Beginners Guide to Nuclear Chemistry: Radioactivity and Nuclear Decay

1. Nuclear Chemistry I. Radioactivity

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. Radioactivity • It is not uncommon for the nucleus of an element to be unstable, or radioactive. • There are several ways a radioactive element can decay into a new element.

4. A. Definitions • Radioactivity • emission of high-energy radiation from the nucleus of an atom • Transmutation • process of changing one element into another via nuclear decay Nuclide • an atom that is identified by the number of protons and neutrons in its nucleus ie. nuclear symbol

5. 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)

6. 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) causing a neutron to turn into a proton

7. 11 6 11 5 0 1 C B e e 0 1 +  Types of Radioactive Decay Positron Emission A particle with the mass of an electron, but with a positive charge. Causes a proton to turn into a neutron.

8. 0 0  Types of Radioactive Decay Gamma Emission Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

9. B. Types of Radiation • Alpha () • helium nucleus paper • Beta-minus (-) • electron lead • Gamma () • Highest energy concrete

10. C. Nuclear Decay TRANSMUTATION • Alpha Emission • Beta Emission

11. Alpha Decay Alpha particle (helium nucleus) is released. Alpha decay only occurs with very heavy elements.

12. Complete the reaction and identify the type of decay.

13. Example Half-lives polonium-194 0.7 seconds lead-212 10.6 hours iodine-131 8.04 days carbon-14 5,370 years uranium-238 4.5 billion years D. Half-life • Half-life (t½) • time it takes for half of the nuclei in a sample to decay

14. D. Half-life • How much of a 20-g sample of sodium-24 would remain after decaying for 30 hours? Sodium-24 has a half-life of 15 hours. GIVEN: total time = 30 hours t1/2 = 15 hours original mass = 20 g WORK: number of half-lives = 2 20 g ÷ 2 = 10 g (1 half-life) 10 g ÷ 2 = 5 g (2 half-lives) 5 g of 24Na would remain.

15. Calculating Amount of Radioactive Material Remaining

16. Sample Problem • Silicon-31 has a half-life of 2.5 hours. If we begin with a sample containing 1000 mg of Si-31, what is the approximate amount remaining after 10 hours?

17. C. Nuclear Decay • Why nuclei decay… • to obtain a stable ratio of neutrons to protons Stable Unstable (radioactive)

18. Neutron-Proton Ratios • Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. • A strong nuclear force helps keep the nucleus from flying apart. • Neutrons play a key role stabilizing the nucleus. • Therefore, the ratio of neutrons to protons is an important factor.

19. Neutron-Proton Ratios For smaller nuclei (Z  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

20. Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

21. Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

22. Stable Nuclei • Nuclei above this belt have too many neutrons. • They tend to decay by emitting beta particles.

23. Stable Nuclei • Nuclei below the belt have too many protons. • They tend to become more stable by positron emission or electron capture.

24. Stable Nuclei • There are no stable nuclei with an atomic number greater than 83. • These nuclei tend to decay by alpha emission.

25. Nuclear Chemistry II. Nuclear Reactions

26. A. Fission • splitting a nucleus into two or more smaller nuclei • some mass is converted to large amounts of energy • E = mc2

27. Fission • Fission occurs when an unstable heavy nucleus splits apart into two lighter nuclei, forming two new elements. • Fission can be induced by free neutrons. • Mass is destroyed and energy produced according to E = mc2.

28. Fission • Fission occurs only with very heavy elements, since fissionable nuclei are too large to be stable. • A charge/mass calculation is performed to balance the nuclear equation. • Mass is destroyed and energy produced according to E = mc2.

29. Sample problem Complete the following reaction

30. A. Fission • chain reaction - self-feeding reaction

31. B. Fusion • combining of two nuclei to form one nucleus of larger mass • produces even more energy than fission • occurs naturally in stars and our sun.

32. 1 1 H H 2 He 1 1 2 Fusion • Fusion occurs when two light nuclei come together to form a new nucleus of a new element. • Fusion of light elements can result in non-radioactive waste.

33. Nuclear Chemistry III. Applications

34. Cooling Tower A. Nuclear Power • Fission Reactors

35. A. Nuclear Power • 235U is limited • danger of meltdown • toxic waste • thermal pollution • Hydrogen is abundant • no danger of meltdown • no toxic waste • not yet sustainable FISSION FUSION vs.

36. B. Other Uses • Medical Uses • PET Scans • Cancer Treatment • Radioactive Tracers • Nuclear Weapons • Smoke Detectors • Dating

37. Nuclear Weapons • Nuclear weapons have been used only twice, although they have been tested thousands of times. • Weapons based on nuclear fission involve slamming together enough material to produce an uncontrolled fission chain reaction. Little Boy was dropped on Hiroshima and contained U-235 produced in Oak Ridge, TN.