1. AP Chemistry Podcast 1.3 Nuclear Chemistry

2. 2 Nuclear Chemistry Nuclear reactions involve changes that originate in the nucleus of the atom. Chemical changes involve changes in the electron cloud. Uses: – 60 Co- gamma ray emitter- ionizing radiation for treatment of cancerous tumors. – 201 Thallium stress test of heart muscle – Radiocarbon dating  14 C ½ life 5730 years – Nuclear power ~ 20% of US electricity production

3. 3 Radioactivity Recall that all atoms of the same element have the same number of protons. The number of neutrons in the atoms nucleus, however, may be different from one atom to the next= Isotopes. Uranium- 234Uranium-235Uranium-238 92 protons92 protons92 protons 142 neutrons143 neutrons146 neutrons Trace 0.7% 99.3% Different isotopes have different abundances Different isotopes have different stabilities

4. 4 Patterns of Nuclear Stability As the atomic number increases, the neutron to proton ratio of the stable nuclei increases. The stable nuclei are located in the shaded area of the graph known as the belt of stability. The majority of radioactive nuclei occur outside this belt.

5. 5 Nuclear Equations Radionuclides are unstable nuclei that emit particles and electromagnetic radiation to transform into a stable nucleus. U 238 92 Th 234 90 + He 4 2

6. 6 Nuclear Equations Mass numbers and atomic numbers must be balanced in all nuclear equations.

7. 7 What product is formed when thorium-232 undergoes alpha decay?

8. 8 Types of Radioactive Decay Alpha decay- nucleus emits 2 protons and 2 neutrons (He nucleus) Beta decay- a neutron in the nucleus decays into a proton and an electron, the electron is emitted Gamma- high energy, short wavelength electromagnetic radiation- accompanies other radioactive emissions.

9. 9 Types of Radioactive Decay Electron Capture- capture by the nucleus of an electron from the electron cloud surrounding the nucleus. Positron- particle with the same mass as an electron, but an opposite charge  collides with an electron and produces gamma radiation.

10. 10 Penetrating Power of Radioactive Decay

11. 11 Radioactive Decay Particles ParticleNuclear Equation Example Alpha= 2 protons and 2 neutrons Nucleus  4 He 226 Ra  222 Rn + 4 He Beta = neutron converts to proton and a high energy electron 1 n  1 p + 0 e 131 I  131 Xe + 0 e Electron Capture= electron captured by nucleus 1 p + 0 e  1 n 81 Rb + 0 e  81 Kr 8886 2 2 0 1 5354 1 0 37 36

12. 12 Radioactive Decay Particles ParticleNuclear Equation Example Positron= proton converted to a neutron and an electron 1 p  1 n + 0 e 11 C  11 B + 0 e Gamma= electromagnetic radiation Not shown in equations, but almost always accompanies other decay. 101 Remember a positron has the same mass as an electron, but the opposite charge 651

13. 13 Radioactive Decay Particles ParticleEffect AlphaDecrease atomic mass by ___ and atomic number by _____. BetaAtomic number _______________. Electron Capture Atomic number _______________. PositronAtomic number _______________. Gamma

14. 14 Half Life- the time required for half of any given quantity of a substance to react / decay. (independent of initial quantity of atoms) Number of Th- 232 atoms in a sample initially containing 1 million atoms as a function of time. Th-232 has a half-life of 14 billion years. Half Life Simulation

15. 15 Half Life Problems Example: An isotope of cesium (cesium-137) has a half- life of 30 years. If 1.0 mg of cesium-137 disintegrates over a period of 90 years, how many mg of cesium-137 would remain?

16. 16 1. A 2.5 gram sample of an isotope of strontium-90 was formed in a 1960 explosion of an atomic bomb at Johnson Island in the Pacific Test Site. The half-life of strontium-90 is 28 years. In what year will only 0.625 grams of this strontium-90 remain? 2. Actinium-226 has a half-life of 29 hours. If 100 mg of actinium-226 disintegrates over a period of 58 hours, how many mg of actinium-226 will remain? 3. Thallium-201 has a half-life of 73 hours. If 4.0 mg of thallium-201 disintegrates over a period of 6.0 days and 2 hours, how many mg of thallium-201 will remain? 4. Sodium-25 was to be used in an experiment, but it took 3.0 minutes to get the sodium from the reactor to the laboratory. If 5.0 mg of sodium-25 was removed from the reactor, how many mg of sodium-25 were placed in the reaction vessel 3.0 minutes later if the half-life of sodium-25 is 60 seconds? 5. Selenium-83 has a half-life of 25.0 minutes. How many minutes would it take for a 10.0 mg sample to decay and have only 1.25 mg of it remain? Half Life Problems

17. 17 Uranium-238: an example of an unstable nucleus decaying to form other unstable nuclei Uranium-238 is radioactive, undergoing alpha decay. But, the daughter nuclide is also radioactive, undergoing beta decay, to produce yet another radioactive nuclide, which decays. The atom goes through a rather involved sequence of radioactive decays (both alpha and beta), until a stable isotope (lead-206) is reached.

18. 18 Fission Reaction Collision of a neutron with a U-235 nucleus can cause the nucleus to split, creating two smaller nuclides and three free neutrons. The three neutrons may travel outward from the fission, colliding with nearby U-235 nuclei, causing them to split as well. Each split (fission) is accompanied by a large quantity of energy.

19. 19 Fission Chain Reaction Collision of a neutron with a U-235 nucleus can cause the nucleus to split, creating two smaller nuclides and three free neutrons. The three neutrons may travel outward from the fission, colliding with nearby U-235 nuclei, causing them to split as well. Each split (fission) is accompanied by a large quantity of energy. If sufficient neutrons are present, we may achieve a chain reaction. If only one neutron were produced with each fission, no chain reaction would occur, because some neutrons would be lost through the surface of the uranium sample. Mousetrap Chain Reaction

20. 20 Fission Reaction

21. 21 Fusion Reaction Tremendous energy needed to overcome the repulsion between nuclei. Heat required for this reaction is on the order of 40,000,000 K. The energy from an atomic bomb could generate this heat (hydrogen or thermonuclear weapon).