Nuclear Chemistry

1. Nuclear Chemistry

2. Nuclear Reactions • Nuclear reactions involve changes in the composition of nuclei. • Radioactive isotopes are isotopes who nuclei are unstable-that is the nuclei spontaneously disintegrate, changing into different nuclei. • Radioactive decay-the process by which radioactive isotopes spontaneously emit small particles and/or energy in order to attain a more stable nuclear state.

3. Radioactive Isotopes (Radioisotopes) • All elements have at least one isotope that is radioactive. • All isotopes with atomic numbers larger than 83 are radioactive. • Some radioactive isotopes occur in nature while others are created in the laboratory.

4. Types of Radioactive Decay • Beta Emission • Electron Capture • Positron Emission • Alpha Emission • Neutron Emission • Gamma Rays

5. Beta Emission • Beta particles are fast moving electrons that are emitted from the nucleus. • A neutron is converted into a proton and an electron-the proton remains in the nucleus and the electron is emitted • Beta particles have a negligible mass, a negative charge, and have a mid-range penetrating ability (stopped by aluminum foil) • Symbol for beta particles include β and 0-1e. • Beta emission occurs when the ratio of neutrons to protons is too high. (Mass number is greater than the atomic mass) • Example: 22888Ra  or 146C 

6. Positron Emission or Electron Capture • Positron emission is most common with artificially radioactive isotopes of lighter elements. • Electron capture occurs most often with heavier elements. • Both types occur when the ratio of neutrons to protons is too low. (Mass number is less than the atomic mass) • Positrons have the mass of an electron but are positively charged. Positrons are emitted when a proton is converted to a neutron. The symbol for a positron is 0+1β. • Example: 3819K  • Electron capture occurs when an electron from the first energy level is captured by the nucleus. The electron combines with a proton to form a neutron. • The symbol for an electron is the same as the beta particle. • Example: 10647 Ag + o-1e 

7. Alpha Emission • Alpha emission occurs most often in heavier radioactive isotopes that are low in neutrons. (Mainly seen in large nuclei with atomic numbers larger than 60) • Alpha particles are helium nuclei, 42He, consisting of two protons and two neutrons. • Alpha particle are 8000 x larger in mass than a beta particle, have very low penetrating ability, and a positive 2 charge. • Example: 20482Pb 

8. Gamma Rays • Gamma radiation has no mass or charge and is unaffected by magnetic or electrical fields. • Gamma radiation accompanies alpha and beta radiation, is the most penetrating, and the most damaging. • Gamma radiation is the energy that is released as a portion of the nuclei disintegrates. • The symbol for gamma radiation is 00γ

9. Rates of Radioactive Decay • The rates of all radioactive decays obey first order kinetics. (Independent of temperature) • The decay rates vary from isotope to isotope-some decay in a fraction of a second while others require millions of years. • See table 18.3 on page 849.

10. Uses of radioactive isotopes • Carbon dating (carbon-14’s half life is 5730 years). • Medical Uses-radioactive tracers • Agricultural Uses • Industrial Uses

11. Other Nuclear Reactions • Fission-Isotopes of atomic number greater than 80 split into nuclei of intermediate masses and emit one or more neutrons (see page 859-862) • Fusion-the joining of light nuclei to form heavier nuclei (see page 863) Much greater amounts of energy are produced in fusion reactions than in fission reactions.

12. Homework • All homework problems listed in the outline for Chapter 18 can now be completed.