Nuclear Chemistry Init 9/16/2008 by Daniel R. Barnes
25.1 Section Assessment, page 802 1. How does an unstable nucleus release energy? An unstable nucleus releases energy by emitting radiation during radioactive decay The amount of energy released is given by Albert Einstein’s famous equation . . . e = mc2 e = energy m = mass c = the speed of light The equation implies that matter is a form of energy. A small amount of an atom’s matter (mass) is turned into a whole bunch of energy during radioactive decay.
25.1 Section Assessment, page 802 1. How does an unstable nucleus release energy? An unstable nucleus releases energy by emitting radiation during radioactive decay If an atom’s nucleus has the right balance of protons and neutrons, it can last pretty much forever. e = mc2 However, if the atom has too many or too few neutrons, the nucleus will be unstable. At some random, unpredictable moment, the nucleus will “decay”, spitting out a piece of itself.
25.1 Section Assessment, page 802 2. What are the three main types of nuclear radiation? The three main types of nuclear radiation are alpha radiation, beta radiation, and gamma radiation. g a b alpha particle beta particle gamma particle g - 0 e 2+ 4 He -1 2 high energy, high frequency photon high speed helium nucleus high speed electron
25.1 Section Assessment, page 802 3. What part of an atom undergoes change during radioactive decay? The nucleus undergoes change during radioactive decay. However, radiation particles fly out of the nucleus with such energy that they probably knock off one or more or even all of the electrons from the atom that spit them out. Also, the nucleus probably recoils away from the radiation particle (Newton’s 3rd Law), shooting off in the other direction, smacking into electrons also. Therefore, I, Mr. Barnes, believe that radioactive decay doesn’t just affect the nucleus. It affects the electron cloud as well.
25.1 Section Assessment, page 802 4. How is the atomic number of a nucleus changed by alpha decay? By beta decay? By gamma decay? In alpha decay, the atomic number goes down by two. In beta decay, the atomic number goes up by one. The emission of gamma radiation does not affect the atomic number of an atom. Atoms usually only spit out a gamma particle in the process of spitting out an alpha or a beta particle. Therefore, although radiating a gamma particle does not affect an atom’s atomic number, it often happens during an event that does change the atomic number.
25.1 Section Assessment, page 802 5. How is the mass number of a nucleus changed by alpha decay? By beta decay? By gamma decay? In alpha decay, the mass number goes down by four. In beta decay, the atomic number stays the same. The emission of gamma radiation does not affect the mass number of an atom. I suspect that the expression “gamma decay” is incorrect. Gamma radiation is often given off when a nucleus goes through alpha decay or beta decay, but never by itself. Since a nucleus does not change its atomic number or mass number as a result of giving off a gamma particle, giving off a gamma particle isn’t really an example of “decay”, so we really shouldn’t use the expression “gamma decay”.
238 U 92 Alpha Decay Uranium-238 undergoes alpha decay to become thorium-234. a e = mc2 The nucleus’ mass number . . . . . . goes down by four. The nucleus’ atomic number . . . . . . goes down by two. a 234 Th + He 4 2 90
Beta Decay Carbon-14 undergoes beta decay to become nitrogen-14. b e = mc2 The nucleus’ mass number . . . . . . stays the same. The nucleus’ atomic number . . . . . . goes up by one. b 14 C 14 N + e 0 6 7 -1
25.1 Section Assessment, page 802 6. Which of the three kinds of radiation described in this section is the most penetrating? g Gamma radiation is the most penetrating. Alpha radiation penetrates 0.05 mm into the human body. Beta radiation penetrates 4 mm into the human body. Gamma radiation shoots right through the human body. In fact, gamma radiation not only shoots right through skin, flesh, and bone, but also through substantial amounts of lead, concrete, or whatever. Gamma rays are very hard to stop. Even air can stop gamma rays, though, if you have enough air. The 50-mile-thick (or so) layer of air that surrounds our planet shields us nicely from gamma rays that come from outer space.
25.1 Section Assessment, page 802 6. Which of the three kinds of radiation described in this section is the most penetrating? g Gamma radiation is the most penetrating. Alpha particles may be the largest radiated particle that we’re studying here, but they stop the quickest. This seems odd, considering that it’s easier to stop a baseball than it is to stop a freight train. Alpha particles lose their energy rapidly by ionizing other atoms . . .
Alpha particles are a form of "ionizing radiation". Here’s an ordinary carbon atom minding its own business. Perhaps it’s part of a keratin protein molecule in your epidermis. It’s got the usual six electrons oribiting its nucleus. 6 positive protons 6 negative electrons neutral atom
Alpha particles are a form of "ionizing radiation". I’m gonna go mess up a DNA molecule! Look out, little carbon atom! Here comes an alpha particle! Aw, gee! That psycho alpha particle just knocked one of your electrons out of orbit! I’m CHARGED already! Look at me! I’m an ION! 6 positive protons 5 negative electrons +1 ION! 6 positive protons 6 negative electrons neutral atom No! Please don’t! You’ll be charged with causing cancer!
Alpha particles are a form of "ionizing radiation". Notice something about the alpha particle’s motion . . . It takes energy to ionize an atom. Knocking electrons out of orbit is hard work. . . . It slows down after it knocks the electron out of orbit. It loses lots of speed by ionizing the atoms it bumps into, so it doesn’t go very far. 6 positive protons 5 negative electrons +1 ION! 6 positive protons 6 negative electrons neutral atom
Alpha particles are a form of "ionizing radiation". Beta particles are a form of ionizing radiation, also. Gamma rays ionize atoms, too. Alpha, beta, and gamma particles all lose energy when they ionize an atom. The thing about beta and gamma particles seems to be that they don’t bump into electrons as easily as those huge alphas do, so they get to penetrate deeper into matter before they bump into an electron, lose their energy, and stop. I think.
25.2 Section Assessment, page 808 9. What determines the type of decay a radioisotope will undergo? The neutron-to-proton ratio determines what kind of decay a radioisotope will undergo. If an isotope has too many neutrons, it will probably undergo beta decay, in which one of its excess neutrons turns into a proton as it spits out a beta particle (an electron). The opposite of this, electron capture, is when a nucleus with too many protons captures an electron, turning one of its excess protons into a neutron. Also, if a nucleus is simply too big (atomic number > 83), the nucleus will be radioactive. Such large, unstable nuclei are often alpha emitters, but some are also beta emitters.
25.2 Section Assessment, page 808 10. How much of a sample of a radioisotope remains after one half-life? After two half-lives? After one half-life, only half of the radioisotope will remain. After two half-lives, only one quarter of the original radioactive atoms will still be present, unchanged. The amount of a radioactive isotope gets cut in half every time a half-life goes by. In general, a radioactive material gets less radioactive as time goes by, and, eventually, turns completely into a stable isotope. The shorter the half life . . . . . . the faster the radioisotope decays The longer the half life . . . . . . the longer the material is radioactive.
WARNING • The following simulation of radioactive decay focusing on the issue of half-life is somewhat oversimpilfied.
234 234 234 234 234 234 234 234 234 234 234 234 234 234 234 234 Th Th Th Th Th Th Th Th Th Th Th Th Th Th Th Th 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 238 238 238 238 238 238 238 238 238 238 238 238 238 238 238 238 U U U U U U U U U U U U U U U U 92 92 92 92 92 92 92 92 92 92 92 92 92 92 92 92 0 years 4.5 billion years 9 billion years 13.5 billion years 18 billion years ? years
25.2 Section Assessment, page 808 11. What are two ways that transmutation can occur? A nucleus can be transmuted by spontaneous decay, or by being bombarded by high-energy particles. SOME EXAMPLES: A nucleus spitting out an electron is called . . . . . . beta decay. The opposite of beta decay is called . . . . . . electron capture. A nucleus spitting out a helium nucleus is called . . . . . . alpha decay.
25.2 Section Assessment, page 808 12. Complete and balance the following nuclear reactions: ? Al a. 27 + He 30 Si + H 4 1 13 2 14 1 27 + 4 = 30 + 1 13 + 2 = 14 + 1
25.2 Section Assessment, page 808 12. Complete and balance the following nuclear reactions: Bi b. 214 4 He ? + 210 Tl 83 2 81 214 = 4 + 210 83 = 2 + 81
25.2 Section Assessment, page 808 12. Complete and balance the following nuclear reactions: ? Si c. 27 0 e 27 P + 14 -1 15 27 = 0 + 27 14 = -1 + 15
25.2 Section Assessment, page 808 12. Complete and balance the following nuclear reactions: ? Cu d. 66 66 Zn 0 e + 29 30 -1 66 = 66 + 0 29 = 30 + -1
25.2 Section Assessment, page 808 • A radioisotope has a half-life of 4 days. How much of a 20 gram sample of this radioisotope remains at the end of each time period? • a. 4 days b. 8 days • 4 days = 1 half-life, so the amount gets cut in half once. 20 grams / 2 = 10 grams. b. 8 days = 2 half-lives, so the amount gets cut in half twice. 20 grams / 2 / 2 = 20 grams / 4 = 5 grams
25.2 Section Assessment, page 808 14. The mass of cobalt-60 in a sample is found to have decreased from 0.800 g to 0.200 g in a period of 10.5 years. From this information, calculate the half-life of cobalt-60. How many times do you have to cut 0.800 in half to get 0.200? 800/2 = 400 (one half-life) (two half-lives) 400/2 = 200 10.5 years must be two half-lives, so one half-life would be . . . 10.5 years / 2 = 5.25 years The half-life of the isotope cobalt-60 is 5.25 years.
25.3 Section Assessment, page 813 15. Explain what happens in a nuclear chain reaction. Neutrons produced by fissioning atoms strike other fissionable atoms, causing them to split, which produces even more neutrons, which can then strike even more fissionable atoms. Lather, rinse, repeat.
Kr-91 Kr-91 Kr-91 Kr-91 Ba-142 Ba-142 Ba-142 Ba-142 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-235 U-236 U-236 U-236 U-236 Nuclear Fission
The atom bomb dropped on Hiroshima contained 64 kg of uranium, of which 0.7 kg underwent nuclear fission, and of this mass only 0.6 g was transformed into energy. The energy released by nuclear reactions is much larger, per gram of explosive material, than the energy relased by chemical explosions. c2 = 34,500,000,000 mi2/s2 c = 186,000 mi/s The 1.5 pounds of uranium that split that day yielded the same explosive energy as 15,000 tons of TNT. e = mc2 1.5 pounds of uranium = 30,000,000 pounds of TNT.
25.3 Section Assessment, page 813 16. Why are spent fuel rods from a nuclear reaction stored in water? Water cools spent fuel rods and provides a radiation shield.
25.3 Section Assessment, page 813 17. How are fusion reactions different from fission reactions? Fission reactions involve splitting nuclei. In fusion reactions, small nuclei combine and release much more energy. In fission reactions, big atoms fall apart. In fusion reactions, little atoms come together. Fission reactions make atoms smaller and more numerous. Fusion reactions make atoms bigger but less numerous.
Kr-91 Ba-142 U-235 U-236 Fusion Fission Gluing atoms together "Splitting the atom"
Protons don’t like other protons. Protons close enough to join and form one nucleus will repel each other with extreme force. Fusion Positives don’t like positives. The closer two charged particles get, the stronger the force between them. To get protons to get close enough to fuse together, you need to overpower this electrostatic repulsion. Remember how small a nucleus is compared to the atom as a whole? Extreme heat and/or extreme pressure, both of which are found in the centers of stars, can make fusion happen. In a nucleus, protons are VERY close together. Gluing atoms together
If protons are smashed together by exterme heat and/or pressure, they will get close enough for something magical to happen . . . The strong force has a strength of zero until protons get VERY close together, and then it gets VERY strong, very suddenly. Fusion The strong force is so powerful, that it overpowers the electrostatic repulsion the protons feel for each other . . . The “strong force” turns on. (The “strong force” is sometimes called the “strong nuclear force”.) Gluing atoms together . . . and it “glues” them together.
He Kr-91 Ba-142 H H H H U-235 U-236 Fusion Fission Gluing atoms together "Splitting the atom"
25.3 Section Assessment, page 813 18. What does nuclear [sic] moderation accomplish in a nuclear reactor? Neutron moderation slows down neutrons so that they can be absorbed by fissile atoms. If you don’t slow down the neutrons, the chain reaction stops. If you throw the baseball too fast, the next guy can’t catch it.
Kr-91 Ba-142 U-235 U-235 U-236 Without a moderator . . . TOO FAST! . . . fission can not continue
Kr-91 Kr-91 Ba-142 Ba-142 U-235 U-235 U-236 U-236 But with a moderator . . . Nice throw! Nice and slow! MODERATOR . . . the chain reaction can continue.
25.3 Section Assessment, page 813 19. What is the source of the radioactive nuclei in spent fuel rods? Unused nuclear fuel and fission products are the radioactive nuclei in spent fuel rods. Because “spent” fuel rods still have some unsplit, fissile nuclei in them (U-235 or Pu-239), some day, when uranium and plutonium are more expensive, we may need to recycle our spent fuel rods to get the remaining U-235 & Pu-239 out of them. “Spent” fuel rods aren’t 100% spent. There’s still unused fuel in them. Now that’s nuclear waste, isn’t it?
25.3 Section Assessment, page 813 20. Assuming technical problems could be overcome, what are some advantages to using a fusion reactor to produce electricity? Potential fuels are inexpensive and readily available. Fusion uses heavy hydrogen, and the world is covered with H2O. Not every hydrogen atom is deuterium or tritium, but when you’ve got as much water as we do in our oceans, it adds up to quite a bit of these less abundant hydrogen isotopes. Also, fusion produces helium instead of radioactive fission products. Helium is a very safe by-product. Perfecting nuclear fusion power will help us fill party balloons.
25.4 Section Assessment, page 813 21. Describe three methods of detecting radiation. If you want to detect radiation, you could use a Geiger counter, a scintillation counter, or a film badge.
25.4 Section Assessment, page 813 22. Describe two applications of radioisotopes in medicine. Radioisotopes can be used to both diagnose and cure disease. “Diagnosis” means figuring out what’s wrong with a sick person. For example, the radioisotope iondine-131 can be used to detect thyroid problems in a patient. As harmful as radiation is, radioisotopes can also be used, ironically, to help cure a sick person. For instance, salts of radioisotopes can be sealed in gold tubes and implanted in tumors. With any luck, this kills tumor cells much more than it kills healthy cells nearby, but, sometimes, the cure is worse than the disease and the patient suffers horribly.
25.4 Section Assessment, page 813 23. If you work regularly near a radiation source, why might your employer want to monitor your exposure to radiation by having you use a film badge rather than a Geiger counter? A Geiger counter only detects radiation being given off by radioactive atoms in you or on you. It doesn’t measure how badly you’ve been hit by radiation in the past. You can be hit by lots of damaging radiation but not become radioactive. Therefore, a dose of radiation might make your film badge change color, but you wouldn’t make a Geiger counter click. Don’t confuse radioactive atoms with radiation. Don’t confuse the gun with the bullet. You can be full of bullet holes, but with no guns anywhere on you.