Chapter 17 Radioactivity and Nuclear Chemistry
The Discovery of Radioactivity (1896) Antoine-Henri Bequerel designed experiment to determine whether phophorescent minerals also gave off X-rays.
Bequerel discovered that certain minerals were constantly producing penetrating energy rays like X-rays not related to fluorescence Bequerel determined that the minerals contained uranium uranic rays Production of uranic rays did not require exposure to outside energy. Energy was being produced from nothing!!!
The Curies Marie Curie (1867-1934) broke down these minerals and used an electroscope to detect uranic rays. She discovered the rays were emitted from specific elements. She also discovered new elements 1. radium named for its green phosphorescence 2. polonium named for her homeland She coined the name radioactivity
Other Properties of Radioactivity Can ionize matter (cause matter to become charged) (basis of Geiger Counter) High energy Can penetrate matter Can cause phosphorescent chemicals to glow (basis for the scintillation counter)
Electroscope Ionizing radiation +++ +++ When charged, the metal foils spread apart due to like charge repulsion. Ionizing radiation knocks e l e c t r o n s o f f t h e a i r molecules, which jump onto the foils and discharge them.
Rutherford (1871-1937) Discovered Three Types of Radiation ++++++++++++ β γ α --------------
Types of Radioactive Rays Alpha Rays (α) charge of +2 and mass of 4 amu essentially the nucleus of a helium atom Beta Rays (β) charge of -1 and negligible mass high-energy electrons Gamma Rays (γ) electromagnetic radiation, not α or β
Penetrating Ability of Radioactive Rays α β γ 0.01 mm 1 mm 100 mm Pieces of Lead
Penetrating Ability of Radioactive Rays
Review of Nuclear Structure Every atom of an element has the same number of protons designated by the atomic number Z Atoms of the same element may have different numbers of neutrons called isotopes have different atomic masses Isotopes and Nuclides are represented symbolically: mass number atomic number A Z X symbol mass # = protons + neutrons
1. Which of the following pairs are isotopes? A. H-2 and He-2 B. C-14 and N-14 C. H-3 and C-14 D. O-16 and O-18 E. Two of the above 16 8 O 18 8 O
2. How many protons and neutrons, respectively, does 210 Po contain? A. 84 protons and 84 neutrons B. 84 protons and 210 neutrons C. 84 protons and 126 neutrons D. 210 protons and 84 neutrons E. 126 protons and 84 neutrons 210 84 Po
Radioactivity Unstable radioactive nuclei spontaneously decompose into smaller nuclei through radioactive decay. PARENT NUCLIDE > DAUGHTER NUCLIDE(S) PARTICLE(S) and/or ENERGY All nuclides with 84 or more protons are radioactive
Important Atomic Symbols Particle Symbol Nuclear Symbol proton p + neutron n 0 electron e - alpha beta positron α β, β β, β +
Transmutation Atoms of one element are changed into atoms of a different element. The number of protons in the nucleus changes. We describe the process with nuclear equations.
Nuclear Equations In nuclear equations, atomic numbers and mass numbers are conserved. conservation of nucleons 238 = 234 + 4 92 = 90 + 2 conservation of charge
Alpha Emission An α particle contains 2 protons and 2 neutrons. a helium nucleus The most ionizing, but least penetrating of radiation types
Alpha Emission 3. Radium-222 decays by alpha emission Loss of an α particle means atomic number decreases by 2 mass number decreases by 4
Beta Emission
Beta Emission A beta particle is like an electron moves much faster (has more energy) produced in the nucleus In β decay, a neutron changes into a proton
Beta Emission 4. Thorium-234 decays by beta emission Loss of an β particle means atomic number increases by 1 mass number remains the same
Gamma Emission Gamma (γ) rays are high energy photons. No loss of particles from the nucleus No change in composition of the nucleus Occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange Least ionizing, but most penetrating
Positron Emission
Positron Emission from the Positron has a charge of +1 and negligible mass Appears to result from a proton changing into a neutron
5. Sodium-22 decays by positron emission When an atom loses a positron from its nucleus, atomic number decreases by 1 mass number remains the same
Electron Capture Be-7 Li-7
Electron Capture An inner orbital electron is pulled into the nucleus No particle emission, but the atom changes
6. Ruthenium-92 undergoes electron capture Proton combines with electron to make a neutron Mass number stays the same Atomic number decreases by 1 The result is the same as positron emission!!
Table 17. 1 Selective types of Radioactive Decay
7. What type of decay occurs in the following nuclear reaction: Bi-210 Tl-206 + A. Alpha B. Beta C. Gamma D. Delta E. Positron 210 83 Bi 206 81 Tl + 4 2 α
8. What is the other product in this nuclear reaction? Bi-211 + one beta particle A. Pb-206 B. Po-211 C. Pb-210 D. At-211 E. Pb-211 211 83 211 Bi 84 Po + 0-1 β
9. Fill in the blank in the following nuclear equation: Rn-222 + one alpha particle A. Rn-222 B. Rn-226 C. Po-222 D. Po-226 E. Ra-226 226 88 Ra 222 Rn + 4 86 2 α
10. Gamma decay of Pb-214 yields A. Bi-214. B. Pb-214. C. Hg-210. D. Pb-210. E. Bi-215. 214 82 Pb 214 + 0 82 Pb 0 γ
11. Fill in the blank in the following nuclear equation: 30 P + one positron A. Si-30 B. S-30 C. P-30 D. P-29 E. S-29 30 15 P 30 14 Si + 0 +1 β
What Causes Nuclei to Break Down? The particles in the nucleus are held together by a very strong attractive force between nucleons, the strong force, which acts only over very short distances.
Neutrons and Protons are Held Together by the Strong Force. Neutrons play an important role in stabilizing the nucleus. They add to the strong force, but don t repel each other like protons.
The Valley of Stability and the N/Z Ratios Valley of Stability for Z = 1 20, stable N/Z 1 for Z = 20 40, stable N/Z approaches 1.25 for Z = 40 80, stable N/Z approaches 1.5 for Z > 83, there are no stable nuclei
Nuclear Decay Series In nature, one radioactive nuclide often changes into another radioactive nuclide. All of the radioactive nuclides that are produced one after another until a stable nuclide is made is called a decay series.
12. What is the product formed when 238 U goes through one alpha decay followed by two beta emissions and then another alpha decay? A. U-235 B. U-234 C. Pa-234 D. Th-234 E. Th-230 α β 238 92 U 234 90 Th 234 91 Pa β α 234 92 U 230 90 Th
A Natural Radioactive Decay Series for U-238 U-238 Decay Series α β β α α α α β α β α β β α or α β α β β α β or other combinations
The Concept of Half-life Half-life - The time it takes for half of the parent nuclides in a radioactive sample to decay to the daughter nuclides The amount that remains after one half-life is always onehalf of what was present at the start. The amount that remains after two half-lives is onequarter of what was present at the start. A radioactive sample does not decay to zero atoms in two half-lives You can t add two half-lives together to get a whole life.
Th-232 has a half-life of 14 billion years. A plot of the number of Th-232 atoms in a sample initially containing 1 million atoms as a function of time.
Half-Life Half of the radioactive atoms in a sample decay each half-life. of the radioactive atoms decay each half
Half-Life Table 17.2 - Each radioactive nuclide has a unique half-life that is not affected by physical conditions or chemical environment.
13. Based solely on the following half-lives, which radioactive atom listed is the most radioactive? A. C-14 (5730 yrs) B. Ra-226 (1620 yrs) C. U-238 (4.5 million yrs) D. Po-214 (1.6 ms) E. P-32 (14 days)
What is meant by Radioactive Decay- Half Life I-131 decays by beta emission with a half life of 8 days.? 131 53 I 0-1 e 131 + 54 Xe 1.000 g 131 53 I 8 days 0.500 g 0.500 g 131 53 I 131 54 Xe 8 days 0.250 g 0.750 g 131 53 I 131 54 Xe 8 days 0.125 g 0.875 g 131 53 I 131 54 Xe
14. The half-life of the beta particle emitter tritium, 3 H, is 12 years. How much of a 1.00 g sample of 3 H remains after 48 years? A. 1.00 g B. 0.750 g C. 0.250 g D. 0.125 g E. 0.0625 g 1.00 g 0.50 g 0.250 g 0.125 g 0.0625 g
Artifact Dating Mineral (geological) Compare amount of U-238 (t½ = 4.5 x 10 9 yr) to Pb-206 Compare amount of K-40 (t½ = 1.25 x 10 9 yr) to Ar-40 Archeological (once living materials) Compare amount of C-14 (t½ = 5730 yr) to C-12 While a substance is living C-14/C-12 ratio is constant (CO2 exchange with the atmosphere continues). When an organism dies, C-14/C-12 ratio decreases. Useful to up to about 50,000 yr
15. An artifact contains 12.5% of the original amount of C-14. How old is this sample? (C-14 half-life is 5730 years.) A. The sample is new. B. 5.73 x 10 3 yr C. 1.15 x 10 4 yr D. 1.72 x 10 4 yr E. 2.29 x 10 4 yr 3 x 5730 % C-14 (relative to living organism) Number of Half-Lives Time (yrs) 100.0 0 0 50.0 1 5,730 25.00 2 11,460 12.50 3 17,190 6.250 4 22,920 3.125 5 28,650 1.563 6 34,380 100 % 50 % 25 % 12.5 % 6.25 %
Chapter 17 Radioactivity and Nuclear Chemistry Part 2
Nonradioactive Nuclear Changes Some nuclei are inherently unstable. If their nuclei are hit by a neutron, the large nucleus splits into smaller nuclei This is called fission Small nuclei can be accelerated to such a degree that they overcome their charge repulsion and smash together. A larger nucleus is formed. This is called fusion Both fission and fusion release large amounts of energy
On January 6, 1939, Meitner, Strassmann, and Hahn reported that the neutron bombardment of uranium resulted in nuclear fission the splitting of the atom. 1 235 n U Ba 0 92 142 91 1 K 3 n 56 36 0 + + +
Fissionable Materials U-235, Pu-239, and Pu-240 Natural uranium is less than 1% U-235 Mostly U-238 Not enough U-235 to sustain a chain reaction To produce fissionable uranium, natural uranium must be enriched in U-235 to about 3% for reactor grade to about 7% for weapons grade
Fission Chain Reaction A chain reaction occurs when a reactant in a process is also a product of the process. In the fission process, the neutrons are both. Only a small number of neutrons are needed to start the chain. Man neutrons produced in fission are either ejected from the uranium before they hit another U-235 or are absorbed by the surrounding U-238 The minimum amount of fissionable isotope needed to sustain the chain reaction is called the critical mass.
Fission Chain Reaction
In essence, the Little Boy design consisted of a gun that fired one mass of uranium 235 at another mass of uranium 235, thus creating a supercritical mass. A crucial requirement was that the pieces be brought together in a time shorter than the time between spontaneous fissions. Once the two pieces of uranium are brought together, the initiator introduces a burst of neutrons and the chain reaction begins, continuing until the energy released becomes so great that the bomb simply blows itself apart.
The initial design for the plutonium bomb was also based on using a simple gun design (known as the "Thin Man") like the uranium bomb. As the plutonium was produced in the nuclear reactors at Hanford, Washington, it was discovered that the plutonium was not as pure as the initial samples from Lawrence's Radiation Laboratory. The plutonium contained amounts of plutonium 240, an isotope with a rapid spontaneous fission rate. This necessitated that a different type of bomb be designed. A gun-type bomb would not be fast enough to work. Before the bomb could be assembled, a few stray neutrons would have been emitted from the spontaneous fissions, and these would start a premature chain reaction, leading to a great reduction in the energy released. Seth Neddermeyer, a scientist at Los Alamos, developed the idea of using explosive charges to compress a sphere of plutonium very rapidly to a density sufficient to make it go critical and produce a nuclear explosion.
Nuclear Fusion Fusion is the combining of light nuclei to make heavier nuclei. The source of the sun s energy Requires a high input of energy to initiate the process Produces 10 times the energy of fission per gram No radioactive byproducts The only currently working application is the H-bomb.
Nuclear Fusion Deuterium-Tritium Fusion Reaction
Artificial Transmutation Bombardment of one nucleus with neutrons or another nucleus causing new atoms to form Requires a particle accelerator Ex: Tc-97 is made by bombarding Mo-96 with deuterium: 2 96 H Mo Tc 1 42 97 1 n 43 0 + +
4 Formation of Transuranium Nuclides 239 240 1 H 95 1 + + + 2 n He Pu Am 2 94 4 239 + + n He Pu Cm 2 94 96 4 244 242 1 0 245 97 1 H 1 246 1 4 98 0 248 1 99 0 + + + 2 n He Cm Bk 2 96 12 238 + + n C U Cf 6 92 2 H Cf Es 1 98 4 253 256 + + n He Es Md 2 99 101 10 249 + + n 252 256 + + 6 n B Cf Lr 5 98 103 1 0 1 0 1 0 1 0 51 hr 163 d 5 d 36 h 3 25 min 76 min 28 sec
Positron Emission β+ C-11 B-11
Positron Emission β+ antimatter Detector photo 511 kev photo 511 kev Detector β- matter
Positron Emission Tomography
Pet Scanning One-quarter of all patients in U.S. hospitals undergo tests using descendants of cameras developed by BER to follow radioactive tracers in the body. PET scanning has been key to a generation of brain metabolism studies as well as diagnostic tests for heart disease and cancer. PET studies above reveal brain metabolism differences in recovering alcoholics (left, 10 days, and right, 30 days, after withdrawal from alcohol).