Nuclear Chemistry(Topic for Regents exam, SAT II exam and AP exam)
Video Animations • Online resources from our TB for ch 21 (discovery: alpha, beta and gamma radiation) • Chem tours chapter 20 of Gilbert’s book see: http://www.wwnorton.com/college/chemistry/gilbert2/contents/ch20/chemtours.asp • Modes of radioactive decay, Balancing nuclear reactions and Fusion of Hydrogen
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 (TB, p. 902) .
Predicting the mode of radioactive decay. • In general: • neutron-rich nuclei tend to emit beta particles • proton-rich nuclei tend to either emit positrons or undergo electron capture • heavy nuclei tend to emit alpha particles. • The presence of magic numbers of nucleons and an even number of protons and neutrons also help determine the stability of a nucleus.
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
238 92 234 90 4 2 4 2 He U Th He + Types of Radioactive Decay Alpha Decay = Loss of an -particle (a helium nucleus) Atomic # decreases by 2 Mass # decreases by 4 # of protons decreases by 2 # of neutrons decreases by 2
Which element undergoes alpha decay to form lead-208? • Analyze • Plan • Solve • Check
131 53 131 54 0 −1 0 −1 0 −1 e I Xe e + or Types of Radioactive Decay Beta Decay = Loss of a -particle (a high energy electron) Atomic # increases by 1 # of protons increases by 1 # of neutrons decreases by 1 Mass # remains the same
11 6 11 5 0 1 0 1 e C B e + Types of Radioactive Decay Positron Emission = Loss of a positron (a particle that has the same mass as but opposite charge than an electron) Atomic # decreases by 1 # of protons decreases by 1 # of neutrons increases by 1 Mass # remains the same
Electron capture • Capture by the nucleus of an electron from the electron cloud surrounding the nucleus (effectively converts a proton to a neutron). • Ex: Rubidium-81 is converted to Krypton-81 by this process (Atomic numbers: Rb = 37, Kr = 36) • Nuclear equation:
0 0 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. • 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.
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 T Equations: fraction remaining = (1/2)(t/T) # of half-lives remaining = t/T Key: t = total time elapsed T = half-life
Sample Half-Life Question 1A - Regents 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- Regents (Hint:1stuse Regents tables to find half-life) (a) 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- Regents 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.
Equations to learn for calculations based on half-life (AP) • Radioactive decay is a first order process; • ln (Nt/No) = -kt • Nt = No e-kt • k = 0.693/t1/2 • or t1/2 = 0.693/k • (kinetics topic) • k = ? (rate constant or decay constant) • N = ? For time interval t
Energy changes in Nuclear Reactions E =mc2 • Einstein E =mc2 • mass defect • For nuclear reactions ∆E =c2∆m • E = energy in Joules (J = kg•m2/s2) • m = mass in kg • C = speed of light • (2.9979 x 108 m/s)
Activities and Problem set X TB ch. 21 – sections 21.1 and 21.4 most impt for AP exam POGIL activity on nuclear chemistry Lab activity: Paper lab on nuclear decay Concept map (Group) • Ch 21 Problems TO DO • all GIST, sample & practice exercises, Visualizing concepts, • Ch 21 end of ch. Red ex:21.7,9,11,13,15,19,23,25,27,29,31,33,41,43,45,57,65,70,