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NUCLEAR CHEMISTRY

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NUCLEAR CHEMISTRY

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  1. NUCLEAR CHEMISTRY

  2. Section 22-1: The Nucleus Objectives Explain what nucleons are. Explain what a nuclide is, and describe the different ways it can be written. Define nuclear binding energy. Explain the relationship between nucleon number and stability of nuclei.

  3. The Nucleus • The nucleus is composed of nucleons • Protons • Neutrons • A nucleus is characterized by two numbers • mass number (A; total # of nucleons) • atomic number (Z; number of protons) ZAE

  4. 1327Al • total number of nucleons is 27 • total number of protons is 13 • the number of neutrons is 14 • in nuclear chemistry, an atom is referred to as a nuclide.

  5. Subatomic Particlesone atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom

  6. Einstein’s Equation • Energy and mass can be interconverted • E = mc2 • E- energy, m-mass, c-speed of light • When protons & neutrons are packed together to form a nucleus, some of the mass is converted to energy and released.

  7. Einstein’s Equation • Nuclear Binding Energy – the energy released when a nucleus is formed from protons and neutrons. • Can also be thought of as the amount of energy required to break apart the nucleus. Of an existing atom. • The higher the binding energy, the more tightly the nucleus is held together.

  8. Binding Energy Curve • graph peaks at A=56 • the more BE released per nucleon, the more stable the nucleus • mass number of 56 is maximum possible stability

  9. How Many Neutrons? • Stable nuclides have certain characteristics • The number of neutrons in a nucleus can vary as we have seen • Range limited by the degree of instability created by: • having too many neutrons • too few neutrons • Stable nuclei do not decay spontaneously • Unstable nuclei have a certain probability to decay

  10. Nuclear Stability Facts • 265 stable nuclides • For light elements (Z  20), Z:N ratio is ~1 • Example: helium-4 (2 neutrons, 2 protons) • Z:N ratio increases toward 1.5 for heavy elements • Example: lead-56 (124 neutrons, 82 protons) • For Z > 83 (bismuth), all isotopes are radioactive

  11. Nuclear Stability Facts • Stable nuclei tend to have an even number of nucleons. • Out of 265 stable nuclides, 159 have even numbers of both protons and neutrons. • Only 4 nuclides have odd numbers of both.

  12. The most stable nuclides are those having: 2, 8, 20, 28, 50, 82, 126 • Protons, neutrons or total nucleons • Examples: • Sn (Z=50) has 10 isotopes; In (Z=49) & Sb (Z=51) have only 2 isotopes • Pb-208 has a double magic number (126n, 82p) & is very stable • “Magic numbers” of protons or neutrons which are unusually stable

  13. Nuclear Reactions • Nuclear Reaction – a reaction that affects the nucleus of an atom. • Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons. • They also give off a large amount of energy and increase their stability in the process.

  14. Nuclear Reactions • In a nuclear reaction, the total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation. • Example: • 49Be + 24He 612C + 01n

  15. 49Be + 24He 612C + 01n • Note that when the atomic number changes, the identity of the element changes. • Transmutation – a change in the identity of a nucleus (element) as a result of a change in the number of its protons.

  16. Sample Problem Identify the product that balances the following nuclear reaction: 84212Po ? + 24He

  17. Classwork Problems 1-3, page 704

  18. Homework Page 724 Problems: 31, 33 and 40

  19. Section 22-2 Radioactive Decay

  20. Radioactivity Objectives Define the terms radioactive decay and nuclear radiation. Describe the different types of radioactive decay. Define the term half-life, and how it relates to stability.

  21. Radioactivity • The spontaneous decay of an unstable nucleus into a more stable nucleus. • Energy is released. • Certain isotopes are just not stable and will spontaneously decay.

  22. Types of Radioactive Decay • All elements with 84 or more protons (Polonium) are unstable, and will undergo radioactive decay. • Naturally occurring radioactive isotopes decay in three primary ways: • Alpha particle emission • Beta particle emission • Gamma radiation emission

  23. Alpha Emission Alpha particle – is two protons and two neutrons bound together and emitted from the nucleus. They are helium nuclei with a charge of +2. It has no electrons. Represented by the symbol: 24He Restricted to heavy elements: Ex. uranium

  24. Alpha Emission Process which is effective to lose a lot of mass form the element. Example: 84210Po 82206Pb + 24He The atomic number decreases by two (a new element) and the mass number decreases by four.

  25. Alpha Emission • 92235U 90231Th + 24 • Quick way for a large atom to lose a lot of nucleons

  26. Beta Emission Beta particle – is essentially an electron that’s emitted from the nucleus: -10e In the nucleus, a neutron is converted (decayed) into a proton and an electron. The electron is emitted as a beta particle. Represented by the symbol: -10e or -10b A good way to decrease the number of neutrons.

  27. Beta Emission By decreasing the number of neutrons you improve the neutron/proton ratio. Example: 53131I 54131Xe + -10b The mass number stays the same in going from I-131 to Xe-131but the atomic number increases by 1. Loss of neutron!

  28. Beta Emission • 1940K 2040Ca + -10b • Identity of atom changes

  29. Gamma Emission Gamma rays – are high energy electromagnetic waves emitted from the nucleus. There is no mass change with gamma emission, only radiation. Usually occurs immediately following other types of decay. Not shown in a balanced nuclear reaction.

  30. Gamma Emission • An example is cobalt-60 (Co-60) which gives off a large amount of gamma radiation. • Co-60 used in the radiation treatment of cancer.

  31. Electromagnetic Radiation • Electromagnetic radiation is a form of energy that can pass through empty space • It is not just a particle, and it is not just a wave. It may be both.

  32. Electromagnetic Radiation • Gamma rays are similar to x-rays – high energy, short wavelength radiation.

  33. Positron Emission Positron particle – is essentially an electron that has a positive charge. A useful way to decrease the number of protons. Represented by the symbol: +10e Doesn’t occur with naturally occurring radioactive isotopes.

  34. Positron Emission Example: 1938K 1838Ar + +10e Notice that the atomic number decreases by one but the mass number stays the same. Can be viewed as the opposite of beta emission.

  35. Half-Life Half-life - the amount of time it takes for one-half of a radioactive sample to decay is called the half-life of the isotope It is given the symbol: t1/2 No two radioactive isotopes decay at the same rate

  36. Half-Life Useful application of half-life is radioactive dating. Used to determine the age of things. Carbon-14 dating can be used to determine the age of something that was once alive. Examples include animal and plant species. Cannot be used to determine the age of rocks.

  37. Half-Life Radium-226 has a half-life of 1599 years. Half of a given amount of radium-226 decays in 1599 years. In another 1599 years, half of the remaining radium-226 will decay. This will continue until there is a negligible amount of radium-226 remaining.

  38. Half-Life Decay of radium-226

  39. Half-Life • Each radioactive element has its own half-life. • More stable elements decay slowly and have longer half-lives. • Less stable elements decay quicker and have shorter half-lives

  40. Radioactive Decay Rates

  41. Half-Life The time required for half of a sample to decay

  42. Half-Life

  43. Half-Life

  44. Half-Life Problem: Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope. First determine the number of half-lives that have elapsed!

  45. Classwork Problems 1-6, page 709

  46. Some nuclides (particularly those Z>83) cannot attain a stable, nonradioactive nucleus by a single emission. • The product of such an emission is itself radioactive and will undergo a further decay process. • Heavy nuclei may undergo a whole decay series of nuclear disintegrations before reaching a nonradioactive product. Decay Series

  47. Trying To Reach Nuclear Stability Decay Series – a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached. The heaviest nuclide of each series is called the parent nuclide. The nuclides produced are called daughter nuclides.