Nuclear Notes. Unit 13, Chemistry Themed

Transcription

1 Nuclear Notes Unit 13, Chemistry Themed

2 What is nuclear chemistry? Why do we care? Burning gives us energy by trading high energy bonds for low energy bonds E = mc 2 lets us convert a small amount of mass into a large amount of energy This process happens in nuclear reactions Use the energy for tremendously destructive bombs Use the energy to make electricity Using nuclear energy has advantages and disadvantages that you can only appreciate by understanding it better

3 History how we discovered radiation and our model of the atom 400 BC: Democritus theorizes that a sample of any element could only divided so many times and then you would come to its essence, where it can be divided no more Greek word atomos means indivisible (not able to be cut) 1808: Part of Dalton s famous atomic theory claimed that atoms are indestructible spheres 1895: Roentgen s X rays: Working with Cathode Ray tubes in November 1895, Roentgen discovered an invisible ray that penetrated many materials. He made an X ray image of his wife s hand, the first every and by early 1896, X rays were being used around the world! 1896: Becquerel discovered radioactivity: Becquerel accidentally exposed photographic film by placing it near a piece of uranium. He carefully shielded the film from the uranium, but it still exposed the film. He deduced that uranium gives off some kind of rays (radiation).

4 History how we discovered radiation and our model of the atom continued 1897: JJ Thomson discovers the electron: Thomson found that the rays coming from Cathode ray tubes were small massed negatively charged particles. These are electrons. Thomson s model of the atom was the plum pudding model, where he imagined atoms have electrons dispersed in them, like plums in pudding. 1898: Curie (Marie & Pierre) discover more radioactive elements: Radium & Polonium. Polonium was named after Poland, where Marie was born; radium give off a million times more radiation than uranium, so it s named after radiation. 1910: Rutherford s gold foil and electrically charged plate experiments: Electrically charged plates were used to find that there are three kind of radiation coming from a radioactive substance: alpha, beta and gamma radiation Gold foil can be made extremely thin so Rutherford using very thin gold foil to show that most radioactive particles pass through with no interaction from gold atoms, but once in a great while, one bounces completely backward. Conclusion: Atoms have very tiny but very dense centers (the nucleus) with vast space between atoms

5 Rutherford s two important experiments In 1899 he found radiation consisted of three types: Alpha, beta and gamma Large mass & positive, small mass and negative, no mass and highly penetrating In about 1910, Rutherford s gold foil led him to conclude that atoms have a dense, positively charged nucleus completely changing our notion of atoms

6 History how we discovered radiation and our model of the atom continued 1913: Bohr planetary model : Niels Bohr developed an atomic model where electrons exist in specific orbits, or electron shells a simple atomic model that explains most common chemistry, so it is still taught today. 1926: Schrodinger s electron cloud: Schrodinger developed an atomic model where electrons don t reside at any particular location, but instead have higher probabilities of being in some locations over other locations. The Schrodinger equation requires sophisticated math. 1935: Chadwick discovers the neutron. Chadwick looked specifically for neutral particle while tracking radiation and found it in In 1935, he was awarded the Nobel prize. The neutron s discovery allowed the possibility of bombarding atoms with a neutral particle which unlike protons, wouldn t be repelled. This discovery led directly to bombarding uranium atoms and nuclear energy. 1964: Gellman/Zweig discover the quark. Particle accelerators were built that allowed high energy physics experimentation that showed fundamental particles were themselves made up of even more fundamental particles.

7 Summary Models of the Atom Democritus Greek model (400 B.C.) Dalton s model (1803) Thomson s plum-pudding model (1897) Rutherford s model (1909) (more detailed notes coming on next slides) Bohr s model (1913) Charge-cloud model (present) 1803 John Dalton pictures atoms as tiny, indestructible particles, with no internal structure J.J. Thomson, a British scientist, discovers the electron, leading to his "plum-pudding" model. He pictures electrons embedded in a sphere of positive electric charge New Zealander Ernest Rutherford states that an atom has a dense, positively charged nucleus. Electrons move randomly in the space around the nucleus In Niels Bohr's model, the electrons move in spherical orbits at fixed distances from the nucleus Erwin Schrodinger develops mathematical equations to describe the motion of electrons in atoms. His work leads to the electron cloud model Dorin, Demmin, Gabel, Chemistry The Study of Matter, 3 rd Edition, 1990, page 125 QUARKS (Gellman/Zweig James Chadwick, a British physicist, confirms the existence of neutrons, which have no charge. Atomic nuclei contain neutrons and positively charged protons.

8 What is radiation? Where does it come from? Radiation is either pure energy (electromagnetic radiation), or fast moving particles Electromagnetic radiation (diagram below, diagram of types based on wavelength) Electromagnetic radiation is a result of how electricity and magnetism interact EMR has no mass and all EMR is fundamentally the same, just energy (no mass) Small wavelengths have lots of energy; larger wavelengths vibrate with lower frequencies and have small quantities of energy UV, X rays and gamma rays are energetic enough to kick out electrons and create ions (damage living tissue) UV, X rays and gamma rays are ionizing radiation, because of their high energy Fast moving particles are also energetic enough to damage living tissue Alpha radiation (particles composed to two protons and two neutrons) and beta radiation (electrons flying solo) Neutron radiation also exists, but we will only study this as part of fission (atomic bomb and nuclear energy)

9 Alpha, beta, gamma radiation Penetrating power & tissue damage Alpha particles: Relatively massive causes largest tissue damage. Large size means poor penetrating power. A sheet of paper or dead skin cells offer enough protection. Inhalation of alpha producing particles a big concern (no protection) Gamma radiation: No mass + tiny wavelength = great penetrating ability. Thick lead or concreted required for shielding. Most gamma radiation passes right through you, since wavelength is so small. Least damage of the three. Beta radiation: In between alpha and gamma for penetration and tissue damage. Clothing or a thin sheet of aluminum are enough shielding protection.

10 Is radiation harmful? Ionizing radiation is what concerns us medically (alpha, beta, gamma and UV) Visible light, microwaves, radio waves, etc. are NOT ionizing radiation Non ionizing radiation causes no direct damage to tissue We would be moles living in dark caves if we demand no exposure to ANY radiation Ionizing radiation creates ions damages tissue Exposure to small doses ionizing radiation: The body can heal the small amount of tissue damage Moderate radiation exposure taxes the body s ability to heal itself (you get very sick, but recover) Exposure to large radiation doses: too much can cause swift death Exposure to small amounts over many years: Increased cancer risk (cumulative) This is a personal risk vs. reward decision How much ionizing radiation exposure is acceptable? What is an acceptable lifestyle to you?

11 Typical Sources of Ionizing Radiation The yearly average radiation exposure is about 360 mrem For most of us: Radon is #1 source of radiation exposure Radon is a gas and can be inhaled. Radon emits damaging alpha radiation. Background radiation: The normal, low risk (low dosage) radiation (right ) Want no radiation exposure at all? Live in a darkened cave lined with thick lead walls. Even then, some radiation is present in your own body you can t escape it entirely. Choose reasonable risk vs. reward!

12 Natural radiation sources typically give us more radiation than manmade Some of these may be avoided, but to what lengths are you willing to go? Radiation man vs. nature

13 Live in an area where low to moderate Radon levels are typical but test your home Get an x ray when recommended by a health professional (x rays increase cancer risks by a tiny amount, but the medical help they give far outweighs the risk, IMO) Go outdoors once in a while (risk cosmic and terrestrial radiation) Are these reasonable risks?

14 How radon gets in. Why we care. Radon comes from a two step nuclear decay of uranium: uranium radium radon Radon is produced in the soil and often enters homes through cracks in walls, etc. Once in homes, concentrations build if homes are poorly ventilated Inhaled radon emits alpha particles leading to increased lung cancer risk

15 Geiger Counter: How is radiation detected? Presence of radiation is not among the five human senses we can detect By the time radiation made you feel sick, you might be exposed to a fatal, or nearfatal dosage A Geiger counter is commonly used to detect the presence and level or radiation Ionizing radiation ionizes a gas (it s ionizing radiation, so this is what it does!)knocking off an electron Electron moves toward positive plate; positive ion moves toward negative plate Current flows to reduce positive ion and flow of current cause a click sound

16 Understanding nuclear reactions Atom anatomy Atoms are composed of three particles and two regions Particles: Electrons, protons, neutrons Regions: Nucleus, electron shell (or orbital) The nucleus is very massive and very tiny compared to overall size of atom Electrons are spaced far apart in shells Classical chemistry (reactions) are determined by interactions of valence electrons Nuclear chemistry is determined by the nucleus (bombardment of particles and stability)

17 Isotopes review & more Isotope: Each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons An element is determined ONLY by the number or protons Isotopes have different numbers of neutrons and different mass numbers MN = P + N Why is this important? Some isotopes are very stable and some undergo nuclear reactions. Three ways to write an isotope Carbon 14 C 14

18 Nuclear reactions Nuclear reactions: Man made or natural Man made: Atomic bomb (fission), H bomb (fusion), Nuclear power (fission) Some nuclei are naturally unstable and particles are randomly emitted Ejection of these particles can change the element itself (transmutation) This process is called nuclear decay (particles are lost, hence decay) Half life The amount of time where a nuclear reaction has a 50% chance of taking place Example: Carbon 11 has a half life of 20 minutes. It decays to B g of C 11 would decay to ABOUT 60 g of C 11 and 60 g of B 11 in 20 minutes There is no way to predict exactly which C 11 atoms will decay (random, like flipping a coin) It s a one way nuclear reaction, the B 11 is stable; the remaining C 11 is still unstable In 40 minutes (the next 20 minutes, two half lives of time), half of the remaining B 11 will decay, only 30 g of C 11 remain In 60 minutes total time, half of the remaining C 11 will decay to B 11 (only 15 g of C 11 left now)

19 Half life problems The math we use in this class is simple multiplication and dividing by 2 All of the problems in this class will involve an integer number of half lives Create a table with columns of: HL, grad, %rad, gdec, time Half lives so far, grams radioactive, % still radioactive, grams decayed, time elapsed Example: 9.38 g will still be Radioactive (or 3.18%)

20 Alpha and Beta Emission (Decay): Nuclei decay to become more stable (Improve Neutron/proton ratio) Transmutation is when a new element is formed by nuclear reaction Alpha decay of Ra-226 Alpha Decay Beta decay of I-131 Beta Decay

21 Writing/balancing nuclear reactions Nuclear reactions are balanced in the same way ordinary chemical reactions are We balance them using isotopic notation for atoms and special radiation symbols (right) Reactant (left) and product sides (right) need to have mass (top part of isotopic notation) and charge (bottom part)

22 Examples of nuclear reaction problems Write a balanced equation for the alpha decay of radium 222 Write what you know: Balance the mass (222 = 4 + x, in other words, left and right sides have same mass) Balance the charge (88 = 2 + x, in other words, left and right sides have same charge) Solution: Write the equation for beta decay of C 14 Step 1: Write out given information: Balance mass and charge

23 Diagnostic Applications To Diagnose Medical Problems more than just X rays Radioisotope Tracers Can be followed through the body Radioisotope is injected into the bloodstream to observe circulation. Absorbed by specific organs Technetium 99m (A) Normal. (B) Graves disease:. (C) TMNG: hot and cold areas of uneven uptake. (D) Toxic adenoma: (E) Thyroiditis Courtesy Christy Johannesson

24 Therapeutic Application Treating a medical condition Irradiators: Destroy living tissue Radiation Treatment for cancer large doses of radioisotopes are used to kill cancerous cells in targeted organs internal or external radiation source Courtesy Christy Johannesson Radiation treatment using -rays from cobalt-60.

25 Atoms for Peace Eisenhower Show nuclear science is not evil Has good uses, too. Food irradiation radiation is used to kill bacteria Cancer treatment PET & CAT scan Destroy ANTHRAX bacteria Consumer Products ionizing smoke detectors 241 Am

26 Others Radioactive Tracers explore chemical pathways trace water flow study plant growth, photosynthesis Optimal composition of fertilizers Abrasion studies in engines and tires Courtesy Christy Johannesson

27 Radioactive Tracers

28 Nuclear Fission The splitting of an atom (E = mc 2 ) Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 620

29 Nuclear Power Plants Most of area electricity comes from it 2008

30 Nuclear Power Plant Fission of U 235 releases heat energy via E = mc 2 Heat is used to make steam, expansion of gas pushes turbine, makes electricity Cooling tower condenses steam to liquid (cycles it) Control rods absorb some of neutrons, control rate (like a gas pedal) Moderator slows down neutrons so they stick instead of bounce off Containment dome prevents accidental release of radioactive material if something goes wrong

31 Difference between fission and fusion Fusion: small nuclei fuse together making bigger nucleus releases a lot of energy Fission: large nucleus splits, leaving two smaller nuclei releases lots of energy, but not as much as fusion Fusion powers the sun and we use it to make the more recent H bomb; needs higher temperatures than we can control, so we can t use it to make electricity; the sun s fusion combined hydrogen nuclei to make helium Fission powers nuclear power plant, was used to make the atomic bomb we used on Japan in WWII, and creates radioactive nuclear waste products Elements smaller than Fe release energy with fusion; elements larger than iron, release energy with fission All of the elements bigger than Li were made in stars via fusion after the Big Bang (original universe contained no elements larger than Li)

32 Three mile island vs. Chernobyl (comparison) Chernobyl Meltdown of nuclear core Killed people (49 immediate deaths and a few thousand deaths attributed to a 3% increase in regional cancer rates blamed on increased radiation levels) Explosion with widespread radiation No confinement vessel Released Plutonium (half life of hundreds of yrs) Three Mile Island Partial meltdown of nuclear core No deaths (very small increases in radiation levels meant no immediate deaths; probably about 1 to 2 additional cancer deaths within 10 mile radius according to one source) No explosion Confinement vessel (Containment dome) No long lived release of radiation