Nuclear Chemistry 1989 D By: Stephanie Chen and Stephanie Ng

1989 D The carbon isotope of mass 12 is stable. The carbon isotopes of mass 11 and mass 14 are unstable. However, the type of radioactivity decay is different for these two isotopes. Carbon – 12 is not produced in either case.

1989 D (a) Identify a type of decay expected for carbon – 11 and write the balanced nuclear reaction for that decay process.

1989 D (a) ANSWER  mass # = 11 (protons + neutrons) atomic # = 6 (protons) ____________________________ 5 neutrons neutrons / protons = 5 / 6 = 0.83  + The expected decay for carbon – 11 is beta decay 11 C 6 0 p +1 1 n 0 00 +1

1989 D (b) Identify the type of decay expected for carbon – 14 and write the balanced nuclear reaction for that decay process.

1989 D (b) ANSWER 14 C 6  + 14 N 7 0 β

1989 D (c) Gamma rays are observed during the radioactive decay of carbon – 11. Why is it unnecessary to include the gamma rays in the radioactive decay equation of (a)?

1989 D (c) ANSWER  + + (e = γ) Unstable isotopes emit α, β, and γ- radiation. Understand that while α and β-radiation represents the release of radioactive particles, γ-radiation is the emission of electromagnetic energy and thus cannot be represented chemically 11 C 6 11 B 5 0 β +1

1989 D (d) Explain how the amount of carbon – 14 in a piece of wood can be used to determine when the tree died.

1989 D (d) ANSWER The life of carbon-14 is 5760 years In order to find the piece of wood, we could use the half life equation k = / 5760 yrs 2.3 log xt/x0 = -kt or ln xt/x0 = -kt