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Unit 3: Nuclear Chemistry

Unit 3: Nuclear Chemistry. Unit Objectives. Describe the makeup of the nucleus Describe the relationships between neutron-proton ratio and nuclear stability Discuss what is meant by the band of stability Calculate mass deficiency and nuclear binding energy

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Unit 3: Nuclear Chemistry

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  1. Unit 3: Nuclear Chemistry

  2. Unit Objectives • Describe the makeup of the nucleus • Describe the relationships between neutron-proton ratio and nuclear stability • Discuss what is meant by the band of stability • Calculate mass deficiency and nuclear binding energy • Describe the common types of radiation emitted when nuclei undergo radioactivity decay • Write and balance equations that describe nuclear reactions

  3. Unit Objectives • Predict the different kinds of nuclear reactions undergone by nuclei, depending on their positions relative to the band of stability • Describe methods for detecting radiation • Understand half-lives of radioactive elements • Carry out calculations associated with radioactive decay • Interpret decay series

  4. Unit Objectives • Discuss some uses of radionuclides, including the use of radioactive elements for dating objects • Describe some nuclear reactions that are induced by bombardment of nuclei with particles • Discuss nuclear fission and some of its applications including nuclear reactors • Discuss nuclear fusion and some prospects for and barriers to its use for the production of energy

  5. So What’s Nuclear Chemistry? • Nuclear chemistry is concerned with the behavior of protons and neutrons in the atomic nucleus Radioactive tracers Radioactivity and radioactive decay Nuclear reactors Carbon dating Nuclear weapons Radiation therapy atomic bomb

  6. Beginning of Nuclear Science • Acceptance of Dalton’s theory convinced scientists that one element could not be converted in another (transmutation) Before there was Chemistry there was Alchemistry

  7. Beginning of Nuclear Science • In 1896, Henri Becqurel accidentally discovered radioactivity in Uranium (U) salts. • In 1898, Marie and Pierre Curie discovered two new radioactive elements in U mine residue. • Po (polonium) and Ra (radium) • In 1898, Ernest Rutherford discovered that radioactivity has two distinct forms. •  and  radiation Marie Curie (1867-1934)

  8. Nuclear Reactions Particles within the nucleus, such as protons and neutrons, are involved in reactions. Elements may be converted from one element to another. Chemical Reactions Usually only the outer most electrons participate in reactions. No new elements can be produced, only new chemical compounds. Comparison Of Chemical and Nuclear Reactions

  9. Nuclear Reactions Release or absorb immense amounts of energy, typically 1000 times more. Rates of reaction are not influenced by external factors. Chemical Reactions Release or absorb much smaller amounts of energy. Rates of reaction depend on factors such as concentration, pressure, temperature, and catalysts. Comparison Of Chemical and Nuclear Reactions

  10. Fundamental Particles of Matter • So… what do you remember about these particles?

  11. The Nucleus • The nucleus consists of protons and neutrons in a very small volume. • Since nearly all the mass of an atom resides in the nucleus  nucleus very DENSE (2 x 1014 g/cm3) • So how can positively charged protons be packed so closely together without causing most nuclei to spontaneously decompose ? • There are over 100 short-lived subatomic particles as products of nuclear reactions (quarks) • They help overcome proton-proton repulsion

  12. Neutron-Proton Ratio and Nuclear Stability • Recall… • Nuclides denotes different nuclei • Nuclide symbol for an element AZE • What is the nuclide symbol for an element with 79p, 118n and 76e? • Isotopes are nuclei that have the same number of protons but different neutron numbers. • Isotopes are the same element.

  13. Neutron-Proton Ratio and Nuclear Stability • Experimentally, it can be shown that nuclei have a preference for even numbers of protons and neutrons Abundance of naturally occurring nuclides

  14. Neutron-Proton Ratio and Nuclear Stability • Special stability is associated with certain proton and neutron numbers (or sum of the two) • Magic numbers are: 2 8 20 28 50 82 126

  15. Neutron-Proton Ratio and Nuclear Stability • Example nuclides with magic numbers of nucleons includes:

  16. Plot of # neutrons versus atomic # • stable nuclei (green dots) are located in the band of stability • all other nuclei are unstable and radioactive (white, blue and pink) • To note: • For low atomic numbers ( 20), the most stable nuclei have equal numbers of protons & neutrons (N =Z) • Above Z=20, the most stable nuclei have more neutrons than protons

  17. Nuclear Stability and Binding Energy • Experimentally shown that masses of atoms other than hydrogen are always LESS than sum of their particles • Referred to as mass deficiency (Δm)

  18. Nuclear Stability and Binding Energy • Example 26-1: Calculate the mass deficiency for 39K. The actual mass of 39K is 39.32197 amu per atom.

  19. Nuclear Stability and Binding Energy • Example 26-1: Calculate the mass deficiency for 39K. The actual mass of 39K is 39.32197 amu per atom. Therefore, in one atom

  20. Nuclear Stability and Binding Energy • You try it! • Questions: 14 (a) , 16 (a) & (b)

  21. Nuclear Stability and Binding Energy • Some what happened to the “lost mass”??? • Einstein’s Theory of Relativity:- • Matter & energy are equivalent (E = mc2) • Therefore the mass lost was converted to energy • ( To this date the reverse has not yet been achieved on a large scale, i.e. converting energy into matter) • The mass defect is the mass of the nuclear particles that has been used to bind the nucleus in the nuclear binding energy or strong nuclear force.

  22. Nuclear Stability and Binding Energy • Due to the Einstein relationship, we can calculate the nuclear binding energy for a nucleus.

  23. Nuclear Stability and Binding Energy • Example 26-2: Calculate the nuclear binding energy of 39K in J/mol of K atoms. 1 J = 1 kg m2/s2. • Step 1: Find mass deficiency for 1 mol of atoms  Δm = 0.00100 amu * 6.022 x 1023 atoms = 6.022 x 1020 amu / mol atom 1 mol • Step 2: Convert amu to grams  This is the SAME value as amu/ atom: • Δm = 0.00100 amu * 6.022 x 1023 atoms * 1 g = 0.0010 g • atom 1 mol 6.022 x 1023 amu mol

  24. Nuclear Stability and Binding Energy • Example 26-2: Calculate the nuclear binding energy of 39K in J/mol of K atoms. 1 J = 1 kg m2/s2. • Step 3: Convert grams to kilograms  • 0.0011 g x 1 kg = 1.1 x 10-6 kg /mol • mol 1000 g

  25. Nuclear Stability and Binding Energy • Example 26-2: Calculate the nuclear binding energy of 39K in J/mol of K atoms. 1 J = 1 kg m2/s2. • Step 4: Substituting for m in Energy equation  • Step 5: Converting kg m2 / s2mol to J  This is a huge amount of energy. Enough to heat 60,000 to 70, 000 tons of water from 0 -100 0C. It’s also the amount of energy needed to separate 1 mole of 39K nuclei into its protons and neutrons.

  26. Nuclear Stability and Binding Energy • You try it! • Questions 14 (b), 16 (c) (d) & (e) • Remember: • 1 J = 1 kg m2/s2 • 1g = 6.022 x 1023 amu

  27. Radioactive Decay • Nuclei whose neutron-to-proton ratio lies outside the band of stability experience spontaneous radioactive decay. • Emit one or more particles, electromagnetic rays or both.

  28. The major types of natural radioactivity: • Alpha emission • Beta emission • Gamma emission • Positron emission • Electron capture • The type emitted depends on where the nucleus is relative to the band of stability

  29. Radioactive Decay or He ion (He 2+) Nucleon = number of particles in the nucleus; protons and neutrons

  30. Radioactive decay Chemical equations show equal numbers of each kind of atom and equal number of charges on either side of the equation Law of conservation of mass In nuclear reactions a proton can be converted to a neutron and vice versa So, the total number of nucleons remain the same

  31. Equations for Nuclear Reactions • Two conservation principles hold for ALL nuclear reaction equations. • The sum of the mass numbers of the reactants equals the sum of the mass numbers of the products. • The sum of the atomic numbers of the reactants equals the sum of the atomic numbers of the products.

  32. For the general reaction: The two conservation principles demand M1 = M2 + M3 and Z1 = Z2 + Z3 Where the M's are mass numbers, And the Z's are atomic numbers. Equations for Nuclear Reactions

  33. Neutron Rich Nuclei (Above the Band of Stability) • These nuclei have too high a ratio of neutrons to protons. • Decays must lower this ratio and include: • neutron emission • beta emission • A beta particle is an electron ejected from the nucleus when a neutron is converted to a proton;

  34. Neutron Rich Nuclei (Above the Band of Stability) • Beta emission simultaneously decreases the number of neutrons (by one) and increases the number of protons (by one). • Efficiently changes the neutron to proton ratio. • Examples of beta emission: NOTE: the SUM of mass numbers are the same on either side of equation. Likewise the atomic numbers Note: Because there is a change in atomic number, the identity of the atom changes: C  N

  35. Neutron Rich Nuclei (Above the Band of Stability) • Neutron emission does not change the atomic number, but it decreases the number of neutrons. • The product isotope is less massive by the mass of 1 neutron. • Examples of neutron emission Because the atomic numbers do not change  identity of the atom remains the same

  36. Neutron Poor Nuclei (Below the Band of Stability) • These nuclides have too low a ratio of neutrons to protons. • Nuclear radioactive decays must raise this ratio • The possible decays include: • electron capture • positron emission

  37. Neutron Poor Nuclei (Below the Band of Stability) • Electron capture involves the capture of an electron in the lowest energy level in the atom by the nucleus. • conversion of a proton to a neutron

  38. Neutron Poor Nuclei (Below the Band of Stability) • A positron has the mass of an electron but has a positive charge. • The symbol is 0+1e. • Positron emission is associated with the conversion of a proton into a neutron.

  39. Nuclei with Atomic Number Greater than 83 • Alpha emission occurs for some nuclides, especially heavier ones. • Alpha () particles are helium nuclei, 42He, containing two protons and two neutrons. • Alpha emission increases the neutron-to-proton ratio.

  40. Nuclei with Atomic Number Greater than 83 • All nuclides having atomic numbers greater than 83 are beyond the belt of stability and are radioactive. • Many of these isotopes decay by emitting alpha particles.

  41. Nuclei with Atomic Number Greater than 83 • The transuranium elements (Z>92) also decay by nuclear fission in which the heavy nuclide splits into nuclides of intermediate mass and neutrons.

  42. Detection of Radiation • Present radiation detection schemes depend on the fact that particles and radiations emitted by radioactive decay are energetic and some carry charges. • Photographic Detection • Radioactivity affects photographic plates or film as does ordinary light. • Medical and dental x-ray photographs are made using this technique.

  43. Detection of Radiation • Fluorescence Detection • Fluorescent substances absorb energy from high energy rays and then emit visible light. • A scintillation counter is an instrument using this principle.

  44. Detection of Radiation • Cloud Chambers contain air saturated with a vapor. • Radioactive decay particles emitted ionize the air molecules in the chamber. • The vapor condenses on these ions. • Then the ion tracks are photographed.

  45. Detection of Radiation • Diagram of a Simple Cloud Chamber

  46. Detection of Radiation • A Cloud Chamber Photo from a Large Detector.

  47. Detection of Radiation • Gas Ionization Counters • The ions produced by ionizing radiation are passed between high voltage electrodes causing a current to flow between the electrodes and the current is amplified. • This is the basis of operation of gas ionization counters such as the Geiger-Mueller counter.

  48. Detection of Radiation • Schematic of Geiger Counter

  49. Detection of Radiation • Picture of a Geiger Counter

  50. Rates of Decay and Half-Life • Radionuclides have different stabilities and decay at different rates ( seconds to millions yrs) • The rates of all radioactive decays are independent of temperature and obey first order kinetics. • Rate proportional to the conc. of only one substance Where: A0 and A can either be molar conc. or masses of reactant Where: A0 = original mass or conc A = remaining mass or conc

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