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Atoms

Atoms. History of Atomic Discovery— Models over Time. Democritus vs. Aristotle or round one of atomic theory. Aristotle, a Greek philosopher living from 384-322 BC, believed that matter was continuously divisible. This theory held until nearly two millennia had passed

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Atoms

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  1. Atoms History of Atomic Discovery— Models over Time

  2. Democritus vs. Aristotleor round one of atomic theory • Aristotle, a Greek philosopher living from 384-322 BC, believed that matter was continuously divisible. • This theory held until nearly two millennia had passed • Democritus (460-370 BC) • Democritus came up with the idea that matter could be divided in half only until it was cutting apart extremely tiny indivisible pieces: atomos.

  3. Dalton (1766-1844) • Dalton rejected the idea that matter is continuous, and he revived the idea of atoms around 1803. • Atoms are solid balls that are capable of being ‘hooked’ together to form bonds. • Molecules have atoms of different of elements in fixed specific ratios.

  4. Dalton’s Atomic Theory Dalton's atomic theory had five main points: • All elements consist of minuscule particles called atoms. • All atoms of a given element are identical to each other. • All atoms of a given element are different than those of other elements. • Atoms of one element combine with other elements to create compounds. They always combine in whole number ratios. • Atoms cannot be created, divided, nor destroyed.

  5. Thomson (1856-1940) • The plum pudding theory • The atom is not a solid ball, but rather something that could be embedded with the negatively charged electrons. • Thomson deduced the charge and character of the electron with his cathode ray experiments. • He discovered isotopes and invented the mass spectrometer. http://www.youtube.com/watch?v=IdTxGJjA4Jw 3 minutes http://www.youtube.com/watch?v=7YHwMWcxeX8

  6. Rutherford (1871-1937)or shall we have plum pudding? • Rutherford’s gold foil experiment demonstrated that much of the atom is empty space. Electrons are low mass and unable to deflect the much heavier alpha particle. • Father of nuclear physics • http://www.youtube.com/watch?v=XBqHkraf8iE&feature=related&safety_mode=true&persist_safety_mode=1&safe=active The experiment 4minutes • http://www.youtube.com/watch?v=wzALbzTdnc8 Rutherford discovery 3 minutes

  7. Rutherford Atomic Model Based on his experimental evidence: • The atom is mostly empty space • All the positive charge, and almost all the mass is concentrated in a small area in the center. He called this a “nucleus” • The nucleus is composed of protons and neutrons (they make the nucleus!) • The electrons distributed around the nucleus, and occupy most of the volume • His model was called a “nuclear model”

  8. Schrodinger (1887-1961) • His thought experiment, gedanken, about the cat in the box changed how we think about quantum events and full scale events. • He discovered the wave properties of electrons which became part of quantum mechanics. http://www.youtube.com/watch?v=JNalMWLnt0o The cat 3:07 minutes http://www.youtube.com/watch?v=HCOE__N6v4o Big Bang version 2:19 minutes http://www.youtube.com/watch?v=CrxqTtiWxs4 Sixty Symbols UK 7:57 minutes

  9. Atom—the basics • Basic particles are proton (+), neutron (no charge), and electron (-). • Large mass particles are in the nucleus • Low mass particles, electrons, are in the cloud around the nucleus.

  10. Atomic Number • Atoms are composed of identical protons, neutrons, and electrons • How then are atoms of one element different from another element? • Elements are different because they contain different numbers of PROTONS • The “atomic number” of an element is the number of protons in the nucleus • # protons in an atom = # electrons When the atom is neutral, net 0 charge.

  11. Complete Symbols • Contain the symbol of the element, the mass number and the atomic number. X mass number (superscript) atomic number (subscript)

  12. Frederick Soddy—Isotopes • Frederick Soddy (1877-1956) proposed the idea of isotopes in 1912 • Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons. • Soddy won the Nobel Prize in Chemistry in 1921 for his work with isotopes and radioactive materials

  13. Isotopes • Dalton was wrong about all atoms of an element are of the same type, being identical. • Atoms of the same element can have different numbers of neutrons. • Thus, different mass numbers. • These are called isotopes. • Periodic table reports average atomic mass units

  14. Naming Isotopes • We can also put the mass number after the name of the element: • carbon-12 • carbon-14 • uranium-235

  15. The particles • Proton p • Neutron n • Electron e- • Alpha particle α or α2+, • Beta particle • Positron +1e+ or β+ This diagram demonstrates the constitution of different kinds of ionizing radiation and their ability to penetrate matter. Alpha particles are stopped by a sheet of paper whilst beta particles halt to an aluminium plate. Gamma radiation is dampened when it penetrates matter. Gamma rays can be stopped from 4 meters of lead. Tungsten and tungsten alloys can stop Gamma radiation with much less mass than lead

  16. Alpha particle • Decay—particles must add up • Total protons equals protons in alpha plus protons in daughter nucleus. • Same for neutrons. • Alpha radiation is the most dangerous of the three types. • Paper can shield us. • Low penetration.

  17. Alpha decay • The nucleus of an atom splits into two parts. • One of these parts (the alpha particle) goes zooming off into space. • Usually occurs in atomic number, Z > 83 • The nucleus left behind has its atomic number reduced by 2 and its mass number reduced by 4 (that is, by 2 protons and 2 neutrons). Note: 1) The atom on the left side is the one that splits into two pieces. 2) One of the two atoms on the right is ALWAYS an alpha particle.3) The other atom on the right ALWAYS goes down by two in the atomic number and four in the mass number.

  18. Example alpha decay • What element is produced by alpha decay of Americium with atomic number of 95 and atomic mass of 241? 241Am95 ZXA + 4He2 241 = z + 4 95 = A + 2 The element is Neptunium, 237Np93

  19. These alpha decay examples

  20. Beta minus particle • In beta decay, a neutron breaks into a proton, an electron, and an anti-neutrino. • The electron and the anti-neutrino are emitted. In the reaction, Ac has one more proton than Ra. Ac has 89 protons compared to Ra’s 88. 228 Ra had one more neutron than 228Ac.

  21. Beta minus • A neutron inside the nucleus of an atom breaks down, changing into a proton. • It emits an electron and an anti-neutrino (more on this later) which go zooming off into space. • The atomic number goes UP by one and mass number remains unchanged. Note 1) The nuclide that decays is the one on the left-hand side of the equation.2) The order of the nuclides on the right-hand side can be in any order.3) The way it is written above is the usual way.4) The mass number and atomic number of the antineutrino are zero and the bar above the symbol indicates it is an anti-particle.5) The neutrino symbol is the Greek letter "nu."

  22. Example • The decay by beta minus of carbon-14 to nitrogen-14 is used in carbon dating. • The electron and anti-neutrino are lost while the new proton is retained. The atomic weight is nearly the same. • Write an equation for carbon-14 14C614N7 + e- + antineutrino

  23. Try these beta minus examples • 60Co27 • 60Co2760Ni28 + e- + antineutrino • 137Cs55 • 137Cs55137Ba56 + e- + antineutrino

  24. Try these beta minus

  25. Positron or Beta plus • In beta plus decay, a proton decays into a neutron, a positron (the antiparticle of the electron) and a neutrino. • The positron and the neutrino are emitted. • The radioactive particle is the positron. • Electron and positron collision results in annihilation.

  26. Positron or Beta Plus • Something inside the nucleus of an atom breaks down, which causes a proton to become a neutron. • It emits a positron and a neutrino which go zooming off into space. • The atomic number goes DOWN by one and mass number remains unchanged. • Same notes as for beta minus. This is like a mirror image to beta minus

  27. Beta plus examples • 10C6 • 10C610B5 + e+ + neutrino • 22Na11 • 22Na1122Ne10 + e+ + neutrino

  28. Try these

  29. Gamma emissions • Gamma radiation is part of the electromagnetic spectrum just as light, UV, and radio waves are. • Gamma readily penetrates most materials including many centimeters of human tissue. • X-rays are like gamma radiation. • Dense elements such as lead or tungsten are needed for shielding.

  30. Gamma Exposures • The sun emits gamma radiation. This is part of the background radiation a person is continually exposed to. • Radon gas in homes is a source of gamma. • Doses are measured in rem, a unit that is based on both the radiation received and the biological effect for that radiation. Common form of the unit is millirem, mrem. • ~damage from one rad of gamma radiation = rem. • Background annual dose averages 360mrem total. 300 from natural sources and 60 from man-made sources.

  31. Median Radon Exposure in Houses, DOE

  32. Your Home and Radon • The danger of radon exposure in dwellings was discovered in 1984 by an employee at the Limerick Nuclear Power Plant in Pennsylvania. The employee set off the radiation alarms on his way into work for two weeks straight. While authorities searched for the source of the contamination. They were shocked to find that the source was astonishingly high levels of Radon in his basement, and it was not related to the nuclear plant. The risks associated with living in his house were estimated to be equivalent to smoking 135 packs of cigarettes every day. • Sumps equipped with ventilation fans pull the gas from under foundations as required by current code.

  33. Common Man-made Sources • Gastrointestinal series (upper & lower): 1400 millirem • CT Scan (head & body): 1100 milliremRadon in average household: 200 millirem/year • Plutonium-powered pacemaker: 100 millirem/year • Natural radioactivity in your body: 40 millirem/yearCosmic radiation: 31 millirem/year • Mammogram: 30 millirem • Smoking Cigarettes (1 pack/day): 15-20 millirem/year • *Maximum possible from normal operations on the Oak Ridge Reservation: 12 millirem/year • Consumer products: 11 millirem/year • Chest X-ray: 10 millirem • Dental X-ray: 10 millirem • Using natural gas in the home: 9 millirem/year • Road construction material: 4 millirem/year • Living near a nuclear power station: 1 millirem/year • Air travel (every 2006 miles): 1 millirem • *Source, 2004 DOE Annual Site Environmental Report Summary

  34. Electron capture • There are two ways in which neutron-deficient / proton-rich nuclei can decay. When the mass change Δm < 0 yet is insufficient to cause spontaneous positron emission, a neutron can form by an alternate process known as electron capture. An outside electron is pulled inside the nucleus and combined with a proton to make a neutron, emitting only a neutrino. • 11p + 0-1e → 10n + ν

  35. Examples of Electron Capture • 8136Kr + 0-1e- → • 8136Kr + 0-1e- → 8135Br + ν • 23192U + 0-1e- → • 23192U + 0-1e- → 23191Pa + ν

  36. Electron Capture Problems • Write an equation for electron capture in 207Bi. • 20783Bi + 0-1e- → 20782Pb + ν

  37. Isotope Stability • Why are some isotopes unstable? • Isotopes all have the same number of protons, but vary in the number of neutrons. Some combinations are unstable. • A nuclide is an atom with a specific number of protons and neutrons in the nucleus. • Unstable nuclides are radioactive. They decay to reach a balance between the neutrons and protons that is stable. • Stable is when an isotope does not decay in an observable manner. The half-life can be over 80 million years. • The nucleus is bound together by the residual strong force.

  38. Comparison of Atomic Number vs. Mass Number The stable line of isotopes are bracketed with others that have more or fewer neutrons and less stability. As stability goes down, the half-life becomes longer. Note that above 80 million is functionally stable.

  39. Pattern of Radioactive Decay

  40. Decay Sequence

  41. Half-life • The time it takes for one half of the radioactive atoms of an element present in a sample to decay is the half-life. • The time for a specific radioactive isotope to decay half of its atoms is a constant. • Decay rate constant = λ (units time-1) • Half-life = t1/2 = ln2/λ (units time)

  42. Graph of Radioactive Decay

  43. Half Life, 1st Order Reaction This experimental data graphed to form a non-linear first order reaction line.

  44. Half-Life Calculations • Remaining radioactive atoms are equal to the starting amount multiplied by the number ½ raised to the number of half-lives that have elapsed. • (starting amount) x 1/2numberhalf-lives = (remainder amount) • Number of half-lives = time elapsed length of 1 half-life

  45. Example • The half-life of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed? 7.2 / 2.4 = 3 half-lives (1/2)3 = 0.125 (the amount remaining after 3 half-lives) 100.0 g x 0.125 = 12.5 g remaining

  46. Try This • Os-182 has a half-life of 21.5 hours. How many grams of a 10.0 gram sample would have decayed after exactly three half-lives? • (1/2)3 = 0.125 (the amount remaining after 3 half-lives) 10.0 g x 0.125 = 1.25 g remain • 10.0 g - 1.25 g = 8.75 g have decayed • Note that the length of the half-life played no role in this calculation.

  47. Example • After 24.0 days, 2.00 milligrams of an original 128.0 milligram sample remain. What is the half-life of the sample? • Time elapsed = 24.0 days • Initial amount = 128.0 mg • Remainder amount = 2.0 mg • 128.0 mgm x (½)n =2 mg • (1/2)n = 2/128 = 1/64 = (1/2)6 • n = 6 half-lives and 24.0 days/6 = 4 days t1/2

  48. Characteristics of Particles

  49. Resources • http://orise.orau.gov/reacts/guide/gamma.htm REAC/TS Oakridge • http://www.oakridge.doe.gov/external/PublicActivities/EmergencyPublicInformation/AboutRadiation/tabid/319/Default.aspx • http://www.chemteam.info/Radioactivity/Radioactivity.html

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