Alpha particles are essentially the same as a helium atom’s nucleus. o 4 2 or 42 He o Because it has two protons but no electrons, it has a charge of 2+. o Alpha particles are slow and penetrate materials much less than the other forms of radiation. A sheet of paper will stop an alpha particle. 226 88 Ra 222 88 Rn + 42 or 226 88 Ra 222 88 Rn + 42 He Beta Radiation o Beta radiation, , is an electron. o Beta particles are represented by the symbols: 131 53 I 131 54 Xe + 0 –1 or 131 53 I 131 54 Xe + 0 –1 e o Electrons are very tiny, so beta particles are assigned a mass of 0. o Beta decay occurs when a neutron changes into a proton + an electron. o Beta radiation penetrates better than alpha particles o It takes a thin sheet of aluminum foil to stop a beta particle. Gamma Radiation o Gamma radiation, , is a ray of high energy, shortwavelength radiation. 0 o Gamma radiation has no charge and 0 no mass, o Gamma radiation is the highest energy form of electromagnetic radiation. It takes thick blocks of lead or concrete to stop gamma rays. o Gamma decay results from energy being released from a high-energy nucleus. 238 92 o o 60 28 U Ni* Read pgs 284-288 Do 1-4, 6 pg 289 234 90 Th + 42 He + 2 60 28 Ni + 00 Radiation and Radioactive Decay Summaries, and Nuclear equations for radioactive decay o Nuclear equations are written like chemical equations, but represent changes in the nucleus of atoms. o o o 1. The sum of the mass numbers should equal.