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NUCLEAR CHEMISTRY

This review provides an overview of nuclear chemistry, covering topics such as atomic number, mass number, isotopes, nucleons, mass defect, nuclear binding energy, binding energy per nucleon, nucleus stability, nuclear reactions, and types of radiation.

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NUCLEAR CHEMISTRY

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  1. NUCLEAR CHEMISTRY THE ULTIMATE IN SPONTANEITY

  2. Review • Atomic number (Z) – number of protons • Mass number (A) – sum of the protons and the neutrons • Isotopesor Nuclides– atoms with the same atomic number but different mass numbers, different numbers of neutrons. • Nucleons – the particles that make up the nucleus. (protons and neutrons = mass #)

  3. Facts about the nucleus • Very small • Very dense • Held together by the nuclear strong force • Location of the protons and neutrons • Most of the mass of an atom is located

  4. Mass Defect • You might expect the mass of an atom to be the same as the sum of it’s parts, protons, neutrons, and electrons. • Protons 1.007276 amu • Neutrons 1.008665 amu • Electrons 0.0005486 amu

  5. Mass Defect • The difference between the calculated mass and the actual mass is known as mass defect.

  6. What causes the lost mass? • According to Albert Einstein, mass and energy can be converted into each other. • Some of the mass is lost during the formation of the nucleus. • The amount of energy can be calculated using Einstein’s famous equation.

  7. Nuclear Binding Energy • The energy released when a nucleus is formed from nucleons. • E = mc2 • E is for energy unit: Joules (J)=kg.m2/s2 • M is for mass unit: kilograms (kg) • C is the speed of light (squared) • 3.00 x 108 m/s

  8. Binding Energy per Nucleon • The binding energy per nucleon is used to compare the stability of different nuclides. • It is the binding energy of the nucleus divided by the number of nucleons that are in the nucleus.

  9. Binding Energy • The higher the binding energy per nucleon, the more tightly packed the nucleons are held together, the more stable the nuclide. "A is for atom" (1952) video

  10. Binding Energy • Elements with intermediate atomic masses have the greatest binding energies per nucleon and are therefore the most stable. Iron is the most stable isotope.

  11. Binding Energy per Nucleon

  12. How does the nucleus stay together? • Relationship between the nuclear strong force and the electrostatic forces between protons. • Like charges repel each other through electrostatic repulsion

  13. How does the nucleus stay together? • The nuclear strong force allows protons to attract each other at very short distances. • As protons increase in the nucleus so does the electrostatic forces, faster than nuclear forces.

  14. Why do atoms want more neutrons than protons? • More neutrons are required to increase the nuclear force and stabilize the nucleus. • > 83 the repulsive forces of protons is so great that no stable nuclides exist.

  15. Band of Stability • Stable nuclides have certain characteristics • When the number of neutrons are plotted against the number of protons a pattern is observed

  16. Band of Stability • The neutron-proton ratio of stable isotopes cluster around a narrow band called the band of stability. • For atoms with low atomic numbers the ratio is 1 : 1 • As the atomic number increases, the ratio increases to 1.5 : 1

  17. Band of Stability

  18. Magic Numbers • Stable nuclides tend to have even numbers of nucleons. • 256 stable nuclides • 159 have both even protons and neutrons • Only 4 have odd numbers of protons and neutrons.

  19. Nuclear Shell Model • Nucleons exist in different energy levels, or shells, in the nucleus. • The number of nucleons that represent completed nuclear energy levels, • 2, 8, 20, 28, 50, 82, and 126 • Called magic numbers

  20. Nuclear Reactions • Unstable nuclei undergo spontaneous changes that change the number of protons and/or neutrons. • Give off large amount of energy by emitting radiation during the process of radioactive decay.

  21. Nuclear Reactions • Eventually unstable radioisotopes of one element are transformed into stable, non-radioactive, isotopes of a different element. • Total of mass number and atomic number must be equal on both sides of a reaction.

  22. Nuclear Reactions • When the atomic number changes, the identity of the element changes. • A transmutation is a change in the identity of a nucleus as a result of a change in the number of protons.

  23. Nuclear Reactions Mass Number’s must equal on both sides of the equation. 14 0 14 C  e + N 6 -1 7 Atomic number’s must equal on both sides of the equation

  24. Nuclear Reactions • Try one! 238 4 U  He + 92 2 _______

  25. Nuclear Reactions • Try one! 238 4 234 U  He + Th 92 2 90

  26. Types of Radiation • Alpha Radiation • Alpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus stripped of it’s electrons

  27. Alpha Radiation • 2 protons and 2 neutrons. • Charge +2 (lost both electrons) • Large mass, 4 amu. • Low penetration power • Shielded by paper or clothing.

  28. Alpha Radiation • Occurs in unstable nuclei that has too many protons and too many neutrons. • Effect on the nucleus: • Mass number is reduced by 4 amu • Atomic number is reduced by 2

  29. Beta Radiation • A fast moving electron • Occurs in an unstable nuclei that has too many neutrons

  30. Beta Radiation • Converts a neutron into a proton and a beta particle • An electron that doesn’t belong in the nucleus and therefore gets thrown out.

  31. Beta radiation • Charge -1 • Mass = 1/1840 or 0.0005486 amu • Moderate penetration power (0.4 cm) • Shielded by metal foil • Effect on nucleus: • Mass number remains the same • Atomic number increases by 1

  32. Positron emission • A positive electron • Has the same mass as an electron, 1/1840 or 0.0005486 amu

  33. Positron emission • Charge +1 • Occurs in unstable nuclei that has too many protons • Converts a proton into a neutron • Effect on the nucleus: • Mass number remains the same • Atomic number decreases by 1

  34. Electron Capture • Occurs in unstable nuclei that has too many protons: same as positron emission • An inner orbiting electron gets captured by the nucleus and is used to convert a proton into a neutron.

  35. Electron Capture • The effect is the same as for positron emission • Mass number remains the same • Atomic number decreases by 1

  36. Gamma radiation • Is high-energy electromagnetic radiation • No charge and no mass: no effect on the nucleus • Penetration power is high and only lead and several centimeters of concrete can slow it down • Always accompanies another form of radiation

  37. Half-Life • No two radioisotopes decay at the same rate. • t1/2 is the symbol for half-life • Half-life is the time required for half the atoms of a radioactive nuclide to decay. • The longer the half-life the more stable the nuclide.

  38. Half-life Variables • Variables • Ao = original amount • A = final amount • T = total time elapsed • t1/2 = half-life • n = number of half-lives

  39. Half-life Equations • n = T t1/2 • Ao = A * 2n

  40. Half-life Calculations • To solve half-life problems first write down all of the data in the problem. • Determine which formula you’re going to use. • Plug in the values and calculate

  41. Half-life problem • Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remains after 57.2 days if you start with 4.0 mg of the isotope? A0 = 4.0 mg A = ? T = 57.2 days n = T / t1/2 t1/2 = 14.3 days A = A0 / 2n

  42. Problem (Con’t) • n = T / t1/2 n = 57.2 days / 14.3 days n = 4 half-lives • A = A0 / 2n A = 4.0 mg / 24 A = 4.0 mg / 16 A =0.25 mg

  43. ½ remain ½ decay ¼ remain ¾ decay 1/8 remain 7/8 decay Half-life graphic • Picture representation of half-life

  44. Total time problem • The half-life of radon-222 is 3.824 days. After what time will one-fourth of a given amount of radon remain? A = ¼ remain n = T / t1/2 T = ? t1/2 = 3.824 days

  45. Total time problem (Con’t) * We don’t need to know the beginning amount. Looking at the picture representation we see that it needs to go through 2 half-lives in order to have ¼ remaining. • n = T / t1/2 • 2 =T / 3.824 days • T = 2 x 3.824 days • T = 7.648 days

  46. Decay Series • One nuclear reaction is not always enough to produce a stable nuclide. • A decay series is a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached.

  47. Decay Series • The heaviest nuclide of each decay series is the parent nuclide and the nuclides produced by the decay is called the daughter nuclide.

  48. Artificial Transmutations • Artificial radioactive nuclides are radioactive nuclides not found naturally on Earth. • They are made by artificial transmutations, bombardment of nuclei with charged and uncharged particles.

  49. Artificial Transmutations • Neutrons have no charge and no mass and can easily penetrate the nucleus of an atom. • Positively charged alpha particles, protons, and other ions are repelled by the nucleus.

  50. Artificial Radioactive Nuclides • A great deal of energy is needed to bombard nuclei with these particles. • Energy may be supplied by accelerating these particles in the magnetic or electric field of a particle accelerator. • Radioactive isotopes of all the natural elements have been produced.

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