Nuclear Chemistry Chemistry involving changes in the nucleus References: Text Chapter 19 Review Book Topic 12

The Nucleus Remember that the nucleus is comprised of protons and neutrons. The number of protons is the atomic number. The number of protons and neutrons together is the mass of the atom.

Isotopes Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms. There are three naturally occurring isotopes of uranium: Uranium-234 Uranium-235 Uranium-238

Stable Nuclei The shaded region in the fig. shows what nuclides would be stable, the so-called belt of stability. It is the ratio of neutrons to protons that determines the stability of a given nucleus.

Radioactivity It is not uncommon for some nuclei to be unstable, or radioactive. There are no stable nuclei with an atomic number greater than 83. Radioisotopes = isotopes that are unstable and thus radioactive There are several ways radionuclides can decay into a different nuclide **a nuclide is a nucleus with a specified number of protons and neutrons

Radioactive Series Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. They undergo a series of decays until they form a stable nuclide (often a nuclide of lead). Transmutation = the reaction by which the atomic nucleus of one element is changed into the nucleus of a different element

Types of Radioactive Decay Alpha Decay = Loss of an -particle (a helium nucleus) He 4 2 U 238 92  Th 234 90 He 4 2 + Atomic # decreases by 2 Mass # decreases by 4 # of protons decreases by 2 # of neutrons decreases by 2

Types of Radioactive Decay Beta Decay = Loss of a -particle (a high energy electron)  −1 e or I 131 53 Xe 54  + e −1 Atomic # increases by 1 # of protons increases by 1 # of neutrons decreases by 1 Mass # remains the same

Types of Radioactive Decay Positron Emission = Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 1 C 11 6  B 5 + e 1 Atomic # decreases by 1 # of protons decreases by 1 # of neutrons increases by 1 Mass # remains the same

Types of Radioactive Decay Gamma Emission = Loss of a -ray (a photon of high-energy light that has no mass or charge & that almost always accompanies the loss of a nuclear particle; often not shown when writing nuclear equations) 

Table N shows the decay mode of selected radioisotopes

Artificial Transmutation = done by bombarding the nucleus with high- energy particles (such as a neutron or alpha particle), causing transmutation 4020Ca + _____ -----> 4019K + 11H 9642Mo + 21H -----> 10n + _____ **Natural transmutation has a single nucleus undergoing change, while artificial transmutation will have two reactants (fast moving particle & target nuclei.**

Nuclear Fission Nuclear fission is the type of reaction carried out in nuclear reactors. = splitting of large nuclei into middle weight nuclei and neutrons

Nuclear Fission Bombardment of the radioactive nuclide with a neutron starts the process. Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. This process continues in what we call a nuclear chain reaction.

Nuclear Fusion 21H + 21H  42He + energy = the combining of light nuclei into a heavier nucleus 21H + 21H  42He + energy Two small, positively-charged nuclei smash together at high temperatures and pressures to form one larger nucleus.

Half-Life HALF LIFE = the time it takes for half of the atoms in a given sample of an element to decay Each isotope has its own half-life; the more unstable, the shorter the half- life. Table N: Selected radioisotopes

Sample Half-Life Question Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. What fraction of a sample of Cr-51 will remain after 168 days? How can we solve this problem? What information do we need?

Sample Half-Life Question If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days?

Sample Half-Life Question How much was present originally in a sample of Cr-51 if 0.75g remains after 168 days?

Sample Half Life Question Four hundred grams of a sample decomposes to 25 grams in 80 days. What is the half life?

Sample Half Life Problem The half –life of a substance is 12 seconds. How many seconds must pass before 128 grams disintegrates into 16 grams?

238U archaeological dating (U-238 to Pb-206 ratio) Some practical uses of Radioisotopes (dating, chemical tracers, industrial applications, medical applications, nuclear power plants) Medical Uses 60Co (cobalt-60) used in cancer treatments and used to kill bacteria in food products 226Ra (Radium-226) used in Cancer treatment 131I diagnosis and treatment of thyroid disorders 11C Positron emission tomography (PET scans) Tc-99 used for brain tumors Other Uses 14C archaeological dating (of once living things) and organic compounds 238U archaeological dating (U-238 to Pb-206 ratio) 241Am (Americium-241) smoke detectors 235U nuclear reactors and weapons