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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 Aristotle and Democritus A Long Time Ago… Antoine Lavoisier (1782) – Used experiments in closed containers to develop the LCM 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 Lavoisier and Proust John Dalton Dalton’s theory had 4 major tenets 1. 2. 3. 4. All matter is composed of atoms Atoms are indivisible Atoms of 1 element are alike, but different from those of other elements Atoms combine in small, whole number ratios to form compounds A Reminder Observation, Observation, Observation Research Hypothesis Experiment, Revise hypothesis lots of times Results match….may become a theory A Little While Ago… 1897- J.J. Thomson discovers the electron using a cathode ray tube Cathode Ray Tube Experiment http://www.aip.org/history/electron/jjappara. htm The Nucleus Exists! 1911- Ernest Rutherford uses the gold foil experiment to “discover” the nucleus The Gold Foil Experiment Results of Gold Foil Experiment… http://online.cctt.org/physicslab/content/Phy1/lesso nnotes/atomic/atomicmodelsandspectra.asp http://micro.magnet.fsu.edu/electromag/java/ruthe rford/ 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 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 Understanding Periodic Blocks 19 K 39.0983 Atomic Number – # of protons Element Symbol Mass Number – Protons + Neutrons 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 Nuclear symbol notation (nuclide symbols) – – – – Example: 2713Al or 27Al # protons = ______ # neutrons = ______ # electrons = ______ Problems that arise – Charges and Ions: only changes the number of – Finding mass number: use symbol, # p+ and no, electrons! or periodic table IN THAT ORDER! Atomic Mass Units Carbon-12 – 6 protons – 6 neutrons 1 amu = 1/12 mass of a C-12 atom Nuclear Stability- too many neutrons or protons causes instability 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 Nuclear Decay Nuclear Particles – Alpha- rapidly moving He nuclei with a (+) charge – Beta – rapidly moving electrons with a (-) charge – Gamma – rapidly moving electromagnetic radiation with no mass or charge 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 Reactions – Transmutation- Changing the nucleus of the atom to create a new element – How is a Nuclear Equation Written? Parent nuclide- initial nucleus that undergoes changes Daughter nuclide- nucleus resulting from decay of parent Types of Nuclear Reactions Alpha Emission- generally occurs if nucleus has too many protons and neutrons Beta Emission- results from conversion of neutron to proton and occurs if nucleus has too many neutrons Positron Emission- results from conversion of proton to neutron and occurs if nucleus has too few neutrons Gamma Emission- often accompanies other decay processes Example: Alpha Decay 25299Es ______ + 4 He 2 Radioactive Decay Half-lives Half-life is the time taken for half of the atoms of a radio-active substance to decay. 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 Tracers Radioactive isotopes used to track pathways Chemistry/biology- pathways of reactions Industry and environment- path of groundwater, durability of containers Medicine- diagnose malfunctions 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 Fission- 92235U fuel used in a chain reaction – – – – – – Limited resource critical massminimum mass to sustain chain reaction Risk of runaway chain reaction Produces radioactive waste products Disposal concerns Reaction: Fusion- 12H and used as fuel 1 3H – extracted from sea water – not a chain reaction – No risk of runaway reaction – Nonradioactive waste: helium – Problem: needs temp of 200 Million K – Reaction: 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 Nuclear Meltdowns Cherynobyl (Soviet Union, 1986) – – – Runaway fission reaction from core meltdown Radiation escaped to atmosphere 31 dead, estimated 15,000 cancer deaths in next 50 years Three Mile Island (Pennsylvania, 1979) – partial meltdown – contained before widespread damages