Chapter 3 Nuclear Properties

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1 CHM 511 Chapter 3 page 1 of 9 Chapter 3 Nuclear Properties Chemistry typically deals with the consideration of electrons: where they are, where they want to go, where they are going. A typical chemical reaction may release anywhere from 0 kj/mol to a few thousand kj/mol. CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(l) ΔHº = -801 kj/mol CH 4 Nuclear processes occur with considerably more energy U 2He 90Th ΔHº = kj/mol U As part of this phenomenal energy exchange, Einstein s famous equation becomes relevant with nuclear reactions: E = mc 2 or better yet: ΔE = Δmc 2 (which is really E 2 = (mc 2 ) 2 + (pc) 2 ) 235 U = amu 4 He = amu 231 Th = amu Sum = amu In fact, this is true even of atomic nuclei. What is the predicted mass of 19 F in amu? proton rest mass = kg neutron rest mass = kg electron rest mass = kg 1 amu = kg Measured 19 F mass = amu

2 CHM 511 Chapter 3 page 2 of 9 Binding Energy The difference in energy between the nucleus and the same number of protons and neutrons as individual particles is known as the binding energy. For 19 F, take the mass and divide by the number of protons and neutrons to get the average binding energy per nucleon. Implications for fission/fusion? Nuclear Reactions (i.e., nuclear transmutations) Unstable nuclei can decompose via several mechanisms Alpha decay (α decay) Beta decay (β or β - decay) Positron emission (written as β + ), postulated in 1931, though neutrons weren t confirmed until 1933

3 CHM 511 Chapter 3 page 3 of 9 Electron Capture Which elements are stable? Depends on the number of protons and neutrons. Of 270 stable isotopes, the breakdown is: # of isotopes Z A 161 Even Even 55 Even Odd 50 Odd Odd 4 Odd Even

CHM 511 Chapter 3 page 4 of 9 An important decay series: Decay rates are set by first order kinetics: If N is one half of N 0, then the amount of time that has passed is called the half life.

4 CHM 511 Chapter 3 page 4 of 9 An important decay series: Decay rates are set by first order kinetics: If N is one half of N 0, then the amount of time that has passed is called the half life. N N 0 e kt Half lives are quite a mystery why do some particles decompose, but others do not? EX. Starting with a 10. g sample of 241 Am (t 1/2 = 433 years), what mass would be left after 1299 years? EX. How long would it take until there was only 1.0 g of 241 Am left?

5 CHM 511 Chapter 3 page 5 of 9 Artificial isotopes Isotopes can be created by bombarding target nuclei with subatomic particles (p +, n) or other nuclei (α or larger). If atoms are hit with neutrons, they may be fast (high energy, ~1 MeV) or slow aka thermal neutrons (low energy, ~0.05 ev). How well a neutron is captured by a nucleus is called the cross section Examples of nuclear bombardment reactions: Short-hand notation: Nuclear fission of 235 U The reaction of 235 U and thermal neutrons results in variable products. What s observed: An average of 2.5 neutrons are released for each neutron- 235 U bombardment resulting in a reaction Daughter nuclei are usually between and mass numbers Daughter nuclei are NOT usually <90, , or >144 mass numbers The sum of all mass numbers must equal 236

6 CHM 511 Chapter 3 page 6 of 9 Because more than one neutron is produced, a branching chain reaction may occur this causes the decay of an ever increasing number of nuclei. The neutrons are coming out FAST, so they pass through a moderator (graphite, D 2 O) to be slowed down, then... To control the neutron concentration (neutron flux), control rods (steel with boron, graphite with boron, boron carbide, or cadmium rods) are placed in the chamber to soak up the neutrons. Chernobyl... Nuclear reprocessing Eventually, the concentration of 235 U is too low to keep enough neutrons in flux the fuel may be reprocessed. Short-lived daughter products are allowed to decay, then a chemical process creates UF 6 gas.

7 Transuranium isotopes Atoms past U are not found naturally, but can be produced by atom bombardment neutrons may induce unstable, β - decaying nuclei small atoms can be used to gain multiple Z at one time CHM 511 Chapter 3 page 7 of 9 Separation of radioactive isotopes As in the case of U, 234 U, 235 U, and 238 U are all types of uranium and therefore have identical chemical properties. Thus we must use a physical means (Graham s Law) to separate isotopes. Daughter products may be separated by chemical means. Another method to separate isotopes employs the bond breaking step from an energetic atom to separate new isotopes: the Szilard-Chalmers Effect. SC Effect: The rupture of the chemical bond between an atom and the molecule of which the atom is part, as a result of a nuclear reaction of that atom. (from the IUPAC Gold Book, Compendium of Chemical Terminology) Creation of a new isotope causes homolytic bond cleavage. The freed isotope may react with another chemical to be sequestered. Applications of isotopes Many medical applications IR spectroscopy: the vibrational constant between two atoms is dependent on the mass of those atoms; change the mass then you change the wavenumber of absorption

8 CHM 511 Chapter 3 page 8 of 9 Kinetic Isotope Effect heavier atoms also change the bond energy of an atom pair. Thus changing a C-H bond for a C-D will make it harder to break (see figure 3.7). If the bond breaking is involved in a rate limiting step, you can investigate the kinetics of the system. Radiocarbon dating: carbon dating is monitored by β decay of 14 C By comparing the ratio of 14 C: 12 C, you can determine when a material stopped gaining new material (i.e., stopped living). Since 14 C radioactively decays (t ½ = 5730 years), why is there any still around?

9 CHM 511 Chapter 3 page 9 of 9 NMR won t specifically be covered in the notes, since you ve had the theory in other classes, but one aspect does need to be addressed: lifetime studies of molecules/complexes undergoing fluxional isomerism. EX. For Fe(CO) 5, how many different carbon signals would you expect? What would be the integration? Model of Berry pseudorotation: How can this mechanism be stopped?

Nuclear Chemistry. The Nucleus. Isotopes. Slide 1 / 43. Slide 2 / 43. Slide 3 / 43

Nuclear Chemistry. The Nucleus. Isotopes. Slide 1 / 43. Slide 2 / 43. Slide 3 / 43 Slide 1 / 43 Nuclear Chemistry The Nucleus Slide 2 / 43 Remember that the nucleus is comprised of the two nucleons, protons and neutrons. The number of protons is the atomic number. The number of protons