1. Chapter 21
Nuclear Chemistry
John A. Schreifels
Chemistry 212

2. Overview Radioactivity and Nuclear Bombardment Reactions
Radiations and Matter: Detection and Biological Effects
Rate of Radioactive Decay
Applications of Radioactive Isotopes
Energy of Nuclear Reactions
Mass – Energy Calculations
Nuclear Fission and Nuclear Fusion
John A. Schreifels
Chemistry 212

3. Nuclear Chemistry
In this chapter we will look at two types of nuclear reactions.
Radioactive decay is the process in which a nucleus spontaneously disintegrates, giving off radiation.
Nuclear bombardment reactions are those in which a nucleus is bombarded, or struck, by another nucleus or by a nuclear particle.
John A. Schreifels
Chemistry 212 2

4. Nuclear Reactions and their characteristics
Nuclear Chemistry: study of changes in structure of nuclei and subsequent changes in chemistry.
Radioactive nuclei: spontaneously change structure and emit radiation.
Differences between nuclear and chemical reactions:
Much larger release in energy in nuclear reaction.
Isotopes show identical chemical reactions but different nuclear reactions.
Nuclear reactions not sensitive to chemical environment.
Nuclear reaction produces different elements.
Rate of nuclear reaction not dependent upon temperature.
John A. Schreifels
Chemistry 212

5. NUCLEAR STRUCTURE & Stability
nucleon: any nuclear particle, e.g. protons, p, and neutrons, n.
Nucleus held together by strong attractive forces; but electrostatic repulsion causes large atoms (>83 protons) to be unstable.
Let Z = atomic # (# of protons) and A = Z + # of neutrons. Isotopes represented as
has 8 p, 8 e, and 8 n;
has 8 p, 8 e, and 9 n;
has 8 p, 8 e, and 10 n.
Structure deduced from emission of radiation from unstable particles:
.a ray = attracted towards negatively charged plate Þ Positively charged.
.b ray = attracted towards positively charged plate Þ Negatively charged.
.g ray = not attracted to either plate Þ Neutral.
John A. Schreifels
Chemistry 212

6. NUCLEAR REACTIONS
Radioactivity: nucleus unstable and spontaneously disintegrates.
Nuclear Bombardment: causes nuclei to disintegrate due to bomdarbment with very energetic particles.
Particles in nuclear reactions:
Positron: positively charged particle with same mass as electron.
Gamma ray: Very high energy photon (l = M; Visible: l = 10-7M).
Nuclear reaction written maintaining mass and charge balance.
E.g. ® g..
John A. Schreifels
Chemistry 212

7. RADIOACTIVITY Types of Radioactive decay:
Beta emission: Converts neutron into a proton by emission of energetic electron; atomic # increases:
E.g. Determine product for following reaction:
Alpha emission: emits He particle.
E.g. Determine product:
Positron emission: Converts proton to neutron:
E.g. Determine product of
Gamma emission: no change in mass or charge but usually part of some other decay process.
E.g.
Electron capture: electron from electron orbitals captured to convert proton to neutron.
John A. Schreifels
Chemistry 212

8. NUCLEAR STRUCTURE and STABILITY
Shell model of nucleus: protons and neutrons exist in energy levels which have optimum # of each in each shell.
Magic # : # of nuclear particles in particular shell (similar to 2,8,18 etc. for electrons.)
Protons : 2, 8, 20, 28, 50, 82
Neutrons: 2, 8, 20, 28, 50, 82 and 126.
E.g. -particles ( ) & are doubly magic.
Nuclei with even # of protons and neutrons most stable. ( Largest # of stable isotopes).
Nuclei with odd # of protons and neutrons least stable. (Least # of stable isotopes).
John A. Schreifels
Chemistry 212

9. Band of Stability
Band of stability = stable isotopes. (above Z = 82:  - or  - emission.)
above: beta emission;
below: electron or positron emission
John A. Schreifels
Chemistry 212

10. NUCLEAR BOMBARDMENT (Transmutation)
Bombard nuclei with nuclear particles to convert element to another one.
Rutherford discovered:
E.g.1. Identify product for electron capture:
E.g.2. Identify products for neutron bombardment of Fe:
E.g.3 Identify the product of
John A. Schreifels
Chemistry 212

11. RATE OF DISINTEGRATION
Rate of disintegration proportional to number of nuclei present.
Rate = k×N or
Half-life-time required for half of original nuclei to undergo decay.
At t1/2 N = 1/2No and , t1/2 = 0.693/k or
E.g.1 The half-life of Cobalt-60 is 5.26 years how much of the original amount would be left after years?
E.g.2 Tritium decays by beta emission with a half-life of 12.3 years. How much of the original amount would be left after 30 years?
E.g.3 If a 1.0 g sample of tritium is stored for 5.0 years, what mass of that isotope remains? k = 0.563/year.
John A. Schreifels
Chemistry 212

12. RATE OF DISINTEGRATION2
Dating ancient objects: Carbon-14 is generated naturally from cosmic rays
is unstable with a half-life of 5730 yr.
Rate of disintegration measured and is proportional to the concentration of 14C:
E.g. Charcoal from a tree killed by the volcanic eruption that formed the crater in Crater Lake (in Oregon) gave 7.0 disintegrations of 14C min.1g1 of total carbon. Present-day carbon (in living matter) gives 15.3 disintegrations min.1g1 of total carbon. Determine the date of the volcanic eruption.
John A. Schreifels
Chemistry 212

13. RADIATION DETECTION
Geiger counters detect charged particles produced from interaction of gas with particles emitted from radioactive material.
Scintillation counters detect particles from radioactive material by measuring intensity of light when these particles hit phosphor.
Units: 1 curie (Ci) = 3.7x1010 disintigrations×s-1
John A. Schreifels
Chemistry 212

14. Energy Changes During Nuclear Reactions
Most nuclear reactions give off a large amount of energy.
The energy required to break an nucleus its individual protons and neutrons is called the binding energy, Eb.
The total mass changes upon combination of protons and neutrons.
E.g. determine the mass change during the formation of Helium nuclei.
Measured mass of He nuclei (excluding electrons) = amu (m = g/mol = called the mass defect).
Energy change calculated from the mass change (decrease) using the Einstein equation: E = mc2.
E.g. determine the binding energy for 1 mol He.
E.g. determine the mass change during the combustion of butane 2878 kJ/mol
John A. Schreifels
Chemistry 212

15. Binding Energies 56Fe has highest Eb and is most stable isotope.
Energy sources:
Fission for large radioactive elements, such as U-235
Fusion for two deuterium producing He. Not yet accomplished.
John A. Schreifels
Chemistry 212