Nuclear Chemistry

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 figure shows what nuclides would be stable, the so-called belt of stability. http://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass Most nuclei are stable. 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.

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

pHET simulations of alpha decay of Polonium-211 to form Lead-207 and of Beta decay of Hydrogen-3 to Helium-3 http://phet.colorado.edu/en/simulation/alpha-decay http://phet.colorado.edu/en/simulation/beta-decay

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 + Correction Atomic # decreases by 2 # of protons decreases by 2 # of neutrons decreases by 2 Mass # decreases by 4

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) 

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. http://phet.colorado.edu/en/simulation/nuclear-fission This process continues in what we call a nuclear chain reaction.

Nuclear Fusion = 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.

Energy changes in Nuclear Reactions E =mc2 Einstein E =mc2 mass defect For nuclear reactions E = energy in Joules (J = kg•m2/s2) m = mass in kg C = speed of light (2.9979 x 108 m/s)

Half-Life fraction remaining = (1/2)(t/T) Key: t = total time elapsed = 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 T Equations: fraction remaining = (1/2)(t/T) # of half-lives remaining = t/T Key: t = total time elapsed T = half-life

PhET simulation of decay and half-life http://phet.colorado.edu/en/simulation/radioactive-dating-game

Sample Half-Life Question 1A Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. (a) What fraction of a sample of Cr-51 will remain after 168 days? Step 1: Determine how many half-lives elapse during 168 days. Step 2: Calculate the fraction remaining.

Sample Half-Life Question 1B (b) If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days? Step 1: Calculate the mass remaining: mass remaining = fraction remaining X original mass (Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each half-life.)

Sample Half-Life Question 2 How much was present originally in a sample of Cr-51 if 0.75g remains after 168 days? Step 1: Determine how many half-lives elapsed during 168 days. Step 2: Multiply the remaining amount by a factor of 2 for each half-life.

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) Other Uses 14C archaeological dating (of once living things) and radiolabelled organic compounds 238U archaeological dating (U-238 to Pb-206 ratio) 241Am (Americium-241) smoke detectors 235U nuclear reactors and weapons