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Chapter 4 Atomic Structure Warm – Up We have not yet discussed this material, but what do you know already?? • What is an atom? • What are electron, neutrons, and protons? • Draw a picture of an atom from what you know today. Today’s Agenda • QOTD: How did we come to understand the atom? • Historical journey of the atom • Scientists and experiments • Atomic Structure • No homework tonight, but warning, you’ll have some this weekend… History of the Atom • 1. Democritus vs. Aristotle pg. 102-103 • 2. John Dalton and conservation of mass pg. 104-105 • 3. Cathode ray tube and Sir William Crookes pg. 107-108 • 4. Mass and charge of electron (J.J. Thompson) and oil drop experiment pg. 108109 • 5. Plum pudding model vs. Rutherford’s experiment pg. 110-112 Democritus • Greek philosopher who asked questions about matter. – Can you divide matter infinitely? • Democritus says no! – Tiny particles called atoms, indivisible! • Matter is composed of atoms, which move through empty space. • Atoms are solid homogeneous, indestructible, and indivisible. • Atoms have different sizes and shapes. These properties, and movement determine properties of matter John Dalton • Matter is composed of small particles called atoms that are indivisible and indestructible. • Atoms of a given element are identical in size, mass, and chemical properties, and are different from those of another element. • Different atoms combine in simple whole number ratios to form compounds. • In a chemical reaction, atoms are separated, combined or rearranged. The Atom • EXTREMELY small particle of an element that retains the properties of that element is an atom. • If atom is the size of an orange, an orange would be the size of the EARTH Subatomic Particles - Electron • Cathode Ray Tube Cathode Thin beam of electrons Vacuum Anode travels from cathode to anode! • Cathode rays are a stream of charged particles. Particles carry a negative charge…now called electrons! J.J. Thompson • Determined that the mass of the charged particle (electron) was much less than that of the hydrogen atom. – Dalton was WRONG about the atom being the smallest particle! Dalton’s Inaccuracies • Atoms are not the smallest type of matter! – Subatomic particles – electrons, protons, and neutrons • Atoms of the same element can have slightly different masses! - isotopes Millikan Oil-Drop Experiment • Determined the charge of an electron. Charge up the oil particles with electrons. Change the electric field changes the rate of oil droplets! (MOVIE!) Charge of electron 1.602 x 10-19 coulombs Mass of electron = 9.1 x 10-28 grams Plum Pudding Model • Matter isn’t all negatively charged, so how do we have negatively charged subatomic particles without positively charged ones?? • J.J. Thompson thought an atom was a positively charged sphere with electrons hanging out within. Warm Up • What were Dalton’s inaccurate theories. Why are they wrong? • What were the correct ones? • What was JJ Thompsons model for the atom? Agenda • Question of the Day: What is an isotope and how do we write an isotopic symbol? • Finish history lesson • Periodic table and isotopes • Ions • Homework is actually due tomorrow…I forgot that we started on a Tues…sorry! Rutherford and the Nucleus • Experiment proved that plum pudding model was incorrect! • Atom is mostly empty space through which ecan move. Almost all of the positive charge and atomic mass resides in the center – NUCLEUS! Nucleus is positively charged to deflect alpha particles and to balance electron charge. Subatomic Particles • Electron – VERY tiny, negatively charged • Proton – located in the nucleus, charge opposite of an electron (positive!) • Neutron – located in the nucleus, same mass as a proton, neutral! Atomic Theory Today • Quantum Mechanical Model • All atoms are made up of electrons, protons, and neutrons. Electrons are located outside of the nucleus, protons and neutrons are located inside the nucleus. • Electrons exist in a cloud surrounding the nucleus. Attracted to the nucleus so they hang around! • Nucleus accounts for 99.97% of the atomic mass, and occupies a VERY small volume. • A neutral atom has the same number of electrons and protons! Current Atomic Model Neutral atom: # Protons = # Electrons Simulation!! Warm Up • Which of John Dalton’s 4 theorems are wrong and why? • In what experiment was the electron discovered? • What was Rutherford’s contribution? • Describe the current model of the atom. Today’s Agenda • Question of the Day: What is an isotope and how do we write an isotopic symbol? • Periodic table and isotopes • Ions • Average atomic mass • Homework Ch 4 58-74 evens due Monday (they’re not too bad) Properties of Atoms Atomic # # of Protons = # of Electrons (in neutral atom) Atomic mass # Protons + # neutrons Practice What is the isotopic symbol for each? Isotopes and Ions • Isotope – Atoms with the same number of protons but different number of neutrons. • Things to remember – – The # of protons of an element NEVER changes, and is ALWAYS the same as the Atomic #. – If the # neutrons is different = ISOTOPE – If the # electrons is different = ION • + = cation Less electrons • - = anion More electrons 70 Isotopic symbol : 32 Ge Warm Up! Element Calcium Oxygen Mercury Atomic # 20 8 80 Mass # 46 17 204 What is the number of protons, electrons, and neutrons for each? What is the isotope symbol (shorthand notation) for each? Today’s Agenda • Question of the day: How do we calculate the average mass of an element? • Finish table from yesterday • Shorthand notation • Average atomic mass calcs! • Ch 4 58-74 evens due tomorrow (they’re not too bad) LEAVE LAB NTBKS tonight!! Al lab! Fill in the table! Name Symbol Atomic # K Atomic # # # Mass protons neutrons electrons 40 Boron Br Sodium Nitrogen Atom/ cation/ anion Atom 6 5 45 36 24 10 8 10 What is the shortand notation (isotopic symbol with charges if needed!) Warm Up! Element Calcium Oxygen Mercury Atomic # 20 8 80 Mass # 46 17 204 Charge 2 cation 2 anion 0 atom What is the number of protons, electrons, and neutrons for each? What is the isotope symbol (shorthand notation) for each? Today’s Agenda • Question of the day: How do we calculate the average mass of an element? • Average atomic mass calcs! • Homework problems • Homework: Quiz review worksheet. Quiz Wednesday on Ch 4. SSW lab due Wednesday. Mass of Atoms • Mass of electron = 1/1840th of a proton • Mass of proton ≈ mass of neutron • 1 atomic mass unit (amu) ≈ mass of proton Carbon 12 atom = 12 amu Why aren’t the masses of elements in whole numbers? Atomic Mass = Average of Isotopes • Weighted average mass – mass of each isotope contributes to total mass according to how much of that isotope exists. Amu = ((Mass of Isotope1) x (Relative Abundance1)) + ((Mass of Isotope2) x (Relative Abundance2))… Amu = ((M1) x (RA1)) + ((M2) x (RA2)) + ((M3) x (RA3))… Atomic Mass = Average of Isotopes • Given info: K Potassium Three isotopes = 39 K 19 Percent Composition: 93.26% K 19 40 0.01% K 41 19 6.73% What is the atomic mass (AMU)?? Calculate the Atomic Mass of K 1. Use % composition and convert to relative abundance decimal (divide by 100) 93.26% composition = .9326 relative abundance 2. Amu = ((Mass of Isotope1)x(Relative Abundance1)) + ((Mass of Isotope2)x(Relative Abundance2))… ((0.9326)x(39)) + ((0.0001)x(40)) + ((0.0673)x(41)) = 39.1347 amu Practice • What element is this? Isotope Mass of Isotope Percent abundance 6X 6.015 amu 7.59% 7X 7.016 amu 92.41% Find atomic mass and ID the element (from table)! • Boron has two isotopes: Boron-10 (% abundance – 19.8%, mass = 10.013 amu) and Boron-11 (% abundance – 80.2%, mass – 11.009 amu). Calculate the atomic mass of Boron. Warm Up • What element is this? Isotope Mass of Isotope Percent abundance 6X 6.015 amu 7.59% 7X 7.016 amu 92.41% Find atomic mass and ID the element (from table)! • Boron has two isotopes: Boron-10 (% abundance – 19.8%, mass = 10.013 amu) and Boron-11 (% abundance – 80.2%, mass – 11.009 amu). Calculate the atomic mass of Boron. Today’s Agenda • Question for the day: How do we calculate average atomic mass and relative abundances? • Average Atomic mass calcs • Prelab Sand, Salt, Water • Prelab for Monday! • Bromine has two isotopes with the first having a mass of 78.918336 amu and occupying 50.69% and the second isotope having a mass of 80.916289 amu and occupying 49.31%. What is the average atomic mass of bromine? • Verify the atomic mass of Magnesium: 24Mg = 23.985042 amu and percent abundance of 78.99% , 25Mg = 24.985837 amu and percent abundance of 10.00%, 26Mg = 25.982593 amu and percent abundance of 11.01%. Warm Up • Boron has an atomic mass of 10.81 amu according to the periodic table. However, no single atom of boron has a mass of 10.81 amu. How can you explain this difference? • Write the shorthand notation for the following: – A particle with 34 protons, 36 neutrons, and 36 electrons – A particle with 44 protons, 54 neutrons, and 41 electrons Agenda! • Review! • Review Ch 4 • Homework questions • Nuclear chemistry intro • Worksheet collected tomorrow before quiz! Block 3 – leave notebooks! Calculate % abundance • Copper has two naturally occurring isotopes, Cu-63 and Cu-65. The atomic mass of Cu is 63.55 amu. Calculate the percent abundances of the two isotopes. • Silver has two isotopes, 107Ag and 109Ag. The average mass of the isotopes is 108.90470. What is the percent abundance of each? Warm Up • What does it mean to be radioactive? • What is an isotope? • What is a radioisotope? Today’s Agenda • Question of the day: What is radioactivity? • Radioisotopes • Types of decay • Writing Reactions • Homework: Ch 4: 80, 82, 83, 85, 86, 88 Due tomorrow! Radioactivity – emit radiation • Nuclear reactions – change an element into a new element!! Lots of energy involved! – Unlike a chemical reaction because we are doing more than rearranging – we CHANGE the identity. • UNSTABLE nuclei are unhappy and lose energy by emitting radiation – radioactive decay. • They form STABLE atoms of a different element. Radioisotopes • Isotopes of atoms with unstable nuclei. • Undergo radioactive decay to attain stability. Emit 3 types of radiation – alpha, – beta, – gamma, What are the charges on radioactive particles? Types of Radiation • Alpha radiation –made up of POSITIVE alpha particles. • particle – helium cation (2 protons and two neutrons, no electrons!) 4He2+ 2 or Alpha decay 238 4 92 2 U 226 4 88 2 247 4 Ra 96Cm 2 He + He + He + 234 Th 90 222 Rn 86 243 Pu 94 Types of Radiation • Beta radiation – negatively charged beta particles • Unstable neutron turns into a proton and ejects 1 electron e- or Beta Decay Reactions 137 0 55 -1 14 0 Cs 6 C + + -1 260 0 104 -1 Rf + 137 Ba 56 14 7 N 260 105 Db Types of Radiation • Gamma radiation – emits gamma rays, high energy photon that has no mass nor charge. • Gamma rays almost always accompany alpha and beta radiation and account for the energy lost in the nucleus. Usually omitted from nuclear equations. 238 4 92 2 U He + 234 Th 90 + 2 Mixed Practice 149 61 Pm 247 96 247 95 209 82 0 4 He + 2 Cm Am Pb + -1 0 -1 4 + He + 2 149 62 Sm 243 94 247 96 205 80 Pu Cm Hg Penetrating Power of Radiation Warm up • Complete the nuclear reactions: 204 81 144 60 Tl Nd 0 -1 4 + He + 2 204 82 140 58 Pb Ce • Identify the TYPE of decay. • Why is a gamma ray more penetrating than an a particle? Warm Up Complete these reactions and indicate the TYPE of radioactive process: 241Pu 94 241 95 Am + ____ 214 Bi 210 81 Tl + ____ 85 Rb 85 Kr + ____ 36 83 37 Why is a gamma ray more penetrating than an a particle? Penetrating Power Least Alpha particles most mass and charge. Isotopic mass 4He2+ 2 Beta particles less mass (only the mass of an electron) and a neg charge. Most Gamma rays have no mass and no charge. Give reactions for: • Carbon-14 undergoes beta decay • Americium-245 undergoes alpha decay. • An alpha decay produces lead-205 • Curium-247 is the product of a beta decay. Warm Up • Polonium-209 undergoes alpha decay • Beta decay produces mercury-203 • Cerium-143 undergoes an alpha decay followed by a beta decay. B A Write a balanced nuclear equation for the decay shown on the right. Identify A and B 212Bi 4He + 208Tl + 83 81 2 208Tl A 81 208Pb B 82 Bismuth -212 Today’s Agenda • Question for today: Why do some atoms undergo decay and how fast do they decay? • Nuclear decay ratio • Half life calculations • HW: Ch 24 problems 34-58 evens Due Wednesday • TEST 4/24 FRIDAY! In the Nucleus • Radioactive decay – transmutation – Atomic # is altered = identity of element changed Nucleons + + + Strong nuclear force between all nucleons. Repulsive force between 2 protons (electrostatic). Neutron attraction have to overcome the repulsive forces – as atomic # increases more neutrons needed for stabilization!! All nuclei with more than 82 protons are radioactive! Low atomic #’s have a 1:1 neutron to proton ratio 4He 2 High atomic #’s are stabilized by a 1.5:1 ratio 200Hg 80 If atom is not in band (belt) of stability it undergoes radioactive decay to get there! Half Life • Time required for one half of the nuclei to decay into its products. • Strontium-90 half life is 29 years. If you had 10 g now, in 29 years you would have 5g. Work together to solve… Kr is used in indicator lights of appliances. Kr-85 has a half life of 11 years. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left? Half Life Formula N = N0 (½)n N – remaining amount of element N0 – initial amount of element n – number of half lives that have passed Kr-85 has a half life of 11 years. Kr is used in indicator lights of appliances. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left? N=? N0 = 2 mg n = 33 years/11 years (years that have passed/half life) Kr is used in indicator lights of appliances. Kr-85 has a half life of 11 years. If a refrigerator light contains 2.0 mg of Kr-85, after 33 years, how much is left? N = 2.0 mg (½)(33/11) N = 2.0 mg (½)3 N = 2.0 mg (⅛) N = 0.25 mg left after 33 years Half Life Practice • The half life of Ra-222 is 3.8 days. How much is left of a 10 mg sample after 15.2 days? N = N0 (½)n N = 10mg (½)(15.2/3.8) N = 10mg (½)4 N = 10mg (1/16) N = 0.625mg Half Life Practice Bandages can be sterilized by exposure to gamma radiation from cobalt-60, which has a half life of 5.27 years. How much of a sample of cobalt-60 did we start with if after 10.54 years we have 0.75 mg? N = N0 (½)n 0.75 mg = N0 (½)26.35/5.27 0.75 mg = N0 (½)5 Half – Life Calculations • Do the problem intuitively… Think about how many half lives have passed and just do the math! 26.35 years/5.27 years in half life = five half lives Multiply 0.75 mg by 2 five times 0.75 mg x 2 x 2 x 2 x 2 x 2 = 24 mg Warm Up! • Write the equations for decay: – krypton-81 undergoes alpha decay – beta decay of uranium-238 – thorium-230 undergoes alpha decay followed by two beta decays • If 100 g of carbon-14 decays until only 12.5 g of carbon is left after 11,460 years, what is the half-life of carbon-14?. Today’s Agenda • QOTD: What is fission and how do we use it for energy? • Nuclear Reactions • Fission vs Fusion • Review! • Homework due tomorrow. Test Friday. Review from Yesterday! • Use your notes to quietly answer the following: • What keeps the nucleons together? • What causes radioactive decay? • Do the following isotopes decay? Nitrogen-15 Carbon-14 Phosphorous-30 Solve! • Americium-241 has a half life of 430 years. How much of a 15 mg sample is left after 2150 years? • A radioisotope has a half life of 17 years. If 0.25 g remains today, how much did you have 85 years ago? • Strontium has a half life of 29 years. How long will it take for a 56 g sample to decay to 0.875 g? Nuclear Reactions • Induced Transmutation – FORCE an element to change its identity by bombarding it with radioactive particles! 1n + 14 N 0 7 15N 7 24He + 11 B 5 • Particle accelerators move particles at extremely high speeds to overcome repulsive forces. Nuclear Fission • Force a nucleus to split into fragments – resulting in a LARGE release of energy! Kr - 92 Particle accelerator neutron 235 236 1 n + U 92 92 U 0 Ba - 141 What products are formed? Neutrons Perpetuate Fission Critical Mass • Fissionable material must have critical mass – Not massive enough – subcritical – no chain reaction – Extremely massive – supercritical - violent nuclear reaction Critical Mass Violent Nuclear Reactions Subcritical masses that get together to form supercritical mass. Equal to 20,000 tons of TNT Nuclear Reactors Control Rods Reactor core controlled by Cd or B to absorb neutrons Warm Up! • Describe the process that occurs during a nuclear chain reaction and explain how to monitor a chain reaction in a nuclear reactor. True/False • Great amounts of energy can be liberated from small amounts of matter in a nuclear reaction. • The amount of U-235 in a nuclear reactor should always be kept subcritical. • Nuclear power plants do not contribute to air pollution. • Nuclear power use is dangerous because plants are commonly are out of control. Today’s Agenda • QOTD: How do you analyze half life data? • Homework questions • Half-life lab! • HMWK – Review sheet due Friday. Start tonight for questions tomorrow! Test Friday! Warm Up! • Curium-248 has a half life of 5.2 minutes. If you have a 6.4 mg sample after 41.6 minutes, how much did you have at the start? • Curium has three isotopes, Cm-248 (15%), Cm-247 (74%), and Cm-246 (?%). Find the percent abundance for Cm-246 and the average atomic mass of Cm. Half life practice • The half life of Braintrium-128 is 48 minutes. If we have a 0.3 g sample left after 8 hours, how much did we have to start with? • How much time has passed if our 0.3 g sample of Btr-128 has decayed to .0188 g? Nuclear Reactions • What is the isotopic symbol for an element with 99 protons, 153 neutrons, and 97 electrons? • If that element undergoes an alpha decay followed by 2 beta decays, what is the nuclear reaction? Half Life Lab • Statement of problem: Given a sample of M&M’s, collect decay data, graph data using excel, and fit with an exponential function. • Background info: What is a half life? What happens to a sample as each half life passes? Sketch a graph that you expect. Nuclear Fusion • Bind low atomic mass (less than 60 because 60 is ideal) to form more stable atom. • Combination of nuclei called fusion. • How the sun works… 411H + 2 energy + 4He 2 • Need very high energy to initiate and sustain. Nuclear Energy vs. Fossil Fuel • Why is nuclear energy considered cleaner than fossil fuels with respect to greenhouse gases? • Write the balanced nuclear fission reaction below: 2 neutrons (10n) are absorbed by a uranium-235 nucleus, which then undergoes alpha decay. Decay Practice 1 decay 2 4He + 238Pu 2 94 234U 92 Thorium-229 is used to increase the lifetime of fluorescent bulbs. What type of decay occurs when thorium-229 decays to form radium-225? Write out the nuclear equation. 4He + 229Th 2 90 225Ra 88 3 B A Write a balanced nuclear equation for the decay shown on the right. Identify A and B 212Bi 4He + 208Tl + 83 81 2 208Tl A 81 208Pb B 82 Bismuth -212 Warm Up Isotope Percent Abundance Mass (amu) X-50 4.35 49.946 X-52 83.79 51.941 X-53 9.50 52.941 X-54 2.36 53.939 Find the atomic mass and identify the element. How many electrons, and protons does this element have? How many electrons protons and neutrons does 52X2+ have? Is it in cationic or anionic form? Warm – Up! • What is an isotope? • What is a radioactive isotope? • What does it mean for something to be radioactive? Warm Up! • What are the three types of radioactive particles? • What are the charges on those particles? • Complete the following nuclear reaction (Remember that = e-) 137 Cs 55 + 137 Ba 56 Warm – Up!! • What happens to the atomic mass number and the atomic number of a radioisotope when it undergoes alpha emission? • High speed electrons emitted by an unstable nucleus are ________ particles. • What isotope of what element is produced if krypton-81 undergoes beta decay? Write out the nuclear reaction. Warm – Up!! • What is the band of stability and how does it relate to the proton to neutron ratio? • How does the neutron to proton ratio change when polonium-210 decays into lead-206? What type of decay does polonium-210 undergo? (Low atomic # elements are happy with a 1:1 ratio of neutrons to protons. Heavier elements need a 1.5:1 ratio and all elements above 82 are radioactive.) Warm Up 1. Silver has two naturally occurring isotopes, Ag-107 and Ag-109. The atomic mass of Ag is 107.868 amu. Calculate the percent abundances of the two isotopes. 2. 240Am + 2 01n _______ 0 + ______ 95 -1 3. The half life of polonium-218 is 3 min. If you start with 26 g, how long will it be before only 2 g remains? Agenda • Review • Homework • Worksheet Lab for today: Rutherford’s Marble Experiment? • Copy into lab notebook • Statement of Problem: Given 10 marbles of approximately equal diameter, use a system of collision and probability to indirectly measure the average diameter of the marbles. Rutherford and the Nucleus Nucleus is positively charged to deflect alpha particles and to balance electron charge. Warm – Up! • What experiment determined the mass and charge of an electron? • Dalton concluded that the atom was the smallest particle of matter. Was he correct? • What did the gold foil experiment prove? Today’s Agenda • Question for today: What does radioactive mean and what makes certain atoms radioactive? • Isotope calcs • Radioactive particles • Decay practice • Homework set 2 due date TBD. Don’t wait until the last minute!! Final background paper due Friday! Today’s Agenda • Question for today: What are the ways that large elements decay? How harmful are these particles? • Alpha, beta, gamma decay, positron emission and electron capture • Nuclear forces • Band of stability • Quiz tomorrow! Final paper due tomorrow! • Homework set due TUES. Test Ch 4&24 Thurs. Other types of decay… • Nuclei with low neutron to proton ratio undergo positron emission and electron capture. • Positron – particle with same mass as an electron but opposite charge (e+ or ). Positron emission = p n + e+ Electron capture = p + e- n both processes decrease # protons to increase stability Positron Emission and e- Capture