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Unit 3 – Atomic Structure and Nuclear Chemistry Accelerated Chemistry I Nuclear Symbols Element symbols are often accompanied by notation that gives information on atomic composition. The subscript, written to the lower left of an element symbol, represents the atomic number. The superscript, written to the upper left of the symbol, represents the mass number or the total number of protons and neutrons. Using this information, complete the following table. Symbol 12 C 6 40 18 Ar 127 I 53 23 11 Na 20 10 Ne 48 22 Ti 40 20 Ca 238 U 92 Atomic # Mass # # of protons # of neutrons # of electrons 1. How many protons and electrons are in a neutral vanadium atom? 2. How many protons and electrons are in a neutral potassium atom? 3. How many protons and electrons are in a neutral platinum atom? 4. What is the name of the element that contains 17 protons? 5. What is the name of the element that contains 82 protons? 6. Write the complete chemical symbol for the atom that contains 84 protons, 125 neutrons and 80 electrons. 7. Write the complete chemical symbol for the atom that contains 27 protons, 32 neutrons and 25 electrons. 8. Write the complete chemical symbol for the atom that contains 73 protons, 108 neutrons and 68 electrons. Unit 3 – Student Handout 1 9. Write the complete chemical symbol for the atom that contains 31 protons, 39 neutrons and 28 electrons? Complete the following table: 59 Symbol 2+ 140 Ni 28 3+ Ce 58 91 Zr 4+ 79 40 2– Se 34 14 4– C 6 45 3+ Sc 21 Protons Neutrons Electrons Hydrogen Isotopes Atoms are made up of subatomic particles, such as protons, neutrons and electrons. The nuclei of atoms that make up isotopes of an element differ. There are three known isotopes of hydrogen. Make a drawing that represents each of these isotopes. e 2p 2n e Helium-4 Protium Hydrogen-1 Deuterium Hydrogen-2 Tritium Hydrogen-3 1. Do the numbers of electrons for neutral isotopes of the same elements differ? ________ 2. Do the number of protons for such isotopes differ? __________ 3. Do the number of neutrons for such isotopes differ? __________ 4. Do the atomic numbers for such isotopes differ? Explain. 5. Do the mass numbers for such isotopes differ? Explain. Unit 3 – Student Handout 2 6. If (1 + 2 + 3)/3 = 2 then why is the average atomic mass for hydrogen 1.0079 and not a whole number? Review Activity - Development of Atomic Theory atom atomic number Bohr Chadwick Conservation of Matter Dalton Definite Proportions Democritis Einstein electron energy level isotope Lavoisier mass number nucleus neutron Multiple Proportions Planck proton Proust quantum Rutherford Thomson subatomic particle Thomson More than 2000 years ago, a Greek philosopher named _____________ proposed the existence of very small, indivisible particles, each of which was called a(n) _____________. The theory that such particles existed was supported much later, by _____________ who proposed, in his law of _______________ _____ __________, that matter could not be created or destroyed. Then ___________ proposed, in his law of ____________ _____________, that the ratio of the masses of elements in any given compound is always the same. The law of _____________ ______________ , proposed soon after, states that the masses of one element that combine with a fixed mass of another element in different compounds are in simple, whole-number ratios. An atomic theory based on these laws was developed by _____________, who is credited with the first modern atomic theory It was later proposed that the atom is in fact divisible into smaller parts each of which is called a(n) ___________ ____________. These particles include the negatively charged ____________, discovered by ____________; the positively charged ____________; and the uncharged ____________, discovered by ____________. The latter two particles are present in the ____________, or center of the atom, which was discovered by ____________ in his gold foil experiments. Unit 3 – Student Handout 3 The number of positively charged particles in an atom is called its _________ __________. The total number of positively charged particles and the uncharged particles is called the atom’s _________ ________. An atom that has the same number of positively charged particles as another atom, but a different number of uncharged particles, is called a(n) ____________. Average Atomic Mass Problems 1. What is the average atomic mass, in amu, of the element copper if it is composed of 69.5% of an isotope of atomic mass 63.00 and 30.5% of an isotope with an atomic mass of 65.00? Consider sig figs when expressing your answer. – Answ: 63.6 amu 2. Calculate the average atomic mass of lithium, which occurs as two isotopes that have the following atomic masses and abundances in nature: 7.30%, 6.017 amu and 92.7%, 7.018 amu. Consider sig figs when expressing your answer. – Answ: 6.95 amu 3. Calculate the average atomic mass of chromium, given the following percent abundances and atomic masses: 4.350% 49.946 amu; 83.790% 51.941 amu; 9.500% 52.941 amu; 2.360% 53.939amu. Consider sig figs when expressing your answer. – Answ: 51.996 amu Unit 3 – Student Handout 4 4. The four isotopes of lead are listed below, each with its percent abundance. Calculate the average atomic mass of lead. – Answ: 207.2 amu Isotope #1 - 82 protons, 122 neutrons - 1.37% Isotope #2 - 82 protons, 124 neutrons - 26.26% Isotope #3 - 82 protons, 125 neutrons - 20.82% Isotope #4 - 82 protons, 126 neutrons - 51.55% Nuclear Chemistry Radiation Unstable atoms undergo nuclear changes in which particles and energy are emitted. All the emissions are called radiation, even though some of the emissions are particles. Complete the following table by identifying whether each type of radiation can originate from nuclear decay and whether it is ionizing radiation. Include the symbol and charge of the radiation if appropriate. Radiation Nuclear Origin? Ionizing Radiation? Greek Symbol Nuclear Symbol Charge 1. alpha 2. microwave 3. beta 4. X-ray 5. gamma Balancing Nuclear Equations Balanced equations are written for nuclear reactions as well as for chemical reactions. In a nuclear equation, the totals of the atomic numbers on the left and right sides of the equation must be equal. The totals of the mass numbers must also be equal. Fill the blank spaces in the following equations for nuclear reactions. 6. 238 92 U + 14 7 N → _______ + 5 01 n 7. 238 92 U + 21 H → _______ + 2 10 n 8. ________ + 14 7 N → Unit 3 – Student Handout 24 12 Mg + 0 −1 9. 14 7 N + 10. 21 H + e 11. 94 Be + 4 2 3 1 He → ________ + H → 4 2 4 2 1 1 H He + ________. He → ________ + 1 0 n 5 Alpha decay _____ decay Alpha Decay An alpha particle has a charge of _________ and a mass of ________. When an alpha particle is emitted the atomic number of the element is ______________ by _________ and its mass number is ______________ by _________. 12. Write the alpha decay of the following elements: a. Lead-214 c. Thorium-230 b. Radium-226 Beta Decay Beta particles are _____________ charged and have identical properties to those of a(n) ______________. Beta emission converts a(n) ________________ in the nucleus to a(n) _______________. When a beta particle is emitted, the atomic number of the element is ______________ by _________ and its mass number is _________________. 13. Write the beta decay of the following elements: a. Thorium-234 b. Bismuth-210 Radioactive Decay Series. A series of nuclear reactions that begins with an unstable nucleus (a parent), produces a series of daughters, and ends with a stable nucleus, is known as a radioactive decay series. Three such series occur in nature. U-238, an unstable isotope of uranium, undergoes a series of 14 transmutations, turning into a different nuclide with each one. Complete the chart by identifying the missing symbols or types of decay. 238 92 ___ 83 214 ___ Alpha decay U !! ! !!→ 234 90 Beta decay Th !! !! !→ ? ___ ←! !decay !!! 214 ___ Alpha decay ___ ←! ! !!! Alpha decay Po !! ! !!→ Unit 3 – Student Handout 210 ___ Beta decay ___ !! !! !→ 234 91 ___ ___ Beta decay Alpha decay 234 Pa !! U !! !! !→ 92 ! !!→ 218 84 ? Po ←! !decay !!! ___ ___ Alpha decay Rn ←! ! !!! Alpha decay ___ ___ !?! !decay ___ !! ! !→ 84 ! !!→ 230 90 Th ___ ___ ___ ___ ___ ___ 6 Transmutations Nuclear reactions can be induced when a nucleus is struck by a neutron or by another nucleus. Bombarding particles include the neutron whose symbol is __________ and alpha particles whose symbol is ___________. Charged particles can be accelerated to high speeds for bombardment in ______________ and ______________ fields. When a charged particle has high velocity, it possesses enough energy to bring about a nuclear reaction despite _____________ with components of the atom. Balance the following transmutation reactions: 1. Bombardment of cobalt-59 with a neutron produces cobalt-60. 2. Bombardment of Plutonium-239 with a neutron produces americium-240 and one other particle. 3. Plutonium-239 can be produced along with three other particles by bombarding uranium-238 with an alpha particle. 4. With what particle would you bombard sulfur-32 to produce hydrogen-1 and phosphorus-32? Write the nuclear equation. 5. With what particle would you bombard bismuth-209 to produce astatine-211 and 2 neutrons? 6. A long cherished dream of alchemists was to produce gold from cheaper and more abundant elements. This dream was finally realized when 198 80 Hg was converted into gold by neutron bombardment. What one other particle is produced? Unit 3 – Student Handout 7 Half-Life Cesium-137 is a radioactive isotope produced during fission reactions. It undergoes beta decay into 0 137 barium-137 as represented by the equation: 137 55 Cs 56 Ba + −1 e The half-life for this disintegration is approximately 30 years. This is the amount of time required for half the atoms in a sample to undergo decay. Assume that a 64-gram sample of Cs-137 is analyzed every 30 years for a 150 –year period. Calculate the grams of cesium and barium present each time the sample is analyzed and record the data in the table below. Note how the fraction remaining of the original isotope is (0.5) raised to the t1/2 power. This relationship holds even if the elapsed time is not a whole number of half-lives. For example, the fraction of Cs-137 fraction of Cs-137 remaining after 45 years (1.5 half lives) is 0.51.5 = 0.354; the amount remaining is 0.354×64 g = 22.6 g. Time # t1/2 f(Cs remaining) Mass Cs-137 Mass Ba-137 0 yrs 0 (½)0= 1 64 g 0g 30 yrs 1 (½)1 = ½ = 0.51 60 yrs 90 yrs 120 yrs 150 yrs 1. A sample that contained 24 grams of C-14 when alive now as a fossil contains 1.5 grams. How old is the fossil? [t1/2 (C-14) = 5730 yrs] – Answ: 2.3x104yrs 2. A 64 gram sample of germanium-66 is left undisturbed for 12.5 hours. At the end of this period, only 2.0 grams remain. What is the half-lie of germanium-66? – Answ: 2.5 hrs 3. With a half-life of 28.8 years, how long will it take for 1.00 g of strontium-90 to decay to 125 mg? – Answ: 86.4 yrs 4. Cobalt-60 has a half-life of 5.3 years. If a pellet that has been in storage for 26.5 years contains 14.5 grams of Cobalt-60, how much Cobalt-60 was present when the pellet was put in storage? – Answ: 464 g Unit 3 – Student Handout 8 5. A 1.000 kg block of phosphorous-32, which has a half-life of 14.3 days, is stored for 100.1 days. How much phosphorous-32 remains at the end of this period. – Answ: 7.813 g 6. A sample of air is collected from a basement to test for the presence of radon-222, which has a half-life of 3.8 days. However, delays prevent the sample from being tested until 7.6 days later. Measurements indicate the presence of 6.5 µg of radon-222. How much radon-222 was present in the sampe when it was initially collected? – Answ: 26 µg 7. The half-life of sodium-25 is 1.0 minutes. If you start with 1.00 kg. How many grams will remain after 3.75 minutes? – Answ: 74.3 g 8. A 0.500 kg sample of iodine-131, which has a half-life of 8.0 days, is prepared. After 42 days, how much iodine is present? – Answ: 13 g 9. What is the half-life of polonium-214 if, after 825 seconds a 1.0 grams sample decays to 31.25 mg? – Answ: 170 s 10. What is the length of the half-life in minutes of an isotope if after 1.00 hours 20% of the starting material remains? – Answ: 25.9 min 11. A sample originally contained 50 µCi (microcuries) of P-32. Today it contains only 15 µCi. If the half-life of P-32 is 14.3 days. How old is the sample? – Answ: 24 days 12. The FAA considered thermal neutron analyzers to detect bombs in passenger baggage. This analyzer bombards baggage with low-energy neutrons, converting N-14 nuclei to N-15, with simultaneous emission of γ rays. Because explosives often are high in nitrogen, detection of γ-rays would suggest that a bomb may be present. Write an equation for this nuclear process. Unit 3 – Student Handout 9 Nuclear Fission and Nuclear Power Plants 1. Explain the composition of fuel rods. 2. Explain the role of control rods in a nuclear reactor. 3. Outline the process by which electricity is produced as a result of nuclear fission in a nuclear reactor. 4. What role does water play in the reactor core? What is the difference between a boiling water reactor and a pressurized water reactor? 5. Why are “spent” fuel rods stored in pools after being removed from the reactor core? 6. How are “spent” fuel rods stored long-term? Why are they stored in this manner? 7. What caused the Fukushima reactor meltdown in Japan? Radiation in Everyday Life 1. Name two radiotracer isotopes and what they are used for. Unit 3 – Student Handout 10 2. Explain a way that radiation can be used in treating cancer. 3. Explain two practical industrial uses for radiation. 4. What is the SI unit for measuring radioactivity? 5. Why is radiation exposure often described in REM rather than rads? 6. What is the average annual dose of radiation for US citizens? How much of this is from natural sources such as cosmic radiation and radioisotopes in minerals? What is the largest man-made contributor? 7. Explain the difference between ionizing and non-ionizing radiation. Why is ionizing radiation harmful to the body? 8. How can our bodies naturally fight the effects of radiation exposure? 9. What causes the audible “click“ or “beep” made by a Geiger counter? Challenge Problems 1. Antimony has two naturally occurring isotopes. 121 Sb has an atomic mass of 120.904 amu; 123 Sb has an atomic mass of 122.904 amu. Find the average atomic mass of antimony on your Periodic Table. What are the relative abundances of these isotopes in nature? Answ: 57.20% Sb-121, 42.80% Sb-123 Unit 3 – Student Handout 11 2. How long will it take for 6.00 x 1020 atoms of Zn-71 to decay until only 6.00 x 1010 atoms remain? (t1/2 Zn-71 = 2.45 min) – Answ: 81.3 min 3. The radioisotope cesium-137 has a half–life of 30 years. Decay rate is proportional to the amount of radioactive material present. A sample decays at the rate of 544 counts/min (cpm) in 1995. In what year will the decay rate be 27 cpm? – Answ: 130 yrs, 2125 4. Explain why the decay of C-11 into B-10 appears to be a valid reaction: 116 C 105 B + 11 H. Show that this reaction actually violates Einstein’s equation and is not likely to occur. The masses of the nuclides are C-11 = 11.0114 amu, B-10 = 10.0129 amu, H-1 = 1.007 amu. – Answ: Δm = +0.009 amu, The mass of the products are greater than the mass of the reactants, which violates Einstein’s E=mc2 equation. Unit 3 – Student Handout 12