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CHAPTER 4/25 Atomic Structure Nuclear Chemistry A Long, Long Time Ago… • Greek Philosophers 4 elements are Earth, Water, Fire, and Air • Aristotle- first recorded atomic thoughts Matter is continuous (no atoms) • Democritus (400 B.C.) - first recorded atomic theory Atoms are smallest part of matter, each type of matter has different atoms A Long Time Ago… • Antoine Lavoisier (1782) Used experiments in closed containers to develop the Law of Conservation of Matter • Joseph Proust (1799) Analyzed water to develop the Law of Definite Proportions • John Dalton (1803) Compiled past research to develop the first useful atomic theory John Dalton Dalton’s theory had 4 major tenets 1. All matter is composed of atoms 2. Atoms are indivisible 3. Atoms of 1 element are alike, but different from those of other elements 4. Atoms combine in small, whole number ratios to form compounds A Little While Ago… J.J. Thomson (1897) discovers the electron using a cathode ray tube Proposes the “Plum Pudding” Model of the atom Cathode Ray Tube Experiment http://www.aip.org/history/electron/jjappara.htm The Nucleus Exists! Ernest Rutherford (1911) uses the gold foil experiment to “discover” the nucleus The Gold Foil Experiment Results of Gold Foil Experiment… Over 98% of the particles went straight through About 2% of the particles went through but were deflected by large angles About 0.01% of the particles bounced off the gold foil About that 0.01% of particles….. …" It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." E. Rutheford Rutherford’s Conclusion Rutherford's Nuclear Model 1. The atom contains a tiny dense center called the nucleus the volume is about 1/10 trillionth the volume of the atom 2. The nucleus is essentially the entire mass of the atom 3. The nucleus is positively charged the amount of positive charge of the nucleus balances the negative charge of the electrons 4. The electrons move around in the empty space of the atom surrounding the nucleus Coworker James Chadwick later adds neutrons Properties of Subatomic Particles Particle Symbol Electron Proton Neutron ep+ n0 Location Space surrounding the nucleus Nucleus Nucleus Relative Electrical Charge Relative Mass Actual Mass (g) 1- 1_ 1840 9.11 x 10-28 1+ 1 1.637 x 10-24 0 1 1.675 x 10-24 Understanding Periodic Blocks • • Atomic Number • # of protons • # of electrons • Element Symbol Mass Number #protons + # neutrons • Atomic Mass The weighted average atomic mass of isotopes of an atom Can 20.0983 Neutrons Exist??? No, 39.0983 is an average mass of all natural K atoms • All K atoms MUST have 19 protons • Some have 20 neutrons, some 19, some 21… • Average is 20.0983 neutrons • Individual isotopes are identified by the number of neutrons Identifying Protons, Neutrons, and Electrons • Chlorine-35 (element-mass number) • # of protons = 17 • # electrons = # protons • # neutrons = mass number – protons • 35-17 = 18 • Why no electrons in atomic mass? • Electron has mass 1/1837 of Proton and Neutron Isotopes Nuclear Notation or Isotope Notation # protons = ______ # neutrons = ______ # electrons = ______ Problems that can arise.. Charges and Ions – Only changes the number of electrons Finding the mass number – use symbol, p+ and no or periodic table in that order! Atomic Mass Units • Carbon-12 • 6 protons • 6 neutrons atomic mass unit is equal to one-twelfth of the mass of the nucleus of a carbon-12 atom Atomic Mass Problems Boron has two naturally occurring isotopes: boron-10 (19.8%, 10.013 amu) and boron 11 (80.2%, 11.009 amu). What is the atomic mass of boron? Find the Abundance x Mass for each isotope and then add products together. 0.198 x 10.013 = 1.98 0.802 x 11.009 = 8.83 1.000 10.81 amu Chemical vs. Nuclear Change • Chemical Change- produces new kinds of matter with new properties • • • Involves breaking and forming BONDS Accomplished by rearrangement of ELECTRONS Nuclear Change- produces a new nucleus that contains less energy • • Involves emission or capture of nuclear particles Accomplished by changing PROTONS and NEUTRONS in nucleus ALPHA (α) BETA (β) GAMMA (γ) Symbol 4 He 2 0 e -1 0 γ 0 Shielding Easy (skin/clothes) Hard (2 cm lead) Energy High Medium (aluminum foil) Med Biological Hazard Actual Hazard High Med Low-Med Low Med High Low Other Radiation • Ionizing Radiation- has significant energy to change atoms and molecules into ions • • • Types: alpha, beta, gamma, x-rays Effects to living organisms: changes in, DNA (cell death/cancer) Nonionizing radiation- does not have significant energy to ionize atoms or molecules (types: microwaves, visible light, radiowaves) Nuclear Stability (Electrostatic Force vs. Nuclear Force) Electrostatic force arises from the interaction between two protons (repulsive force) Nuclear force arises between protons and neutrons due to their close proximity to one another Neutron to Proton Ration and the Band of Stability 1.5:1 1:1 As the atomic number increases, more and more neutrons are needed to create a strong nuclear force to oppose and increasing electrostatic force Nuclear Reactions • Nuclear Decay • Radioactive Decay Half-lives • Half-life is the time taken for half of the atoms of a radio-active substance to decay. Number of half-lives TE/t1/2 5 variables to consider 1. Initial quantity 2. Final quantity 3. # of half-lives 4. Value of a half life 5. Time Elapsed Sample Problem A sample of radioactive iridium has a half life of 12 years. In 60 years, how much iridium would remain from a 50g n sample? Final 1 Initial 2 Initial= 50g Final 1 50 2 Final= ? Time Elapsed= 60 years # of half lives= 60/12 = 5 Value of 1 half life= 12 years n Final 1 50 2 5 Final 1 50 32 32 x 50 x 50 / 32 1.6 g Half-life • Half-lives can range from a millionth of a second to millions of years Radioactive Dating • Uses carbon-14 to tell age of fossils • • • • C-14 is present constantly in atmosphere 15.3 decays/min in living organism decays/min decreases by ½ every 5370 years an organism is dead. Only useful to 60,000 yrs ago Transmutation • All nuclear reactions are: transmutation reactions o Some transmutation reactions are induced o All transuranium elements (atomic #93 and greater) have been produced through induced transmutation. Conversion of an element into an atom of another element Writing Induced Transmutation Reactions Write the balanced nuclear equation for the induced transmutation of aluminum-27 into sodium-24 by neutron bombardment. An alpha particle is released in the reaction. 27 13 Al n Na He 1 0 24 11 4 2 Write the balanced nuclear equation for the alpha particle bombardment of Pu-239. One of the reaction products is a neutron. 239 94 Pu He Cm n 4 2 242 96 1 0 Nuclear Reactions for Energy • Fission- nucleus broken into 2 smaller nuclei • Fusion- smaller nuclei join to form a larger, more stable nuclei Reactants and Products • • Nuclear Powergenerated by a controlled fission chain reaction Control rods- absorb neutrons to slow the chain reaction • Made of cadmium • Inserted or withdrawn to keep temp of reaction steady Moderators- slow neutrons down so they DO hit uranium fuel rods • Made of water, beryllium, or graphite • Intended to allow neutrons to be absorbed by uranium Cooling and Shielding • Water- acts as a coolant and transfers heat between reactor and turbines that produce electricity • Steel & concrete- surround core and protect personnel by absorbing radiation