2. Radioisotopes The nuclei of some atoms are unstable and undergo spontaneous changes called radioactive decay. One such change is called beta decay. During beta decay a neutron changes into a proton and an electron transforming the atom to an element with an atomic number which is one higher while the atomic mass barely changes.

3. + Tritium atoms, H-3, undergo spontaneous beta decay. Shown below is a tritium nucleus. 1 proton 2 neutrons

4. + 2 proton 1 neutron 1 electron + v v v v v The highly energetic electron is ejected from the nucleus as radiation. It travels at a speed of 1.3 x 10 5 km/s. The equation is: 1H1H 3 2 He 3 + 1- e 0

5. Two other forms of radiation from radioactive decay are: alpha particle emission and gamma rays. An alpha particle contains 2 protons and 2 neutrons while gamma rays do not result in the release of particles. The rate of release of radiation is expressed as a half-life. A half-life is the length of time required for half of the original material to decay.

6. Tritium has a half-life of 12.26 years. 12.26 a (annum is latin for years) If 10 g of tritium were left for 12.26 a there would be 5 g left. After 24.52 a there would be 2.5 g left. Here is a table showing the quantity of tritium remaining after different time periods.

9. Here is an example of Alpha decay. Alpha decay involves the emission of a helium-4 nucleus. Write an equation which shows how uranium-235 undergoes alpha decay. + 90 231 Th

10. Different radioactive isotopes decay at different rates. If 100 g of a radioactive material decays for 10 years and 50 g remains this substance is said to have a half life of 10 years. 5 y After 10 y only 50 g remain If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g -------> 50 g

11. If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 200 g

12. If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g 100 g left after 5 years

13. If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g -------> 50 g 50 g left after 10 years

14. If 200 g of a radioactive material with a half-life of 5 years, is left to decay for 10 years how much of the original material is left? 200 g -------> 100 g -------> 50 g ----> 25g 25 g left after 15 years

15. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g

16. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 256 g 512 g ---> 256 g 25 da

17. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 128 g 512 g ---> 256 g ---> 128 g 25 da Total - 50 da

18. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 64 g 512 g ---> 256 g ---> 128 g ---> 64 g 25 da Total - 75 da

19. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 32 g 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da Total - 100 da

20. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 16 g 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da Total - 125 da 16 g 25 da

21. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da Total - 150 da 16 g 25 da 8 g 25 da

22. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da Total - 175 da 16 g 25 da 4 g 8 g 25 da 4 g 25 da

23. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da Total - 200 da 16 g 25 da 2 g 8 g 25 da 4 g 25 da 2 g 25 da

24. Show the decay sequence for 512 g of a substance with a half-life of 25 da. 512 g ---> 256 g ---> 128 g ---> 64 g ---> 32 g 25 da Total - 225 da 16 g 25 da 1 g 8 g 25 da 4 g 25 da 2 g 25 da 1 g 25 da

25. If U-235 has a half-life of 7.1 x 10 8 y. How many years would it take 32 g to decay to 2 g? 32 g --> 16 g --> 8 g --> 4 g --> 2 g 4 half lifes 2.84 x 10 9 y. Cs-136 has a half-life of 13 da. If 1024 g was left to decay for 65 da how much of the original material would be left? 65/13 = 5 hl 1024 g -> 512 g -> 256 g -> 128 g -> 64 g -> 32 g or 1024 g x (1/2) 5 = 32 g

26. To find the quantity of material remaining use this formula Mass remaining = Original Mass x 1 2 # of Half-lives Pb-212 has a half-life of 10.6 h. If 12.5 g of Pb-212 is left for 84.8 h how much of the original material is left? Mass remaining = Original Mass x 1 2 # of Half-lives 12.5 g x (0.5) 84.8/10.6 = 0.0488 g