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Transcript
Nuclear Chemistry DHS Chemistry Chapters 4 and 25 Little House on the Prarie Review: Structure of an Atom Structure of an Atom An atom consists of three types of subatomic particles, protons, neutrons, and electrons. Structure of an Atom • Protons and neutrons are located in the nucleus • Electrons are in a cloud surrounding the nucleus. • The number of protons is equal to the atomic number. Structure of an Atom If the atom is neutral, the number of electrons is equal to the number of protons. The mass number is equal to number of protons + number of neutrons. Structure of an Atom • Elements with the same number of protons, but have different numbers of neutrons is called an isotope. Structure of an Atom Protons Neutrons Electrons Symbol P+ no e- Charge + 0 - Location Nucleus Nucleus Electron cloud Relative mass 1 amu 1 amu negligible What’s a chemical reaction? Chemical Reactions Why Call it Nuclear Chemistry? See unitedstreaming.com Chemical VS. Nuclear • Nuclear reactions are caused from unstable nuclei becoming stable through radioactive decay. • Releasing particles and high-energy waves • Alters the number of nuclear particles (neutrons and protons). • Nuclear reactions are very different from chemical reactions. Chemical Changes 1. Occur when bonds are broken and formed. 2. Atoms remain unchanged, though they may be rearranged. 3. Involve only valence electrons. 4. Associated with small energy changes. 5. Reaction rate is influenced by temperature, pressure, concentration, and catalyst. Nuclear Changes 1. Occur when nuclei emit particles and/or rays. 2. Atoms are often converted into atoms of another element. Thus their identity changes. 3. May involve protons, neutrons, and electrons. 4. Associated with large energy changes. 5. Reaction rate is not normally affected by temperature, pressure, or catalysts. What is an isotope? Isotope Examples 6 Mass Numbers Li 3 Atomic # = protons Lithium - 6 7 Li 3 Lithium - 7 Isotopes • Isotopes are atoms of the same element that vary in their number of neutrons, thus they have different mass numbers. • The convention for writing isotope names is first the element name dash and then the mass number. For example: Carbon-14, carbon-12 carbon-13 How many protons and neutrons does each of the isotopes of carbon have? • Carbon – 14 • Carbon – 12 • Carbon – 13 8 neutrons, 6 protons 6 neutrons, 6 protons 7 neutrons, 6 protons Mass Number (protons + neutrons) More on Isotopes • When an isotope is unstable it is called a radioisotope. • To gain a more stable configuration, the nuclei emit radiation. • The resulting stable atom is called the daughter product. • This is called radioactive decay. Strong Nuclear Force • The dense nucleus has two different kinds of nuclear particles (protons + neutrons) closed packed together. • The protons are positively charged nucleons. • The neutrons are neutral nucleons. • All of the protons in the nucleus repel each other and cause an electrostatic force that pushes the nucleus apart. • However, there is a force holding the nucleus together. • The strong nuclear force is a force that acts only on subatomic particles that are extremely close together. • If the strong nuclear force overcomes the electrostatic force, the nucleus stays together. Neutron to Proton Ratio • The strong nuclear force is not always strong enough to overcome the electrostatic force. When this happens the nucleus breaks apart. The stability of the nucleus can be determined by the ratio of neutrons to protons.(n/p) Neutron to Proton Ratio • Elements with low atomic numbers (< 20) are most stable when the neutron to proton ratio is 1:1. • As the atomic number increases, more and more neutrons are needed to overcome the electrostatic force. • Thus, the stable ratio of neutron to proton increases as the atomic number increases. • 1.5:1 is the largest ratio for a stable nucleus. Example: Determine the neutron to proton ratio for Lead-206. Is it stable? Lead – 206 82 protons 124 neutrons (206 – 82) 124 = 1.51 stable 82 1 ratio Nuclear Stability Neutron : Protons Stable if: • Smaller than Fe ~1 1 • Between Fe & Pb 1-1.5 1 • Bigger than Pb 1.5 1 Extra Practice • Calculate the neutron to proton ratio, and determine if the isotope is stable. 1. 2:2 1:1 stable 201:92 2.18:1 unstable 3. Lead-206 stable 124:82 1.5:1 2. U- 293 The Band of Stability • The graph on the right plots the neutrons versus protons. The band created is called the band of stability. Anything that falls outside of that band is radioactive. • All elements with an atomic number greater than 83 are radioactive. III. Radioactivity Radioactive Substances • Bananas • Atomic fire ball candies • Colored gemstones (blue topaz) • Fiesta Ware • Table ware that contained unsafe amounts of radioactive Uranium • Uranium for color glaze. Up to 14% can be Uranium Geiger Counter Types of Radiation The experiment Types of Decay • Nuclear reactions change an atom of one element to an atom of another element. This process is called transmutation. In a nuclear reaction there are three common types of radiation that are emitted: alpha, beta, and gamma. The first two are involved in transmutation, changing the identity of the atom. Properties of Alpha, Beta, and Gamma Radiation Property Alpha(α) Beta (β) Gamma (γ) Alpha particles Beta particles High-energy electromagnetic radiation Composition Helium Nuclei Electrons Photons Charge 2+ 1- 0 Relative Mass heaviest lightest 0 Relative Penetrating Power Blocked by paper Blocked by metal foil Not completely blocked by lead or concrete Description of Radiation • Emit means to give off or release Alpha Particles • Alpha particles contain the same composition as a helium nucleus. Out of all of the radiation particles, alpha particles move the slowest and are the least penetrating. As a result of alpha decay, the mass number decreases by 4 and the atomic number decreases by 2. 4 2 He or 4 2 Alpha decay Beta Particles • Beta particles are similar to an electron except they come from the unstable nucleus of the atom. Beta particles are formed and ejected when a neutron decays to a proton and an electron. The proton stays in the nucleus and the electron is the beta particle. Beta emission is a constant flow of quick moving electrons that can be stopped by a metal foil. As a result of beta decay the atomic number increases by one. The mass number does not change. Gamma Rays • Gamma rays are short wavelengths (photons) that move the quickest of all the types of decay. They are very highenergy electromagnetic radiation. These rays often are released at the same time as an alpha or beta particle. The ray is the energy lost in the reaction. Gamma emission does not affect the atomic number or the mass number of the isotope. Gamma Rays Are very dangerous!! (write that down) C. Gamma ( ) Emission • usually occurs along with other forms of radiation. • Gamma particle is emitted • No change in mass number • No change in atomic number Practice Problems 1. Why is radiation given off? 2. What is the most penetrating particle? 3. What is a main difference between a nuclear reaction and a chemical reaction? 4. Is Carbon-14 radioactive? Why or Why not? III. Balancing Nuclear Reactions Balancing Nuclear Reactions • In nuclear reaction equations we account for all of the changes in the mass number and atomic mass that occur through the decay of the nucleus. • To verify this, we include the mass number and atomic mass of every particle or atom involved in the reaction. Mass number Atomic number Mass number 4 2 He Atomic number 0 1 Solving Problems • When solving/balancing a nuclear reaction, • Look to find the difference of the mass numbers and atomic numbers between the reactants and the products. • This will indicate the particle that was released or the atom that was formed. • Make sure you have the same total mass number and atomic number on both sides of the equation. Nuclear Equations Nuclear Reactions Ex 1: Write a balanced equation for the alpha decay of polonium-210 210 210 Po Po 84 84 206 206 4 Pb + He Pb 82 82 2 Ex 2: Write a balanced equation for the beta decay of carbon-14 Practice Fill in the blank with the proper radiation particle or isotope 120 120 1) Cs Ba + ____ 55 2) 150 64 3) 241 95 4) Gd Am _____ 56 146 62 Sm + _____ 4 2 _____ + He 210 81 Tl + 4 2 He Examples • Ex 1. Write an equation for the alpha decay of Protactinium-231 • Ex. 2 Bi ____ Tl 211 83 207 81 III. Radioactive Decay Rates Radioactive Decay Rates • A. Half-Lives • We measure radioactive decay in terms of half- lives. • A half life is the time it takes for half of a radioactive sample to decay. Candy Bar Bandits • There is a candy bar left in the teacher’s lounge. Every 5 minutes a teacher walks in, looks at the candy bar, breaks the candy bar in half and eats it. If the candy bar originally had a mass of 20 grams, how much is left after 4 teachers have a taste? Keeping Track 0 1 2 3 4 5 6 7 8 A T 20g 10g 5g 2.5g 1.25g 0 5 min 10 min 15 min 20 min Candy Bar Keeping Track 0 1 2 3 4 5 6 7 8 A T 20g 10g 5g 2.5g 1.25g 0 5 min 10 min 15 min 20 min Example • The half life of Carbon-14 is approximately 5730 years. If you had 12g of Carbon-14 today, in 5730 years you would only have 6g. The missing 6g decayed and turned into Nitrogen 14. And, in another 5730 years you will have 3g of Carbon-14 left and then in 5730 more years you would only have 1.5g of Carbon-14 left. Every 5730 years your sample is cut in half. Practice • What is the half-life of the sample in the graph? ~11 billion years Half-Lives # of HLs Remaining mass Remaining mass Ratio of remaining mass to original mass 1 Original Mass x (1/2) Org. Mass x (.5)1 0.5 2 Original Mass x (1/2) x (1/2) Org. Mass x (.5)2 0.25 3 Original Mass x (1/2) x (1/2) x (1/2) Org. Mass x (.5)3 0.125 4 Original Mass x (1/2) x (1/2) x (1/2) x (1/2) Org. Mass x (.5)4 0.0625 5 Original Mass x Org. Mass x (.5)5 0.03125 Original Mass x Org. Mass x (.5)6 0.015626 (1/2) x (1/2) x (1/2) x (1/2) x (1/2) 6 (1/2) x (1/2) x (1/2) (1/2) x (1/2) x (1/2) In the box Amount Remaining = initial amount (1/2)HL = initial amount (0.5HL) HL = # of half lives Total Time Passed = # of Half Lives Time of one HL Ex. 1 If gallium-68 has a half-life of 68.3 minutes, how much of a 10.0 mg sample is left after 342 minutes? HL A T 0 1 2 3 4 5 6 10mg 5mg 2.5mg 1.25mg .625mg 0.3125 0 68.3 136.6 204.9 273.2 341.5 0.3125 mg Ex. 1 If gallium-68 has a half-life of 68.3 minutes, how much of a 10.0 mg sample is left after 342 minutes? Amount Remaining = (Initial amount) (0.5) n n = # of half lives that have passed 342 total minutes / 68.3 minutes = Ans = 0.3125 mg half lives More Examples Ex. 2 If the passing of 116 years leaves 25.00 mg of an original 400 mg sample of Strontium90, what is the half life of Strontium-90? 116/4 = 29 years HL 0 1 2 3 4 5 6 7 8 A T 400 0 200 100 50 25mg 116y Don’t get it? Here it is in words. • If you are trying to solve for the amount of element left, divide the time passed from the half life and that will give you the amount of half lives your sample has had. You can then divide your original mass of sample by 2 as many times as you have half lives. • If you are trying to solve for the half life of your sample, take the remaining mass and count how many times you have to multiple it by 2 to get your original mass. That will tell you how many half lives have elapsed. Take that number and divide it by the total time that has elapsed to get the length of just one half life. More on Half-lives • Because the half-life is constant, radioisotopes can be used to date objects. • Radiochemical dating is the process of determining the age of an object based on the amount of a particular radioisotope is remaining in the object. Carbon Dating • Carbon dating is a specific type of radiochemical dating • All living organisms have the same ratio of carbon-14, carbon-13, and carbon-12 as the in atmosphere. • However, once an organism dies and there is no new carbon intake, the unstable carbon-14 starts to break down. Practice HL A 0 1 2 3 4 5 6 7 8 2000 1000 500 250 1. Iron-59 is used in T medicine to diagnose blood circulation 0 disorders. The half44.5 life of iron-59 is 44.5 89 days. How much of a 2000 mg sample will 133.5 remain after 133.5 days? 250 mg of the sample Practice • After 2 years, 1.99 g of a radioisotope remains from the sample that had an original mass of 2.00 g. What is the half-life of this isotope? 200 years More on Carbon dating • Scientists calculate the ratio of carbon12 and carbon-13 to carbon-14 in dead organisms. • Then they compare the ratio to the atmospheres ratio and determine how many half-lives have passed. • With that information they can calculate how long the organism has been dead. More Practice Solving Half-Life Problems 1. Strontium’s half life of the radioisotope strontium-90 is 29 years. If you had 10.0g of strontium-90 today, how much would you have left after 87 years? 1.25g 2. If you start off with 64 grams of some substance, how much will you have after 3 half lives? 8g 3. Iodine-131 has a half life of 8 days. What fraction of the original sample would remain at the end of 32 days? 1/16 1) Strontium’s half life of the radioisotope strontium-90 is 29 years. If you had 10.0g of strontium-90 today, how much would you have left after 87 years? Sr-90 half-life = 29 years How many half-lives is 87 years? 87years ÷ 29 years = 3 half-lives 1 half-life How much is left after 3 half-lives? 10.0g 5.0g 2.5g 1.25g start after 1 half-life after 2 half-lives after 3 half-lives 1) Strontium’s half life of the radioisotope strontium-90 is 29 years. If you had 10.0g of strontium-90 today, how much would you have left after 87 years? HL A T 0 1 2 3 4 5 6 10g 5g 2.5g 1.25g 0 29 58 87 2) If you start off with 64 grams of some substance, how much will you have after 3 half lives? HL A 0 1 2 3 4 5 6 64g 32g 16g 8g T 3) Iodine-131 has a half life of 8 days. What fraction of the original sample would remain at the end of 32 days? HL A T 0 1 2 3 4 5 6 1/1 1/2 1/4 1/8 1/16 0 8 16 24 32 IV. Nuclear Energy A. Fission Fission • Fission means to break apart. Nuclear fission occurs when a nucleus splits apart into different fragments. • This generally occurs with atoms that have a mass number heavier than 60. • The nuclei do not always split the same way. Scientists have found 200 different products from the fission reaction of Uranium-235. More on Fission • Another important factor of fission reactions is that they cause a chain of reactions. • The products of the initial reaction can collide with other molecules and cause a new fission reaction to occur. • This domino affect could go on for a long time. This is how an atomic bomb works. Fission Reaction Nuclear Power • Nuclear power plants harness the energy released in fission reactions and turn it into electricity. • One of the main issues the power plant has to deal with is keeping the chain reactions going, but not letting them speed out of control. To this date there have been two large nuclear accidents. Nuclear Power • Nuclear power plants have to be very precise in their regulations of the reactions. Some of the products of the fission reaction are extremely radioactive. • To ensure safety of all living things, the waste must be properly stored. • It can take up to twenty half-lives for such radioactivity to reach levels safe enough for exposure. • For some waste products this can be thousands of years. Fusion Fusion • Fusion means to come together. • Nuclear fusion is the combining of atomic nuclei. • In fusion reactions, scientists bring together nuclei of atoms that have mass numbers less than 60. Fusion • This kind of a reaction also releases large amounts of energy. • It useful to know, that the sun is powered by fusion reactions.