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NUCLEAR CHEMISTRY Discovery of Radioactivity Radiation - energy emitted in the form of electromagnetic waves (i.e. light, microwaves, x-rays, etc..) • 1895 – Roentgen – discoverer of x-rays - Nobel Prize 1901. video clip • Becquerel – followed-up on Roentgen’s work story • Marie and Pierre Curie • 1899 – Rutherford – alpha and beta rays Radioactivity – spontaneous emission of particles and energy from atomic nuclei. ELECTROMAGNETIC SPECTRUM LOWER ENERGY HIGHER ENERGY Ionizing and Non-ionizing radiation Non-ionizing radiation (long-wavelength) Ionizing radiation (shorter wavelength)Both transfer energy to matter! UV radiation damage Isotopes • Atoms of the same element that contain different numbers of neutrons. • What chemical symbol do these atoms have? What is their atomic number? What is the mass number of the atom on the left? Isotope Tables • http://en.wikipedia.org/wiki/Isotope_table_% 28divided%29 NOTATION Uranium – 238 238 mass number ( p + n ) U 92 atomic number ( # p ) Nuclear Radiation • It is a form of ionizing radiation that is caused by changes in the nucleus of an atom. – Remember: unstable nucleus = radioactive atom change in identity of atom as the nucleus breakdown (radioactive decay). How is this dangerous? - Unstable nuclei can emit high-speed particles from the nucleus thus releasing energy (nuclear radiation). Stable vs. Unstable isotopes • Radioactive Isotopes: unstable atoms, due to a nucleus with too many or too few neutrons relative to the number of protons. • No amount of neutrons can hold a nucleus together once it has more that 82 protons. All of the elements with an atomic number greater than 82 have only unstable isotopes. • Unstable atoms emit energy in the form of radiation when they break down (decay) • Large nucleus (unstable) nucleus + energy • Reaction is EXTREMELY exothermic; called nuclear energy; mass is lost in the process Nuclear Radiation Alpha radiation, α Beta Radiation, β Gamma Radiation, γ low penetrating power Moderate penetrating power Very high penetrating power 2+ -1 0 Ex: Radium-226 Ex: carbon-14 Cobalt -60 3 types of nuclear radiation: Alpha & beta particles and gamma rays Charge of each Gamma rays – highest energy – similar to x-rays frequency. UVA, UVB, UVC rays from the sun UVC (100-290 nm – wavelength) UVA (320 -400nm – wavelengths) ALPHA PARTICLE • The same as a helium nucleus • 2 protons and 2 neutrons 4 mass of 4 amu He 2 Has a 2+ charge because of 2 protons LOWEST PENETRATING POWER BETA PARTICLE • The same characteristics as an electron (not from an energy level—emitted from the nucleus) 0 essentially no mass e -1 charge of -1 HIGHER PENETRATING POWER Gamma Emission • gamma emission usually occurs in concert with other forms of decay that produce nuclei in ‘nuclear excited states’ ignoring the alpha decay event, gamma emission looks like this: GAMMA RADIATION • Pure electromagnetic energy with high frequency and short wavelength • No mass or charge associated with it MOST PENETRATING POWER Strength of alpha, beta, and gamma radiation PENETRATING POWER OF NUCLEAR RADIATION Background radiation • Background radiation: a constant level of radioactivity that results from natural sources and from sources related to human activity. o You can’t completely eliminate background radiation it’s essentially everywhere: soil, water, food, rocks, etc… o Currently, 500 mrem (millirem) is the established US background radiation limit for the general public • EPA_Cosmic Radiation – ~8% of our annual radiation exposure comes from outer space. The closer we get to outer space the more we are exposed to cosmic radiation (such as during flights). • Full Body Scanners at Airports Fyi’s….. • BED (Banana Equivalent Dose) The equivalent dose for 365 bananas (one per day for a year) is 3.6 millirems(36 μSv). • Bananas are radioactive enough to regularly cause false alarms on radiation sensors used to detect possible illegal smuggling of nuclear material at US ports. • Other foods that are above avg. for radioactivity in food include: potatoes, kidney beans, nuts, and sunflower seeds. Ionizing radiation in you! UNITS OF RADIATION DOSAGE • GRAY= SI unit that is equivalent to the dose absorbed: 1 gray = 1 Joule per 1 kilogram of body tissue • SIEVERT: SI unit that is used to express the ability of radiation to cause ionization in human tissue • RAD: abosrbed dose similar to gray • REM: absorbed dose similar to Sievert Copyright© by Houghton Mifflin Company. All rights reserved. 24 Radiation Doses Causing Death Effect of Ionizing Radiation Ionizing radiation worksheet • • Units How much is safe? – Look at 3 main factors: • • • • 1. Radiation Density – (how much per given volume) 2. Dose – (how much total received) 3. Energy associated with that type of radiation Biological effects of ionizing radiation: – Large doses (but less than 200 rems) – can causes changes in DNA or proteins in the body than can result in mutations leading malfunctioning proteins that are unable to do their job in the body, to cancer , or birth defects, etc.. – • • • Low level radiation – does not produce as unique effects or obvious mutations attributable to the radiation exposure that is easy to distinguish from other injuries or illnesses – it can take years for symptoms to even show (making it nearly impossible in some cases to attribute to radiation damage) Chemotherapy vs. radiation therapy Radiation Risk from Medical Imaging Cancer and Radiation TRANSMUTATION INVOLVES THE RELEASE OF RADIATION • A Radioisotope emits, or gives off, radiation from its nucleus • Each isotope emits a certain type of radiation; it has a specific decay mode • The radiation can be in the form of particles or rays NUCLEAR TRANSMUTATION (natural) Original Radioisotope 238 U 92 New Isotope + Radiation 234 Th 90 4 + He 2 A CLOSER LOOK… U-238 has alpha decay Total mass before and after must be “equal” 238 U 92 234 Th 90 + 4 He 2 Total of atomic numbers must also balance How can you use decay mode information? Can you determine the new isotope if you know the decay mode? For example, Th-232 is an alpha emitter. 232 Th 90 4 ? + He 2 STRATEGY… *mass numbers must balance 232 = 228 + 4 Th ? + He 90 = 88 + 2 *atomic numbers must balance How do we know what the new isotope is? CHECK THE PERIODIC TABLE! The new isotope has a mass number of 228 and an atomic number of 88. 228 ? 88 each element has a unique atomic number… The new isotope is Radium-228 which is a beta emitter! What happens to Ra-228? 228 Ra 88 0 ? + e -1 (beta particle) Balance the mass numbers and the atomic numbers…careful of that -1) And the result of the decay is… mass number stays same 228 Ac atomic number increases by 1 89 Actinium a new element with a greater atomic # This is a beta emitter as well! When will it stop??? …not until a stable isotope is formed! The change from one radioisotope to another in a specific sequence is called a decay series We will explore some transmutations that are part of the uranium-238 decay series... You will be given two radioisotopes and their decay modes. Determine what the product of each transmutation will be… Start off examples are: U-238 is an alpha emitter: 238 234 U Th + 92 90 4 He 2 Th-234 is a beta emitter: 234 234 Th Pa + 90 91 0 e -1 U-238 decay series U-238 Th-234 Th-230 Pa-234 Ra-226 Pb-214 Bi-210 U-234 Rn-222 Po-218 Bi-214 Po-214 Po-210 Pb-210 Pb-206 (stable) Take your time and think it out… 1. Write the symbol of the radioisotope in the proper notation on the reactant side 2. Write the notation for the radiation type emitted on the product side 3. Total up the mass numbers on top so the total mass is balanced on each side 4. Total up the atomic numbers on the bottom so they are balanced as well 5. Use Periodic Table to identify the element by its atomic number Copyright© by Houghton Mifflin Company. All rights reserved. 42 One very special isotope… U-238 Th-234 Pa-234 Th-230 Pb-214 Bi-210 U-234 Ra-226 Rn-222 Bi-214 Po-214 Po-218 Pb-210 Po-210 Pb-206 (stable) Rn-222 is a gas! Figure 6.34 – Page 626 Figure 6.35 – Page 626 Figure 6.36 – Page 627 Copyright© by Houghton Mifflin Company. All rights reserved. 47 THIS IS NOT A CHEMICAL CHANGE! • Chemical reactions involve atoms rearranging by breaking and forming bonds involving electrons • TRANSMUTATION involves changes in the nucleus that change the actual identity of the element • These reactions are called NUCLEAR because they involve the atom’s nucleus Transmutation • Some isotopes are naturally unstable and spontaneously change to another isotope of a different element • This change from one element to another is called TRANSMUTATION and can happen 2 ways: 1)by radioactive decay (natural) OR 2) when particles bombard the nucleus of the atom. (nuclear reactor) Figure 19.5: Representation of a fission process. Copyright© by Houghton Mifflin Company. All rights reserved. 52 are a large fraction of potassium atoms radioactive? are a large fraction of tellurium atoms radioactive? no, a tiny fraction is radioactive! yes, more than 1/2 of tellurium atoms in nature are radioactive. are a large fraction of uranium atoms radioactive? ALL uranium atoms are radioactive! Why is there still uranium on earth? It takes a long time to decay. (need to consider how long it takes for a radionuclide to decay) HALF LIVES Half Lives • Half-life - is the time required for one half of original atoms to decay • The half-life is unique to that isotope. • Knowing the half-life of a radioisotope can determine how harmful or useful that substance can be. Half-Life: A Radioactive Clock the time during which half of a sample of radioactive atoms will decay into their daughter atoms. after one half-life 50% of the radioisotopic nuclei will have decayed into their daughter nuclei Common Radionuclide Half-Lives Common Radionuclide Half-Lives Parent Isotope Half-Life (yr) Lead-210 22 Carbon-14 5730 Uranium-235 704 million Daughter Product Bismuth 210 Nitrogen-14 Lead-207 Potassium-40 Uranium-238 Thorium-232 1.25 billion 4.5 billion 14.0 billion Argon-40 Lead-206 Lead-208 Rubidium-87 48.8 billion Strontium-87 Copyright© by Houghton Mifflin Company. All rights reserved. 62 Decay Chain of Uranium Half-Life: A Radioactive Clock Rutherford says: half-life is a time during which there is a 50-50 chance a radioactive atom will undergo nuclear decay If the half life of a radioactive atom is a small amount of time will it be more of less likely to decay than a different radioactive atom with a long half life? MORE LIKELY! Radiometric Dating Radiocarbon Dating Why do living things maintain a constant concentration of carbon-14? Shouldn’t it be consistently decaying in all material? Wh Radiocarbon Dating Radiometric Dating Example Calculations • How long will it take for a sample containing 1 gram of radium 223 to reach a point where it only contains 0.25 grams of radium 223 if a halflife is 12 days? • Answer: – 1 gram 0.5 grams = 12days – 0.5 grams 0.25 grams = 12 days – 12 days + 12 days = 24 days Copyright© by Houghton Mifflin Company. All rights reserved. 70 Storing High Level Radioactive Waste • Store for 10 half lives! • Store in special canisters. • Store in special, secured containment site Copyright© by Houghton Mifflin Company. All rights reserved. 71 Calculations with half-life • A particular plant is 50,000 year old, how many carbon-14 half-lives have occurred? – A: 8.7 half-lives • If the plant original had 30o counts of C-14 atoms, roughly how many radioactive C-14 atoms are left in the plant after 50,000 yrs? – ~0.7 counts Diagnostic & Therapeutic Uses NUCLEAR CHEMISTRY IN MEDICINE Nuclear Chemistry in Medical Field • • • • X-rays CT or CAT scan MRI – no radiation PET scan • Radiation Therapy Therapeutic • http://www.cancer.gov/cancertopics/factshee t/Therapy/radiation NUCLEAR REACTORS Fission vs. Fusion • Fission: Splitting a heavy nucleus into two nuclei with smaller mass numbers. = Nuclear transmutation! • Fusion: combining two light nuclei to form a heavier nucleus. Copyright© by Houghton Mifflin Company. All rights reserved. 78 Nuclear Power Plant http://www.nrc.gov/reactors/power.html