7.1 Atomic Theory and Radioactive Decay Natural background radiation exists all around us. Radioactivity is the release of high energy particles or waves When atoms lose high energy particles and waves, new atoms can be formed. Radiation is useful but it damages DNA in cells: X-rays, radiation therapy and electricity generation The Electromagnetic Spectrum Early discoveries of radiation relied on photographic equipment Marie Curie and her husband Pierre named the energy radioactivity Radium salts, after being placed on a photographic plate, leave behind the dark traces of radiation. Isotopes 39 40 41 19K, 19K, 19K Representing Isotopes are atoms of the same element, with a different number of neutrons in the nucleus. 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 using standard atomic notation Ex. 40 19K Mass number Atomic number 39 40 41 19 K, 19 K, 19 K Atomic Mass (the decimal # s) Atomic mass = average of the mass numbers for all isotopes of an element. 19 19 19 20 21 22 19 19 19 1
Radioactive Decay Uranium goes through many decay steps before it becomes stable: Can 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) 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 radiation did not move towards any plate. Alpha Radiation: is a stream of alpha particles, (shown as ) positively charged weighs the most the same as a helium nucleus Alpha particles are represented by the symbols or He 4 4 2 2 2 protons and 2 neutrons make a mass number of 4 it has a charge of 2+ because of the protons 2
Alpha particles are big and slow. A sheet of paper will stop an alpha particle. Example: the alpha decay of Radium - 226 Ra Rn + or Ra Rn + He 226 222 4 226 222 4 88 86 2 88 86 2 Beta Radiation: A Beta particle,, is a high energy electron. negatively charged, and weigh less than alpha particles. Beta particles are represented by the symbols or e 0 0-1 -1 electrons are very tiny, so beta particles are assigned a mass of 0. one electron gives 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. Example: The beta decay of iodine - 131 I Xe + 131 131 0 53 54 1 or I Xe + 131 131 0 53 54 1 It takes a thin sheet of aluminum foil to stop a beta particle. e Gamma Radiation: Gamma radiation,, is a ray of high energy, shortwavelength 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. Ni * Ni + 60 60 0 28 28 0 Shows unstable nucleus for gamma decay 3
Often, other kinds of radioactive decay will also release gamma radiation. U Th + He + 2 238 234 4 92 90 2 Uranium-238 decays into an alpha particle and also releases gamma rays. Nuclear Equations: 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: 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. 7.2 Half-Life Summary Tables the time it takes for half of a radioactive sample to decay is a constant rate (always the same half life for each element) Example: Strontium-90 has a half-life of 29 years. If you have 10 g of strontium-90 today, there will be 5 g (half) remaining in 29 years. Terminology: Parent isotope: the original radioactive material Daughter isotope: the stable product that remains after decay has happened 4
Decay Curves: Show the rate of decay for radioactive elements show the relationship between half-life and percentage of original substance remaining. Radioactive Dating: a method to determine age of objects compares amount of parent isotope to daughter isotope. Example: Carbon dating measure the ratio of carbon-12 and carbon-14. Stable carbon-12 and radioactive carbon-14 exist naturally in a constant ratio. In nature, carbon-12 appears 98.9% of the time, while carbon-14 is very rare. There are many radioisotopes that can be used for dating When an organism dies, carbon-14 stops being taken in and continues to slowly decay. Comparing the amounts of carbon-12 to carbon-14 is called radiocarbon dating and gives us an age for the object. Radiocarbon dating only works for organisms less than 50,000 years old because the half-life of carbon-14 is 5730 years. (after 50,000 years, there isn t enough C-14 left!) 5
The Potassium-40 Clock some elements require one step to decay, while others decay over many steps before reaching a stable daughter isotope. Carbon-14 decays into nitrogen-14 in one step Uranium-235 decays into lead-207 in fifteen steps. Thorium-235 decays into lead-208 in ten steps. Radioisotopes with very long half-lives can help determine the age of very old things. The potassium-40/argon-40 clock has a half-life of 1.3 billion years. Argon-40 produced by the decay of potassium-40 becomes trapped in rock. Ratio of potassium-40 : argon- 40 shows age of rock. Half-life Problems: 1. What mass of a 200g sample of carbon-14 remains after 22,920 years? 2. A rock has 420g of radioactive isotope. What percentage would remain after 5 half-lives? 7.3 Nuclear Reactions: Nuclear reactions are different than chemical reactions Chemical Reactions Nuclear Reactions Mass is conserved (doesn t change) Small changes in mass Small energy changes Huge energy changes No changes in the nuclei protons, neutrons, electrons and gamma rays can be lost or gained 6
Induced Nuclear Reactions Symbols to Remember: Scientists can also force ( = induce) nuclear reactions by smashing nuclei with alpha, beta and gamma radiation to make the nuclei unstable + N O + p 4 14 17 1 2 7 8 1 or 4 14 17 1 2 7 8 1 He + N O + H Nuclear Reactions Two types: Fission = the splitting of nuclei Fusion = the joining of nuclei (they fuse together) Both reactions involve extremely large amounts of energy Albert Einstein s equation E = mc 2 illustrates the energy found in even small amounts of matter Nuclear Fission: is the splitting of one heavy nucleus into two or more smaller nuclei, as well as some sub-atomic particles and energy. A heavy nucleus is usually unstable, due to many positive protons pushing apart. When fission occurs: 1.Energy is produced. 2.More neutrons are given off. 7
Induced Nuclear Fission Neutrons are used to make nuclei unstable It is much easier to crash a neutral neutron than a positive proton into a nucleus to release energy. Induced Nuclear Fission of Uranium-235 is the origin of nuclear power and nuclear bombs. 1 A neutron, 0n, crashes into an atom of stable uranium-235 to create unstable uranium-236, which then decays. After several steps, atoms of krypton and barium are formed, along with the release of 3 neutrons and huge quantities of energy. Chain Reactions: The neutrons released in the induced reaction can then trigger more reactions on other uranium-235 atoms causing a CHAIN REACTION A chain reaction can quickly get out of control materials that absorb some neutrons can help to control the chain reaction. Nuclear reactors have complex systems to ensure the chain reaction stays at safe levels. An uncontrolled chain reaction can result in the release of excess energy as harmful radiation It is on this concept that nuclear bombs are created. Nuclear meltdown occurs if the chain reactions cannot be controlled 8
Nuclear Energy Nuclear power plants can generate large amounts of electricity using fission reactions. In Canada, Ontario, Quebec and New Brunswick currently use nuclear power. Canadian-made nuclear reactors are called CANDU reactors. CANDU reactors are considered safe and effective, and are sold throughout the world. Hazardous Wastes Hazardous wastes produced by nuclear reactions are problematic. Some waste products, like fuel rods, can be re-used Some products are very radioactive, and must be stored away from living things. Most of this waste is buried underground, or stored in concrete It takes 20 half-lives (thousands of years) before the material is safe. Nuclear Fusion The fusion of hydrogen nuclei joining of two light nuclei into one heavier nucleus. In the core of the Sun, two hydrogen nuclei join under tremendous heat and pressure to form a helium nucleus. When the helium atom is formed, huge amounts of energy are released. Scientists cannot yet find a safe, and manageable method to harness the energy of nuclear fusion. cold fusion would occur at temperatures and pressures that could be controlled (but we haven t figured out how to get it to happen) 9