Ch. 22: Nuclear Chemistry

1. Ch. 22: Nuclear Chemistry • Nuclear forces, and radioactive decay • Alpha, beta, and gamma particles • Transmutation, disintegration series, & carbon dating • Fission vs. fusion • Effects of radiation San Onofre Nuclear Power Plant San Clemente, CA (61 miles south of Los Angeles)

2. The Nucleus • Atomic nuclei are made of protons and neutrons, which are collectively called nucleons • An atom is referred to as a nuclide and is identified by the number of protons and neutrons in its nucleus. • Two types of notations: Radium-228 or 228Ra 88

3. Factors Affecting the Stability of Nuclei • Why are some atoms radioactive and others are not? • There are three major factors: 1. Nuclear Binding Energy (Mass Defect) 2. Band of Stability (n/p ratios) 3. Magic Numbers

4. Mass Defect and Nuclear Stability • The experimental observations show that the mass of a nucleus is always less than the sum of masses of its constituent protons and neutrons. This “missing mass” is called as Mass Defect. This “missing mass” is converted to energy according to Einstein’s E=mc2 and this energy is called as “Nuclear Binding Energy”. The greater the nuclear binding energy, the more stable is the atom.

5. Sample Problem • Calculate the nuclear binding energy of a sulfur-32 atom. The measured atomic mass of a sulfur-32 is 31.972 070 amu. • Find mass defect: • Convert amu to kg; 1 amu=1.6605 *10-27 kg ANSWER: 4.36*10-11 J.

6. Stable, naturally occurring isotopes Band of Stability For low atomic numbers the stable nuclei are those with a ratio of protons to neutrons approx. 1:1 As the atomic #increases, The stable ratio increases to about 1.5: 1. The trend explained by nuclear forces. 1

7. Nuclear Shell Model • Nucleons exist in different energy levels, or shells, in the nucleus. • The numbers of nucleons that represent completed nuclear energy levels -2, 8, 20, 28, 50, 82, and 126- are called magic numbers.

8. Nuclear Reaction • a reaction that affects the nucleus of an atom. • Occurs to increase stability • Give off large amounts of energy • Total of atomic #s & mass #s is equal on both sides of equation. • Identity of atom does not change till atomic # changes, When atomic #changes, the identity of element changes=transmutation

9. Natural Radioactivity Discovered in 1896 by Antoine Henri Becquerel, who saw a uranium salt produce an image on a photographic film. The term radioactivity was coined in 1898 by Marie Curie, a Polish physicist, who was doing research with her husband Pierre. (They did much of the initial work on radioactivity, and eventually died of radiation-related illnesses.) Radioactive Decay: spontaneous disintegration of a nucleus into a slightly lighter & more stable nucleus, accompanied by emission of particles, electromagnetic radiation or both.

10. Half Life: The time required for ½ the amount of a radioactive material to disintegrate. • Phosphorus-32 radioactively decays to form Sulfur-32 • Half life 32P = 14 days

11. Types of Radioactive Decay

12. Alpha particle emissions: • Helium nucleus: 2 protons and 2 neutrons, +2 charge. • For large, unstable nucleus which needs to reduce both the number of protons and the number of neutrons. • Example:

13. Alpha emission

14. Beta particle emissions: • Electron emission, -1 charge. • For unstable nucleus which needs to reduce the number of neutrons. • A neutron is converted into a proton and an electron, the electron is given off as a beta particle. • Example:

15. Beta emission

16. Positron emissions: • Positron emission, +1 charge. • For unstable nucleus which needs to reduce the number of protons. • A proton is converted into a neutron and a positron, the positron is emitted. • Example:

17. Positron emission

18. Electron capture: • An inner orbit electron combines with a proton and forms a neutron. • For unstable nucleus which needs to reduce the number of protons. • Example:

19. Electron capture

20. Gamma emissions: • High energy electromagnetic waves (photons) like visible light, except with a shorter wavelength. • For high energy nucleus when it jumps down from an excited state to a ground state. • Example:

21. Gamma emission

22. Electromagnetic spectrum

23. Arificial Transmutation • First accomplished by Rutherford in 1919, even though alchemists tried for hundreds of years. • Transmutation of lead into gold was achieved by Glenn Seaborg, who succeeded in transmuting a small quantity of lead in 1980. He also first isolated plutonium for the atomic bomb and discovered/”created” many elements. (NY Times, Feb 1999) • There is an earlier report (1972) in which Soviet physicists at a nuclear research facility in Siberia accidentally discovered a reaction for turning lead into gold when they found the lead shielding of an experimental reactor had changed to gold. • Accomplished with particle accelerators like the Stanford Linear Accelerator (SLAC)

24. Atomic Number & Symbol Mass Number Disintegration Series (Decay Series) • “Heavy” atoms (greater than Bismuth, #83) naturally decay to smaller atoms along a consistent path, or series, of decays. Radioactive U-238 → Th-234 + a Th-234 → Pa-234 + b Pa-234→ U-234 + b U-234→ Th-230 + a Th-230 → Ra-226 + a Ra-226 → Rn-222 + a Rn-222 → Po-218 + a Po-218 → Pb-214 + a Pb-214 → Bi-214 + b Bi-214 → Po-214 + b Po-214 → Pb-210 + a Pb-210 → Bi-210 + b Bi-210→ Po-210 + b Po-210 → Stable Pb-206 + a Source: http://www.frontiernet.net/~jlkeefer/uranium.html

25. 1 235 139 94 1 n + Ba + Kr + 3 n + energy U 0 92 56 36 0 Nuclear Fission • Fission: process in which the nucleus of a large, radioactive atom splits into 2 or more smaller nuclei • Caused by a collision with a energetic neutron. • A neutron is absorbed by a U-235 nucleus. The nucleus is now less stable than before. It then splits into 2 parts and energy is released. Several neutrons are also produced; they which may go on to strike the nuclei of other atoms causing further fissions. Fission animation: http://www.howstuffworks.com/nuclear-bomb3.htm

26. *In a fission reaction that is working properly, more than one neutron ejected from the fission reaction causes another fission to occur. This condition is known as supercriticality. • *The process of neutron capture and nucleus splitting happens very quickly (takes about 1 x 10-12 seconds). • An incredible amount of energy is released: • As heat and gamma radiation • Because the product atoms and neutrons weigh less than the original U-235 atom; the “missing mass“ has been converted to energy by E=mc2

27. *A Fission Chain Reaction

28. Harnessing Fission: A nuclear power plant • Nuclear power plants utilize the energy released in a controlled fission reaction in the core to heat water in one pipe, which is used to vaporize water into steam in another pipe, which drives a turbine and generates electricity. • The vaporized H2O is in a closed circuit, and is never exposed to the radiation itself. • The speed of the fission chain reaction is regulated using carbon control rods which can absorb extra neutrons.

29. The Atomic Bomb • Uses an unregulated fission reaction in a very fast chain reaction that releases a tremendous amount of energy. *Critical mass: the minimum amount of radioactive, fissionable material needed to create a sustainable fission chain reaction * Site of fission reaches temperatures believed to be about 10,000,000°C. • Produces shock waves and a, b, g, x-rays, and UV radiation.

30. *The classic “mushroom cloud” is a result of dust and debris lifted into the air as a result of the detonation. US Army aerial photograph from 80 km away, taken about 1 hour after detonation over Nagasaki, Japan, August 9, 1945.

31. 1 0 4 n + energy He + 2 3 2 H + H 1 1 Nuclear Fusion Fusion: process in which 2 nuclei of small elements are united to form one heavier nucleus • Requires temperatures on the order of tens of millions of degrees for initiation. • The mass difference between the small atoms and the heavier product atom is liberated in the form of energy. • Responsible for the tremendous energy output of stars (like our sun) and the devastating power of the hydrogen bomb.

32. *Stars & the Hydrogen Bomb • The first thermonuclear bomb was exploded in 1952 in the Marshall islands by the United States, the second was exploded by the Russia (then the USSR) in 1953. • “H bombs” utilize a fission bomb to ignite a fusion reaction.

33. Mass-Energy Relationship • *The energy that can be released from 2 kg of highly enriched U-235 (as used in a nuclear bomb) is roughly equal to the combustion of a million gallons of gasoline. 2 kg of U-235 is smaller than a baseball; a million gallons of gasoline would fill a rectangular tank that is 50 feet per side.

34. Measurement of Radioactivity ionizing radiation- • radiation from radioactive sources • When it strikes an atom or a molecule, one or more electrons are knocked off and an ion is created • Measured with a Geiger counter, film badge or a Scintillation counter • Curie-measures radioactivity emitted by a radionuclide. • Roentgen & Rad - measures exposure to gamma rays or X-rays • Rem- takes into account the degree of biological effect caused by the type of radiation exposure.

35. *Biological Effects of Radiation: Acute • High level radiation (gamma ray & x–ray) causes death • Damage centered in the nuclei of the cells; cells that are undergoing rapid cell division most susceptible • Gamma rays from a Co-60 source are often used to treat cancer (cells that multiply rapidly) Ionizing radiation:energy emitted from radioactive matter; it can directly affect (ionize) the structure of materials which it passes through, including human tissue.

36. *Effects of Radiation: Long Term • Long term exposure can weaken an an organism and lead to onset of malignant tumors, even after fairly long time delays. • Largest source: X–rays • Sr-90 isotopes are present in fallout from atmospheric testing of nuclear weapons. • Contaminated foods can increase incidence of leukemia and bone cancers.