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## Chapter 21 Nuclear Chemistry

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**Nuclear chemistry is the study of nuclear reactions, with an**emphasis on their uses in chemistry and their effects on biological systems.**2.1 Radioactivity**The Nucleus • Review Chap 2.3 for basic atomic structure. • Remember that the nucleus is comprised of two subatomic particles (nucleons) protons and neutrons. • The number of protons is the atomic number. • The number of protons and neutrons together is effectively 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**A nuclide is a nucleus with a specific number of protons and**neutrons. • It is not uncommon for some nuclides of an element to be unstable, or radioactive. • We refer to these as radionuclides. • There are several ways radionuclides can decay into a different nuclide.**Nuclear Equations**• Radionuclides are unstable and spontaneously emit particles and electromagnetic radiation. • Emission of radiation is a way for an unstable nucleus to be transformed into a more stable one with less energy. • When a nucleus spontaneously decomposes in this way it is said to have decayed.**He**238 92 234 90 4 2 4 2 He U Th + 1. Alpha Decay Types of Radioactive Decay (see tables 21.1 and 21.2) Loss of an -particle (a helium nucleus)**131**53 131 54 0 −1 0 −1 0 −1 e I Xe e + or 2. Beta Decay Loss of a -particle (a high energy electron)****0 0 3. Gamma Emission • Loss of a -ray -- high-energy photon that almost always accompanies the loss of a nuclear particle. • Generally, not shown in a nuclear equation.**11**6 11 5 0 1 0 1 e C B e + 4. Positron Emission • Loss of a positron -- a particle that has the same mass as but opposite charge than an electron.**0**−1 1 1 1 0 p e n + 5. Electron Capture • The addition of an electron to a proton in the nucleus. • As a result, a proton is transformed into a neutron.**21.2 Patterns of Nuclear Stability**Neutron-Proton Ratios • Any element with more than one proton (anything but hydrogen) will have repulsions between the protons in the nucleus. • A strong nuclear force helps keep the nucleus from flying apart. • Neutrons play a key role stabilizing the nucleus. • Therefore, the ratio of neutrons to protons is an important factor.**For smaller nuclei (Z 20) stable nuclei have a**neutron-to-proton ratio close to 1:1. • As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus. • The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.**Nuclei above this belt have too many neutrons.**• They tend to decay by emitting beta particles.**Nuclei below the belt have too many protons.**• They tend to become more stable by positron emission or electron capture. • There are no stable nuclei with an atomic number greater than 83. • These nuclei tend to decay by alpha emission.**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).**Some Trends. . .**• Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons. • Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.**21.3 Nuclear Transmutations**• Nuclear transmutations can be induced by accelerating a particle (neutron or another nucleus) and colliding it with the nuclide. • Examples of particle accelerators:**Synthetic isotopes used in medicine and research are made**using neutrons as projectiles. • Example: cobalt-60**Nt**N0 = kt ln 21.4 Rates Radioactive Decay • Nuclear transmutation is a first-order process. • The kinetics of such a process, you will recall, obey this equation:**0.693**k = t1/2 • The half-life of such a process is: • Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.**Calculations Based on Half-life**• Because the half-life of any particular nuclide is constant, the half-life serves as a nuclear clock to determine the ages of different objects. (See Table 21.4) • The rate at which a sample decays is called its activity. • The SI unit for activity is the becquerel (Bq). • Another unit of activity is the curie (Ci), defined as 3.7 x 1010 disintegrations per second.**A wooden object from an archeological site is subjected to**radiocarbon dating. The activity of the sample that is due to 14C is measured to be 11.6 disintegrations per second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations per second. The half-life of 14C is 5715 yr. What is the age of the archeological sample?**0.693**5715 yr 0.693 k 0.693 k = k = t1/2 = 5715 yr = k 1.21 10−4 yr−1 First we need to determine the rate constant, k, for the process.**11.6**15.2 ln = (1.21 10−4 yr−1) t Nt N0 = kt ln ln 0.763 = (1.21 10−4 yr−1) t = t 6310 yr Now we can determine t: