Nuclear Chemistry

1. Nuclear Chemistry

2. Definitions • Nuclear radiation: radiation emitted from a nucleus during nuclear decay • alpha particle (a): a helium nucleus, He2+; contains two protons and two neutrons, has mass of 4 amu, and atomic number 2 • beta particle (b): an electron; has a charge of -1, and a mass of 0.00055 amu • positron (b+): has the mass of an electron but a charge of +1 • gamma ray (g): high-energy electromagnetic radiation

3. Electromagnetic Radiation • All electromagnetic radiation consists of waves • the wavelength, l is the distance between each crest. • frequency, n: the number of crests that pass a given point in a second

4. Electromagnetic Radiation • The electromagnetic spectrum

5. Nuclear Radiation • Table 9.1 summarizes the types of nuclear radiation we deal with in this chapter

6. Nuclear Radiation • There are more than 300 naturally occurring isotopes • of these 264 are stable, they do not give off radiation • among the lighter elements, stable isotopes have approximately the same number of protons and neutrons; this is the case of 126C, 168O, and 2010Ne • among the heavier elements,stability requires more neutrons than protons • More than 1000 artificial isotopes have been made in the laboratory; all are radioactive

7. Beta Emission • beta emission: a type of nuclear decay in which a neutron is converted to a proton and an electron, and the electron is emitted from the nucleus • emission of a beta particle transforms the element into a new element with the same mass number but an atomic number one unit greater • phosphorus-32, for example, is a beta emitter • note in this nuclear decay equation that the sum of both the mass numbers and atomic numbers are the same on each side of the equation

8. Beta Emission • Problem: carbon-14 is a beta emitter. When it undergoes beta emission, into what element is it converted?

9. Beta Emission • Problem: carbon-14 is a beta emitter. When it undergoes beta emission, into what element is it converted? • Solution: it is converted into nitrogen-14

10. Alpha Emission • alpha emission: a type of nuclear decay in which a helium nucleus is emitted from the nucleus • in alpha emission, the new element formed has an atomic number two units lower and a mass number four units lower

11. Positron Emission • positron emission: a type of nuclear decay in which a positive electron is emitted from the nucleus • in positron emission, the new element formed has an atomic number one unit lower but the same mass number

12. Gamma Emission • In pure gamma emission, there is no change in either the atomic number or the mass number of the element • a nucleus in a higher-energy state emits gamma radiation as it returns to its ground state (its most stable energy state) • in this example, the notation “11m” indicates that the nucleus of boron-11 is in a higher-energy (excited) state

13. Half-Life • half-life of a radioisotope, t1/2: the time it takes one half of a sample of a radioisotope to decay • iodine-131 decays by beta, gamma emission

15. Characteristics of Radiation • Intensity • to measure intensity, we take advantage of the ionizing property of radiation • instruments such as a Geiger-Müller or proportional counter contain a gas such as helium or argon • when a radioactive nucleus emits beta particles, these particles ionize the gas in the instrument; it registers the ionization by indicating that an electric current has passed between two electrodes • another measuring device, called a scintillation counter, has a phosphor that emits a unit of light when a beta particle or gamma ray strikes it • intensity is recorded in counts/min or counts/s

16. Characteristics of Radiation • Energy and penetrating power

17. Radiation Dosimetry • average exposure to radiation from common sources

18. Nuclear Medicine • Radioisotopes have two main uses in medicine; diagnosis and therapy

19. Nuclear Medicine

20. Nuclear Fusion • The transmutation of two hydrogen nuclei into a helium nucleus liberates energy in the form of photons • this process is called nuclear fusion • all transuranium elements (elements with atomic number greater than 92) are artificial and have been prepared by nuclear fusion • to prepare them, heavy nuclei are bombarded with lighter ones

21. Nuclear Fusion • examples are the preparation of Bk, Cf, and Lr • these transuranium elements are unstable and have very short half-lives; that of lawrencium-257, for example, is only 0.65 second

22. Nuclear Fission • Nuclear fission: the fragmentation of larger nuclei into smaller ones • when uranium-235 is bombarded with neutrons, it is broken into two smaller elements • more importantly, energy is released because the products have less mass than the starting materials • the mass decrease in fission is converted into energy • this form of energy is called atomic energy

23. Nuclear Fission • Nuclear fission is a chain reaction

24. Nuclear Fission • today more than 15% of the electrical energy in the United States is supplied by nuclear power plants • disposal of spent but still radioactive fuel materials is a major long-term problem • spent fuel contains high-level fission products together with recoverable uranium and plutonium • in addition, there are radioactive wastes from nuclear weapons programs, research reactors, and so forth • recently the government gave its final approval to store nuclear wastes at a site deep under Yucca Mountain in Nevada