1. Nuclear Changes Chapter 7

2. 7.1 What is Radioactivity? Large atoms are unstable. When the nucleus is crowded with protons and neutrons, it’s just ”too much.” The nucleus begins to emit (shoot out) particles and/or energy.

5. Radioactivity Penetrating power of different forms of radiation:

6. Radioactivity Marie (1867-1934) and Pierre Curie (1859-1906) isolated polonium and radium from pitchblende both elements more radioactive than pure uranium discovered that the source of energy (radiation) were the atoms themselves nature of radioactivity was still unknown

7. Radioactivity Ernest Rutherford (1871-1937) studied absorption of 'rays' emitted by uranium-containing minerals two types of rays:  - and  -rays  -rays are more penetrating than  -rays  - and  -rays are not rays at all (like X-rays or light) but streams of particles

8. Radioactivity  - and  -rays are streams of charged particles: How can you test if a particle is positively or negatively charged?

9. Radioactivity  - and  -rays are streams of charged particles: How about their mass? light particles are easier to deflect than heavy ones (pushing a freight train versus a bicycle!)

10. Radioactivity Ernest Rutherford (1871-1937)  -particles behave like electrons, (1 negative charge) - move very fast  -particles and have 4 times the mass of a hydrogen nucleus and twice the charge (2 positive charges)  -particle = Helium nucleus (2 protons, 2 neutrons)

11. Radioactivity  - and  -radiation are made up of particles,  -radiation is not!  -radiation is electromagnetic radiation (just like light and X-rays): no mass, no charge

12. Radioactivity Radioactive decay:  -decay U 92 238 the atomic number counts the number of protons the mass number counts protons and neutrons

13. Radioactivity Radioactive decay:  -decay U 92 238  2 4 + Th 90 234 the atomic number decreases by 2 (loss of 2 protons) the mass number drops by 4 (loss of a total of 2 protons and 2 neutrons)

15. Radioactivity Radioactive decay:  -decay Ra 88 226  2 4 + Rn 86 222 Rn 86 222  2 4 + Po 84 218 Cm 96 245  2 4 + Pu 94 241

16. Radioactivity Radioactive decay:  -decay Proton Neutron a Neutron may split into a Proton plus an Electron Electron

17. Radioactivity Radioactive decay:  -decay Proton Neutron Electron the electron is ejected from the nucleus as  -radiation......leaving behind a nucleus with an extra proton

18. Radioactivity Radioactive decay:  -decay Bi 83 210  1- 0 + Po 84 210 the atomic number increases by 1 amu (1 more proton) the mass number is unchanged (the electron mass in negligible)

19. Radioactivity Radioactive decay:  -decay C 6 14  1- 0 + N 7 14 H 1 3  1- 0 + He 2 3 Pb 82 214  1- 0 + Bi 83 214

21. Nuclear vs Chemical Reaction Na NaOH + HCl  H 2 O + NaCl O H H Cl Na O H H Cl  *** Not a true representation of this reaction in solution Chemical Reaction Nuclear Reaction  212 Po  4  + 82 Pb 2 208 84 *** Not a true representation of the nuclei

22. The Half-Life (t 1/2 ) of a Nuclear Reaction Half-life (t 1/2 ): The time it takes for half of the radioactive nuclei in a sample to decay. 48 radioactive particles at t=0 24 radioactive particles at t=1 (1 half life) 12 radioactive particles at t=1 (2 half life) 6 radioactive particles at t=1 (3 half life) # of radioactive nuclei

23. The Half-Life (t 1/2 ) of a Nuclear Reaction Half-life (t 1/2 ): The time it takes for half of the radioactive nuclei in a sample to decay. 48 radioactive particles at t=0 24 radioactive particles at t=1 (1 half life) 12 radioactive particles at t=2 (2 half lifes) 6 radioactive particles at t=3 (3 half lifes) # of radioactive nuclei Fraction of nuclei 48/48 = 1 @ t 1/2 = 1 24 = 1 48 2 @ t 1/2 = 2 12 = 1 * 1 = 1 48 2 2 4 @ t 1/2 = 3 6 = 1 * 1 * 1 = 1 48 2 2 2 8

24. The Half-Life (t 1/2 ) of a Nuclear Reaction Half-life (t 1/2 ): The time it takes for half of the radioactive nuclei in a sample to decay. 48 radioactive particles at t=0 24 radioactive particles at t=1 (1 half life) 12 radioactive particles at t=2 (2 half lifes) 6 radioactive particles at t=3 (3 half lifes) # of radioactive nuclei Fraction of nuclei 48/48 = 1 @ t 1/2 = 1 24 = 1 48 2 @ t 1/2 = 2 12 = 1 * 1 = 1 48 2 2 4 @ t 1/2 = 3 6 = 1 * 1 * 1 = 1 48 2 2 2 8 General Formula Fraction remaining = 1 2 n where n is the # of half lifes

25. Let’s go over all that again!

26. Phenomenon of Radioactivity Some elements, such as uranium (U) and thorium (Th), are unstable: They decay spontaneously.

27. Uranium Nucleus spontaneously emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

28. Alpha Particle emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).

29. Uranium - Thorium Decay U He + Th 238 92 4 2 234 90 spontaneous decay “parent”“daughter product” alpha particle = 2 protons + 2 neutrons = positively charged ion of Helium Thorium: 90 protons + 144 neutrons

30. Beta Particle Emission But, Th is also unstable, and it emits a beta particle … 234 90

31. Th + Pa 234 90 234 91 Thorium - Protactinium Decay beta particle beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron Protactinium: 91 protons + 143 neutrons

32. Title beta particle = an electron discharged from the nucleus when a neutron splits into a proton and an electron

33. U Pb Series This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

34. U Pb Series The rate at which this process occurs is measured in terms of the “half life”.

35. Half Life Half Life = Number of years for 1/2 of the original number of atoms to decay from U to Pb

36. Carbon-14 Dating