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Chapter 4 Atomic Structure 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. Dalton’s Inaccuracies • Atoms are not indivisible! – Subatomic particles – electrons, protons, and neutrons • Atoms of the same element can have slightly different masses! - isotopes The Atom • EXTREMELY small particle of an element that retains the properties of that element is an atom. • If the 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! 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! 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. 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! 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? 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!! Make sure you can answer… • What are John Dalton’s 4 theorems • How does John Dalton’s theory relate to conservation of mass? • How was the electron discovered? • Who discovered the mass of an e-? HOW? • What was Rutherford’s contribution? • Describe the structure of the atom. Properties of Atoms Atomic # # of Protons = # of Electrons (in neutral atom) Atomic mass a weighted average 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 Isotopic symbol : 70 32 Ge 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. K Potassium Three isotopes = 39 K 19 Percent Composition: 93.26% K 19 40 0.01% K 41 19 6.73% Calculate the Atomic Mass of K 1. Use % composition and convert to relative abundance (divide by 100) 93.26% composition = 0.9326 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 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? Agenda • Question for today: What does radioactive mean and what makes certain atoms radioactive? • Isotope calcs • Radioactive particles • Decay practice Amu = (R.A.)x(Mass) + ((R.A.)x(Mass))… • What element is this? Isotope Mass of Isotope Percent abundance 6X 6.015 amu 7.59% 7X 7.016 amu 92.41% Find the atomic mass What element is this? (Use the Periodic 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. • 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%. One more… • 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. 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. – Change in the atom’s nucleus. • 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, a – beta, b – gamma, g What are the charges on radioactive particles? Types of Radiation • Alpha radiation – (remember the gold foil experiment?!?!) made up of POSITIVE “alpha particles”. • 2 protons and two neutrons (no electrons!) 4He2+ 2 or a 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 b 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. g Usually omitted from nuclear equations. 238 4 92 2 U He + 234 Th 90 + 2 g Penetrating Power of Radiation 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. 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 we need more neutrons to stabilize the nucleus!!! 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! Decay Practice 4He + 238Pu a decay 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 B A Write a balanced nuclear equation for the decay shown on the right. Identify A and B 212Bi 4He + 208Tl b 83 81 2 + 208Tl A 81 208Pb B 82 b Bismuth -212 a 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.) 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. Half Life Calculations 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-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 = 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 10 mg sample of cobalt-60 is left over after 10.54 years? After four half lives? N = N0 (½)n N = 10 mg (½)10.54/5.27 N = 10 mg (½)4 Half – Life Calculations • Do the problem intuitively… Think about how many half lives have passed and just do the division Two half lives (10 mg/2)/2 = 2.5 mg Four half lives 10 mg/2/2/2/2 = 0.625 mg Warm - Up! • 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 197 years. How much remains of a 2.0 g sample after 10 years? • Strontium has a half life of 29 years. How long will it take for a 56 g sample to decay to 1.75 g? Nuclear Reactions • Induced Transmutation – FORCE an element to change its identity by bombarding it with radioactive particles! 4He + 14 N 2 7 17O 8 + 11 H • Particle accelerators move particles at extremely high speeds to overcome repulsive forces. Nuclear Fission • Splitting of a nucleus into fragments - LARGE release of energy! • Large atoms want to be smaller for stability (Atomic # larger than 60 = large atoms) Kr - 92 neutron U-235 U-236 Ba - 141 Neutrons Perpetuate Fission Critical Mass • Fissionable material must have sufficient mass to split – Not massive enough – subcritical – no chain reaction – Extremely massive – violent nuclear reaction Violent Nuclear Reactions Subcritical masses that get together to form supercritical mass. Equal to 20,000 tons of TNT Nuclear Reactors Reactor core controlled by Cd or B to absorb neutrons Assignment : Look up Chernobyl Nuclear Power Plant! 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 + 2b energy + 42 He • Need very high energy to initiate and sustain. Nuclear waste • What happens to all the radioactive waste accumulated at a power plant? • Treated with advanced technologies so the material doesn’t deteriorate • Sealed and buried underground • 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. 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 b = e-) 137 Cs 55 b + 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. Agenda: Nuclear Fission Nuclear Power Plants Homework: # 34 – 79 odds pg 894 due Thursday Test Thursday Today’s Question What are the main components of a nuclear power plant? How does a power plant control the chain reaction? Nuclear Fission • The splitting of an atom into parts Uranium-235 when hit with a neutron at high velocity breaks apart into 2 new elements as well as additional neutrons. Those neutrons go on to strike other U-235 atoms and cause a chain reaction Fission reactions produce a ton of energy. If a chain reaction is uncontrolled the consequences can be extreme Nuclear Reactors • Nuclear fission produces the energy generated in a nuclear power plant • Uses fuel rods made out of enriched uranium • Control rods made out of other metals are used to control the chain reaction by absorbing extra neutrons • Heat generated from fission boils water and the steam spins a turbine which produces electricity Nuclear waste • What happens to all the radioactive waste accumulated at a power plant? • Treated with advanced technologies so the material doesn’t deteriorate • Sealed and buried underground • What happens when we run out of U-235? • Breeder Reactors -cost inefficient -technically challenging Nuclear Fusion • Process by which 2 or more atomic nuclei join together to form a single heavier nucleus • Generally produces large quantities of energy • Stars • Hydrogen bomb • Possible answer to energy source with the least amount of waste products Nuclear Weapons • 2 types Fission based and Fission/Fusion Based • Limited development by a few developed countries globally Fission Weapons • Chemical explosives start a chain reaction Fission/Fusion Weapons • Fission reaction acts as a trigger for the fusion reaction