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The Electromagnetic Spectrum 7.1 Atomic Theory and Radioactive Decay Natural background radiation exists all around us. Radioactivity is the release of high energy particles or waves that we call radiation – When atoms lose high energy particles and waves, new atoms can be formed. – Radiation can be good or bad: – Early discoveries of radiation relied on photographic equipment Marie Curie and her husband Pierre named the energy radioactivity X-rays, radiation therapy and electricity generation are beneficial. High energy particles and waves can do damage to DNA in our cells. Isotopes 39 19 K, 40 19 K, 41 19 Radium salts, after being placed on a photographic plate, leave behind the dark traces of radiation. Representing Isotopes K are atoms of the same element, with a different number of neutrons. – changing the # of neutrons changes the mass number Remember: mass # = # protons + # neutrons – isotopes still have the same number of protons and the same element symbol Isotopes are written two ways – with the mass number at the end – With its chemical symbol Mass number Atomic number 39 19 K, Ex. Potassium – 40 Ex. 40 K 19 40 19 K, 41 19 K Atomic Mass (the decimal #’s) Atomic mass = average of the mass numbers for all isotopes of an element. Radioactive Decay Can 19 19 19 20 21 22 19 19 19 Uranium goes through many decay steps before it becomes stable: result in new atoms forming. – Radioactivity results from having an unstable nucleus. – Radioactive decay = when nuclei break apart + release energy from the nucleus. Radioactive decay continues until a stable element forms. An element may have isotopes that are radioactive called radioisotopes – Ex. carbon-12, carbon-13 and carbon-14 (only C-14 is radioactive) 1 Rutherford identified three types of radiation using an electric field. Alpha Radiation: is a stream of alpha particles, (shown as ) – positively charged – weighs the most – are the same as a helium nucleus – Alpha particles are represented by the symbols – Positive alpha particles were attracted to the negative plate. – Negative beta particles were attracted to the positive plate. – Neutral gamma particles did not move towards any plate. 4 2 or 42 He 2 protons and 2 neutrons make a mass number of 4 it has a charge of 2+ because of the protons Beta Radiation: Alpha particles are big and slow. A sheet of paper will stop an alpha particle. A Example: the alpha decay of Radium - 226 Beta particle, , is a high energy electron. – negatively charged, and weigh less than alpha particles. – Beta particles are represented by the symbols 0 -1 226 88 Ra 222 86 Rn + 24 or 226 88 Ra 222 86 Beta decay occurs when a neutron changes into a proton + an electron. 131 53 I 131 54 Xe + 0 –1 or 131 53 I 131 54 Xe + 0 –1 It takes a thin sheet of aluminum foil to stop a beta particle. e e Gamma Radiation: Example: The beta decay of iodine - 131 0 -1 electrons are very tiny, so beta particles are assigned a mass of 0. one electron gives a beta particle a charge of 1–. Rn + 24 He – The proton stays in the nucleus, and the electron is released. or Gamma radiation, , is a ray of high energy, short-wavelength radiation. – has no charge and no mass. – is the highest energy form of electromagnetic radiation. – It takes thick blocks of lead or concrete to stop gamma rays. – Gamma decay results from energy being released from a high-energy nucleus. 60 28 Ni* 60 28 Ni + 00 Shows unstable nucleus for gamma decay 2 Nuclear Equations: Often, other kinds of radioactive decay will also release gamma radiation. 238 92 U 234 90 Th + 42 He + 2 – Uranium-238 decays into an alpha particle and also releases gamma rays. are written like chemical equations, but represent changes in the nucleus of atoms. – Chemical equations represent changes in the position of atoms, not changes to the atoms themselves. Remember: Summary Tables 1.The sum of the mass numbers on each side of the equation should equal. 2.The sum of the charges on each side of the equation should equal. Take the Section 7.1 Quiz 3