Nuclear Chemistry

Radioactivity Radioisotopes are unstable isotopes whose nuclei gain stability by spontaneously undergoing changes. These changes are accompanied by the emission of large amounts of energy. Radioactive decay is the process by which materials give off this energy. The penetrating rays and particles that are emitted during these changes are called radiation. Eventually unstable radioactive isotopes are transformed into stable isotopes of a different element.

Radioactive Isotopes All elements consist of at least one radioactive isotope. Isotopes that have too many or too few neutrons (atomic mass larger or smaller than the average) tend to be radioactive. All isotopes with an atomic number greater than 83 are radioactive.

Identify the radioactive isotope 10 11H 614C 816O 714N 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Identify the radioactive isotope 10 Chlorine-35 Carbon-12 Lead-207 Potassium-40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Types of Radiation Alpha radiation Beta radiation Gamma radiation

Alpha Radiation Alpha radiation consists of helium nuclei that are emitted from a radioactive isotope. Alpha particles consist of two protons and two neutrons. Alpha particles have a 2+ charge. The symbol for an alpha particle is 42He or α. Alpha particles are the most massive of the radioactive particles (4 amu), the most damaging, and are the least penetrating (easily stopped by a piece of paper). http://phet.colorado.edu/en/simulation/alpha-decay http://www.hpwt.de/Kern2e.htm

What is the product when plutonium-238 undergoes alpha decay? 10 Uranium-234 Thallium-206 Lead-206 Radium-226 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

What is the product when bismuth-210 undergoes alpha decay? Radium-226 Lead-206 Thallium-206 Uranium-232 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

What is the product when polonium-210 undergoes alpha decay? Bismuth-206 Lead-206 Radium-206 Thorium-206 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Beta Particles Beta particles consist of fast moving electrons formed by the decomposition of a neutron in an atom. The neutron decomposes into a proton and an electron-the proton remains in the nucleus and the electron is emitted. Beta particles have a 1- charge. The symbol for a beta particle is o-1e or β. Beta particles are 8000 x lighter than an alpha particle, are less damaging, but are more penetrating (stopped by aluminum foil or thin pieces of wood). http://phet.colorado.edu/en/simulation/beta-decay

What is the product when carbon-14 undergoes beta decay? 10 Carbon-13 Nitrogen-14 Oxygen-14 Boron-10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

What is the product when strontium-90 undergoes beta decay? 10 Rubidium-90 Krypton-91 Strontium-91 Yttrium-90 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

What is the product when potassium-40 undergoes beta decay? 10 Calcium-40 Scandium-40 Argon-40 Chlorine 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Gamma Radiation Gamma radiation is high energy electromagnetic radiation. Gamma rays are emitted along with alpha or beta particles. Gamma rays have no mass or charge. The symbol for gamma rays is ooγ Gamma rays are extremely penetrating and potentially dangerous (stopped only by several meters of concrete or several centimeters of lead).

Nuclear Decay Puzzle Uranium-238 is a radioactive isotope. Through a series of 14 nuclear reactions, the unstable uranium isotope undergoes radioactive decay until it forms a more stable isotope of lead-206.

Unstable isotopes formed during the process: Uranium-234 Thorium-234 Thorium-230 Protactinium-234 Radium-226 Radon-222 Polonium-218 Polonium-214 Polonium-210 Lead-214 Lead-210 Bismuth-214 Bismuth-210

Radiation emitted during the process: Eight alpha particles Six beta particles

Procedure Write the isotope symbol for each of the radioactive isotopes involved in the problem. Put one symbol on each card. (There should be 15 total) On eight cards, write the symbol for alpha radiation. On six cards, write the symbol for beta radiation. Put the cards in the correct order to determine the steps in going from uranium-238 to lead-206. After they are in the correct order, write the 14 nuclear equations that illustrate the steps.

Radioactivity and Half-Lives Purpose: To simulate the conversion of a radioactive isotope over a period of time. Data: Trial # Number of atoms decayed Number of atoms remaining 100

Analysis Use graph paper and plot the “number of isotopes remaining” (y-axis) vs. the trial number (x-axis). Examine your graph. Is the number of isotopes remaining over time linear or nonlinear? Is the rate constant over time or does it change? By approximately how much did the number of isotopes remaining decrease with each trial? Define half-life. What represented one half-life during this lab? Answer the “You’re the Chemist” questions on page 852 in your textbook.

Half-Life A half-life is the time it takes for one -half of the nuclei of a sample of radioactive isotopes to undergo radioactive decay. Half-lives may be as short as a fraction of a second or as long as billions of years.

Half-lives of selected isotopes Half-life Hydrogen-3 12.3 years Carbon-14 5730 years Iodine-131 8.07 days Lead-212 10.6 hours Polonium-194 0.7 seconds Polonium-210 138 days Uranium-235 710 million years Uranium-238 4.5 billion years Plutonium-236 2.85 years

Graph the following data Time elapsed (days) Amount of sample remaining 100 g 5 74 g 10 50 g 15 35 g 20 25 g 25 18 g 30 12.5 g

Use the graph to answer the following questions: 1. How much remains after 3 days? What is the half-life of this isotope? If 25 g remains, how much time has elapsed? How many half lives have occurred when 25 g remains?

Determining Half-Lives In order to solve problems involving half-lives, the following equation may be used: # of half-lives = total time/time of one half-live To determine the amount of sample left, the following equation may be used: amount left = starting amount/ 2# of half-lives Solve Practice Problems on page 849.

The half-life of carbon-14 is 5700 years The half-life of carbon-14 is 5700 years. If a 10 gram sample undergoes decay for 17,100 years, how many half-lives has the sample undergone? 10 10 5 3 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

How much of the sample from the previous problems remains after 17,100 years? 10 g 5 g 2.5 g 1.25 g 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Cobalt-60 is a radioactive element used as a source of radiation in the treatment of cancer. Cobalt-60 has a half-life of five years. If a hospital starts with a 1000-mg supply, how much will remain after 10 years? 10 1000 mg 750 mg 500 mg 250 mg 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

If 62.5 g of the original sample of cobalt-60 remains, how much time has elapsed? 10 15 years 20 years 25 years 30 years 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Fission Fission-when a large nucleus is bombarded with neutrons, a division of the nucleus into 2 smaller nuclei occurs resulting in a large release of energy. Example: This energy is used in nuclear power plants and in atomic bombs.

Fusion Fusion-nuclei with small masses combine to form a nucleus with a larger mass.

Uses of Fusion This type of reaction occurs in the sun and in hydrogen bombs. The high temperature needed to start a fusion reaction is produced by a fission reaction. More energy is released per gram of reactant in fusion than in fission.

Other Uses of Nuclear Reactions Dating of fossils (Carbon-14) Dating of geological time (Potassium-40) Industrial Uses (Radiation used to monitor and control the thickness of aluminum foil and plastic wrap) Nuclear imaging (MRI) Food preservation Medical applications