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Nuclear Changes

Nuclear Changes. Chapter 7. 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. Radioactivity. Penetrating power of different forms of radiation:.

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Nuclear Changes

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  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.

  3. Radioactivity Penetrating power of different forms of radiation:

  4. 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

  5. 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

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

  7. 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!)

  8. 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)

  9. 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

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

  11. 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)

  12. 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

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

  14. 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

  15. 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)

  16. 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

  17. 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

  18. 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)

  19. 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)

  20. 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)

  21. Let’s go over all that again!

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

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

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

  25. 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

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

  27. 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

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

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

  30. U PbSeries The rate at which this process occurs is measured in terms of the “half life”.

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

  32. Carbon-14 Dating

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