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Unit: Nuclear Chemistry

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1 Unit: Nuclear Chemistry
Day 2 – Notes Unit: Nuclear Chemistry Half-Life Pictures starting from upper right moving left: Radioactivity symbol, particle accelerator, mushroom cloud (nuclear explosion), Marie and Pierre Curie, cooling tower of nuclear power plant

2 After today you will be able to…
Identify the factor that nuclear stability is dependent on. Calculate the half-life for a given radioisotope. Calculate how much of a radioisotope remains after a given amount of time.

3 Nuclear Stability Close to 2,000 different nuclei are known.
Approximately 260 are stable and do not decay or change with time. The stability (resistance to change) depends on its neutron:proton ratio.

4 Nuclear Stability Plotting a graph of number of neutrons vs. number of protons for each element results in a region called the band of stability.

5 Nuclear Stability For elements with low atomic numbers (below 20) the ratio of neutrons:protons is about 1. Example: (6n/6p = 1) C 12 6

6 Nuclear Stability For elements with higher atomic numbers, stable nuclei have more neutrons than protons. The ratio of n:p is closer to 1.5 for these heavier elements. Example: 124n/82p = approx. 1.5 Pb 206 82

7 Nuclear Stability Ever wonder why some atomic masses listed on the Periodic Table have ( ) around them? Because their atomic masses are estimated due to radioactive decay! The neutron:proton ratio is important because it determines the type of decay that occurs. All nuclei that have an atomic number greater than 83 are radioactive.

8 Half-Life Half-life: (t1/2) the time required for half of the nuclei of a radioisotope sample to decay to products. Example: If you have 20 atoms of Radon-222, the half life is ~4 days. How many atoms remain at the end of two half lives? 0 t1/2 1 t1/ t1/2 4 days 8 days 20 atoms initially 10 atoms 5 atoms remain

9 Half-Life We can represent half-life graphically as well.
Example: Carbon-14

10 However, very seldom do we count atoms
However, very seldom do we count atoms. Therefore it is more appropriate to calculate amount that remains in terms of mass.

11 Half-Life Example: Carbon-14 emits beta radiation and decays with a half-life (t1/2) of 5730 years. Assume you start with a mass of 2.00x10-12g of carbon-14. How long is three half-lives? How many grams of the isotope remain at the end of three half-lives? 3(5730) = 2.00x10-12g x 1/2 x 1/2 x 1/2 = 17,190 years 2.5x10-13g

12 Half-Life b. How many grams of the isotope remain at the end of three half-lives? Alternatively, part b can also be calculated like this: x 1/2 x 1/2 x 1/2 0 t1/2 1 t1/2 2 t1/2 3 t1/2 2.00x10-12g initially 1.00x10-12g 5.00x10-13g 2.50x10-13g remains

13 Half-Life Example: Manganese-56 is a beta emitter with a half-life of 2.6 hours. How many half-lives did the sample go through at the end of 10.4 hours? What is the mass of maganese-56 in a 1.0mg sample of the isotope at the end of hours? 10.4 h/2.6 h = 1.0mg x 1/2 x 1/2 x 1/2 x 1/2 = 4 half-lives 0.063 mg

14 Complete and turn in the exit ticket.
Questions? Complete and turn in the exit ticket.


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