Nuclear Changes

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 238 U 92 themass number counts protons and neutrons the atomic number counts the number of protons

13. Radioactivity Radioactive decay: -decay + 238 4 234  U Th 92 2 90 • 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 + 226 4 222  Ra Rn 88 2 86 + 222 4 218  Rn Po 86 2 84 + 245 4 241  Cm Pu 96 2 94

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

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

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

21. Nuclear vs Chemical Reaction Chemical Reaction NaOH + HCl  H2O + NaCl Na Cl Cl Na  H H H H O O *** Not a true representation of this reaction in solution Nuclear Reaction 208 212Po  4a + 82Pb 2 84  *** Not a true representation of the nuclei

22. The Half-Life (t1/2) of a Nuclear Reaction Half-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay. # of radioactive nuclei 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)

23. The Half-Life (t1/2) of a Nuclear Reaction Half-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay. # of radioactive nuclei Fraction of nuclei 48 radioactive particles at t=0 48/48 = 1 @ t1/2 = 1 24 = 1 48 2 24 radioactive particles at t=1 (1 half life) @ t1/2 = 2 12 = 1 * 1 = 1 48 2 2 4 12 radioactive particles at t=2 (2 half lifes) @ t1/2 = 3 6 = 1 * 1 * 1 = 1 48 2 2 2 8 6 radioactive particles at t=3 (3 half lifes)

24. The Half-Life (t1/2) of a Nuclear Reaction Half-life (t1/2): The time it takes for half of the radioactive nuclei in a sample to decay. # of radioactive nuclei Fraction of nuclei General Formula Fraction remaining = 1 2n where n is the # of half lifes 48 radioactive particles at t=0 48/48 = 1 @ t1/2 = 1 24 = 1 48 2 24 radioactive particles at t=1 (1 half life) @ t1/2 = 2 12 = 1 * 1 = 1 48 2 2 4 12 radioactive particles at t=2 (2 half lifes) @ t1/2 = 3 6 = 1 * 1 * 1 = 1 48 2 2 2 8 6 radioactive particles at t=3 (3 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 4 234 238 92 90 2 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. Thorium - Protactinium Decay beta particle Th + Pa 234 234 90 91 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 PbSeries This process is called radioactive decay, and eventually uranium (parent) decays to lead (daughter product).

34. U PbSeries 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