Radioactive Decay Radioactivity results from having an unstable nucleus. When these nuclei lose energy and break apart, decay occurs. Radioactive decay releases energy from the nucleus as radiation. Radioactive atoms release energy until they become stable, often as different atoms. An element may have only certain isotopes that are radioactive. (C-12 & C-13 are stable, C-14 is unstable) These are called radioisotopes By emitting radiation, atom of one kind of element can change into atoms of another element Radioactivity = release of high energy particles & rays of energy from a substance as a result of changes in the nuclei of its atoms Radiation = high energy rays and particles emitted by radioactive sources Unstable = likely to decay - goes from unstable radioactive to stable non radioactive Unstable atoms gain stability by losing energy, and lose energy by radiation Radioisotopes – isotopes capable of radioactive decay Most carbon in body C-12  1 carbon atom in 1 trillion in your body is unstable & can release radiation  still huge number of C-14 atoms – C-14 atoms decay without being replaced when you die Th – thorium Pa – protactinium Ra - Radon Radioisotope uranium-238 decays in several stages until it finally becomes lead-206 See page 293

Three Types of Radiation Rutherford identified three types of radiation using an electric field. 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. Rutherford later discovered the nucleus & created model of atom Rutherford placed a radioactive source inside a lead block  allowed radiation to pass thru tiny hole Then, radiation travelled thru slot between electrically charged plates that deflected electrically charged particles +ive charged particles deflected toward negative plate  called alpha particles -ive charged particles deflected toward positive plate  called beta particles Gamma  no electric charge, passed through electric field unaffected See page 294

Alpha Radiation R a ® n + o r H e Alpha radiation is a stream of alpha particles, . Positively charged, and are the most massive of the radiation types. Alpha particles are essentially the same as a helium nucleus. Alpha particles are represented by the symbols . Because it has two protons, it has a charge of 2+. The release of alpha particles is called alpha decay. Alpha particles are slow and penetrate materials much less than the other forms of radiation. A sheet of paper will stop an alpha particle. Alpha particle – mass of 4 & atomic # of 2  2 protons and 2 neutrons Radioactive decay – unstable nuclei lose energy by emitting radiation – release of alpha particle from nucleus alpha decay Radium had mass of 226 and loses alpha particle so loses 2 protons, loses 2 neutrons – turns into radon-222 8 2 6 R a ® n + 4 o r H e Radium-226 releases an alpha particle and becomes Radon-222. Radon has two less protons than radium. See page 294 - 295

Beta Radiation Beta radiation, , is an electron (from the nucleus.) Negatively charged, and are fast moving. Beta particles are represented by the symbols . Electrons are very tiny, so beta particles are assigned a mass of 0. Since there is only an electron, a beta particle has a charge of 1–. Beta decay occurs when a neutron changes into a proton + an electron. The proton stays in the nucleus, and the electron is released. It takes a thin sheet of aluminum foil to stop a beta particle. During beta decay  proton stays in nucleus while electron shoots out so the atomic # increases by one – it has become the next higher element on periodic table Ex. Iodine (at # 53) changes to xenon (at #54) Iodine-131 releases a beta particle and becomes Xenon-131. A neutron has turned into a proton + the released electron. See page 296

Gamma Radiation Gamma radiation, , is a ray of high energy, short-wavelength radiation. Gamma radiation has no charge and no mass, . Gamma radiation 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. Often, other kinds of radioactive decay will also release gamma radiation. Uranium-238 decays into an alpha particle and also releases gamma rays. Gamma radiation does not change atomic # or mass # of a nucleus Gamma decay – redistribution of energy within the nucleus The * means extra energy (extra energy is released as a gamma ray)  high energy gamma ray is given off as isotope falls from high energy to low energy state ex. Nickel Th  Thorium See page 297

Radiation Summary Some isotopes release alpha, beta, and gamma radiation all at once

Nuclear equations for radioactive decay Nuclear equations are written like chemical equations, but represent changes in the nucleus of atoms. The sum of the mass numbers should equal. The sum of the atomic numbers should equal. 230 = 4 + ____ 226 90 = 2 + ____ 88 230 4 226 α 2 4 Th He Ra 90 2 88 137 137 Reactans on the left – products on the right Arrow means “produces or changes to” Th – Thorium Cs – Cesium, Ba – Barium, Lr – Lawrencium, Mendelevium β -1 Cs Ba e 55 56 -1 256 252 4 α 2 4 Lr Md He 103 101 2

Radioactive Decay Summary