Nuclear Chemistry

1. Nuclear Chemistry

2. The Nucleus • Remember that the nucleus is comprised of protons and neutrons. • The number of protons is the atomic number. • The number of protons and neutrons together is the mass of the atom.

3. Isotopes • Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. • There are three naturally occurring isotopes of uranium: • Uranium-234 • Uranium-235 • Uranium-238

4. Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability. http://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass Most nuclei are stable. It is the ratio of neutrons to protons that determines the stability of a given nucleus.

5. Radioactivity • It is not uncommon for some nuclei to be unstable, or radioactive. • There are no stable nuclei with an atomic number greater than 83. • Radioisotopes = isotopes that are unstable and thus radioactive • There are several ways radionuclides can decay into a different nuclide.

6. Radioactive Series • Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. • They undergo a series of decays until they form a stable nuclide (often a nuclide of lead). • Transmutation = the reaction by which the atomic nucleus of one element is changed into the nucleus of a different element

7. pHET simulations of alpha decay of Polonium-211 to form Lead-207 and of Beta decay of Hydrogen-3 to Helium-3 • http://phet.colorado.edu/en/simulation/alpha-decay • http://phet.colorado.edu/en/simulation/beta-decay

8. 238 92 234 90 4 2 4 2 He U Th He +  Types of Radioactive Decay Alpha Decay = Loss of an -particle (a helium nucleus) Correction Atomic # decreases by 2 # of protons decreases by 2 # of neutrons decreases by 2 Mass # decreases by 4

9. 131 53 131 54 0 −1 0 −1 0 −1 e I Xe e  +  or Types of Radioactive Decay Beta Decay = Loss of a -particle (a high energy electron) Atomic # increases by 1 # of protons increases by 1 # of neutrons decreases by 1 Mass # remains the same

10. 11 6 11 5 0 1 0 1 e C B e +  Types of Radioactive Decay Positron Emission = Loss of a positron (a particle that has the same mass as but opposite charge than an electron) Atomic # decreases by 1 # of protons decreases by 1 # of neutrons increases by 1 Mass # remains the same

11.  0 0 Types of Radioactive Decay Gamma Emission = Loss of a -ray (a photon of high-energy light that has no mass or charge & that almost always accompanies the loss of a nuclear particle; often not shown when writing nuclear equations)

13. Artificial Transmutation = done by bombarding the nucleus with high-energy particles (such as a neutron or alpha particle), causing transmutation 4020Ca + _____ -----> 4019K + 11H 9642Mo + 21H -----> 10n + _____ **Natural transmutation has a single nucleus undergoing change, while artificial transmutation will have two reactants (fast moving particle & target nuclei.**

14. Nuclear Fission • Nuclear fission is the type of reaction carried out in nuclear reactors. • = splitting of large nuclei into middle weight nuclei and neutrons

15. Nuclear Fission • Bombardment of the radioactive nuclide with a neutron starts the process. • Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. • http://phet.colorado.edu/en/simulation/nuclear-fission • This process continues in what we call a nuclear chain reaction.

16. Nuclear Fusion • = the combining of light nuclei into a heavier nucleus • 21H + 21H 42He + energy • Two small, positively-charged nuclei smash together at high temperatures and pressures to form one larger nucleus.

17. Energy changes in Nuclear Reactions E =mc2 • Einstein E =mc2 • mass defect For nuclear reactions • E = energy in Joules (J = kg•m2/s2) • m = mass in kg • C = speed of light • (2.9979 x 108 m/s)

18. Half-Life = the time it takes for half of the atoms in a given sample of an element to decay • Each isotope has its own half-life; the more unstable, the shorter the half-life. • Table T Equations: fraction remaining = (1/2)(t/T) # of half-lives remaining = t/T Key: t = total time elapsed T = half-life

19. PhET simulation of decay and half-life • http://phet.colorado.edu/en/simulation/radioactive-dating-game

20. Sample Half-Life Question 1A Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. (a) What fraction of a sample of Cr-51 will remain after 168 days? Step 1: Determine how many half-lives elapse during 168 days. Step 2: Calculate the fraction remaining.

21. Sample Half-Life Question 1B (b) If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days? Step 1: Calculate the mass remaining: mass remaining = fraction remaining X original mass (Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each half-life.)

22. Sample Half-Life Question 2 How much was present originally in a sample of Cr-51 if 0.75g remains after 168 days? Step 1: Determine how many half-lives elapsed during 168 days. Step 2: Multiply the remaining amount by a factor of 2 for each half-life.

23. Some practical uses of Radioisotopes (dating, chemical tracers, industrial applications, medical applications, nuclear power plants) Medical Uses • 60Co (cobalt-60) used in cancer treatments and used to kill bacteria in food products • 226Ra (Radium-226) used in Cancer treatment • 131I diagnosis and treatment of thyroid disorders • 11C Positron emission tomography (PET scans) Other Uses • 14C archaeological dating (of once living things) and radiolabelled organic compounds • 238U archaeological dating (U-238 to Pb-206 ratio) • 241Am (Americium-241) smoke detectors • 235U nuclear reactors and weapons