10/19/2020 Nuclear Stability and Radioactive Decay Review The Nucleus Atomic Number is the number of protons in the nucleus. Nucleotide refers to a specific atom Mass Number is the sum of the protons and neutrons. Atomic Number (Z) – number of protons Nucleons refer to protons and Mass Number (A) – sum of protons and neutrons. neutrons Isotopes are atoms with the same atomic number but a different mass number because of a change in the A number of neutrons. Z X Copyright © Cengage Learning. All rights reserved 3 Radioactive decay Important observations Radioactive decay is the process of All nuclides with 84 or more forming a different nucleus. protons are unstable with respect to Of the approximately 2000 known radioactive decay. nuclides, only 279 are stable with Light nuclides are stable when # of respect to radioactive decay. protons equals # of neutrons, that is, A plot of the positions of the stable when the neutron/proton ratio is 1. nuclei as a function of the number of However for heavier elements, the protons and the number of neutrons neutron/proton ratio must be greater reveals a zone of stability where the than 1. In other words you need more stable nuclides reside. neutrons than protons. Types of Radioactive Decay Types of Radioactive Decay Showing Alpha Decay Alpha-particle production is a mode + o Alpha production (): ~expulsion of 2 p and 2 n of decay in which an alpha ( or He Show the alpha decay of Radon-222. nucleus) particle is produced. 222 α decay 218 This is very common for heavy 86 Rn 84 Po + α radioactive nuclides. lose 2 p+ so the atomic number is now 84 lose 2 p+ and 2 no so the mass number is now 218 element # 84 is Polonium The particle is also released 4 alpha particle can also be written as 2 He Copyright © Cengage Learning. All rights reserved 8 1 10/19/2020 Show the alpha decay of… Types of Radioactive Decay Plutonium-244 Beta-particle production is a mode decay of decay in which a beta ( or 244 α 240 94 Pu 92 U + α electron) particle is produced. Beta production (): Polonium-210 This is very common for nuclides 210 α decay 206 above the zone of stability (those 84 Po 82 Pb + α nuclides whose neutron/proton ratios Technetium- 98 are too high). decay 98 α 94 43 Tc 41 Nb + α Copyright © Cengage Learning. All rights reserved 12 Showing Beta Decay Show the beta decay of… ~conversion of a neutron to a proton and Potassium-40 Gamma-particle production ()refers an electron, and expulsion of the electron. to a high energy photon that 40 β decay 40 The beta decay of Lead-214 19 K 20 Ca+ β accompanies other nuclear decays 214 β decay 214 and particle reactions. 82 Pb 83 Bi + β Carbon-14 14 β decay 14 gain 1 p+ so the atomic number is now 83 6 C 7 N + β lose 1 no and gain 1 p+ so the mass number is the same Thorium-234 element # 83 is Bismuth The particle is also released 234 β decay 234 0 90 beta particle can also be written as Th 91 Pa + β -1 e Types of Radioactive Decay Types of Radioactive Decay Positron production occurs when a The following is an alpha decay accompanied positron, the antiparticle of the with gamma ray production electron is produced. Gamma ray production (): Positron production: This occurs for nuclides below the zone of stability (when the neutron/proton ratio is too small). Copyright © Cengage Learning. Copyright © Cengage Learning. All rights reserved 16 All rights reserved 18 2 10/19/2020 Showing positron production Electron capture Electron capture occurs when a nuclide captures an Positron production- a proton converts into a Electron capture occurs when a inner energy level electron. neutron and a positron, a particle with the nuclide captures an inner energy level same mass of an electron but opposite charge. electron. The positron production from sodium-22 22 22 0 Inner-orbital electron 11 Na 10 Ne + 1 e lose 1 p+ so the atomic number is now 10 gain 1 no and lose 1 p+ so the mass number is the same element # 10 is Neon The particle is also released Note that is it positive 1 not negative 1 like in beta Copyright © Cengage Learning. All rights reserved 21 Positron production and Electron Electron Capture capture commonly occur together ~An inner orbital electron is captured by the Show the positron production, then Nuclear Equations nucleus converting a proton into a neutron electron capture of… Write balanced equations for each of Electron capture of Aluminum-26 Pb-201 the following processes. 26 26 0 0 C-11 produces a positron. 13 12 201 201 201 Al + -1 Mg -1 e 82 Pb 81 Tl + e 80 Hg Bi-214 produces a particle. 0 + Cs-129 Np-237 produces an particle. lose 1 p so the atomic number is now 12 + 1 e o + gain 1 n and lose one 1 p so the mass number is the same 0 129 129 129 -1 element # 12 is Magnesium 55 Cs 54 Xe + e 53 I 0 + 1 e Nuclear Equations II Decay Series Decay Series In each of the following nuclear Often a radioactive nucleus cannot reactions, supply the missing particle. reach a stable state through a single Au-195 + ? -----> Pt-195 decay process and must occur in K-38 -----> Ar-38 + ? several steps until a stable nuclide is produced. 27 3 10/19/2020 Types of radiation diagram (alpha) radiation- 2 protons and 2 neutrons Stopping radiation (helium nucleus) are released by the atom particle-2 p+ 2 no type of How to stop it Danger Level radiation (beta) radiation 1 neutron breaks into a proton and an electron, the electron is released radiation a sheet of Most paper, or skin damaging particle- an electron (gamma) radiation – An energetic atom radiation a sheet of Damaging aluminum foil releases energy as a photon (gamma ray). radiation several cm of Still There is no particle, just a light pulse. lead damaging Continuing Where does radiation come from Most Dangerous only about 18% of the radiation that hits If it hits something nonliving (dead cells or the average person comes from manmade particles are the most dangerous if they molecules), it will damage the nonliving sources. are not stopped. structure. However, it was already dead. The majority of that comes from X-rays They are exceptionally large compared to particles are much smaller and more likely to or related procedures. the other particles squeeze through gaps, penetrating much It is like a cannon ball ripping through a cell. deeper before hitting and damaging something. The rest are naturally occurring on the Earth. If hit, a cell will most likely die. Once the radiation is stopped, it is no longer Because of their size they damage the first dangerous. It is only dangerous when it is Mainly Radon gas (naturally occurring) thing they hit (they aren’t likely to squeeze moving at a high velocity. through gaps) The Kinetics of Radioactive Decay Half life The Rate of Radioactive Decay. Half-life (t ) of a radioactive sample Radioactive decay is a FIRST ORDER 1/2 process is defined as the time required for the number of nuclides to reach half the ln ( N / N ) = k t o t original value. Where No is the number of nuclides at t = 0, t = ln 2 / k for a first-order process. Nt represents the number of remaining 1/2 nuclides at time t, and k is the decay constant Or This is the same as integrated first order t = .693 / k rate law. 1/2 ln [A]t – ln[A]o = - k t . 4 10/19/2020 Kinetics of Nuclear Decay Kinetics of Nuclear Decay II Radio-dating Technetium-99m is used to form The half-life of molybdenum-99 is 67.0 The age of old materials may be dated by pictures of internal organs in the body h. How much of a 1.000-mg of Mo-99 is radioactive isotopes present. and is often used to assess heart left after 335 h? To do this you need a radioactive isotope damage. The m for this nuclide present with a known half life, and a way indicates an excited nuclear state that to compare the amount present, to the decays to the ground state by gamma amount that was present at some date in emission. The rate constant for decay -1 the past. of Tc-99m is known to be 1.16 x10 h-1, What is the half-life of this nuclide? Carbon dating Applications of Radioactivity Finding an age Radiation on this planet causes radioactive Carbon-14 Dating is the most isotopes to form. The amount of C-14 in an object is commonly used dating processes for measured, and is compared to the amount A known percentage of the carbon dioxide in samples 10,000-years old or less. the air contains the radioactive C-14 isotope. that is known to be in all living things, Dating with C-14 produces errors of which is assumed to be there when it This carbon dioxide is used to “build” all up to 3,000 years for 20,000- to died. living things (plants use it for food, animals eat 30,000-year old samples. the plants etc.) while they are alive. Once Using the half lives to determine how The half-life of C-14 is 5730 years. the organism dies it stops taking in new C-14.