1. Unit 13
Nuclear Chemistry

2. Nuclear radioactivity
Natural radioactivity
Radioactivity - emission of particles or energy from an unstable nucleus
Discovered by Henri Bequerel
Three types of radioactive decay

3. The nature of the nucleus
Strong nuclear force
Binds protons and neutrons
Very short ranged, less than m
Overcomes proton-proton Coulomb repulsion
Band of stability

4. Generalizations - nuclear stability
Atomic number > 83:
Nucleon number = 2, 8, 20, 28, 50, 82 or 126: added stability
Pairs of protons and pairs of neutrons: added stability
Neutron: proton ratios for added stability
1+increasing:1 with increasingly heavy isotopes

5. Nuclear equations Atomic number = Isotopes: same atomic number;
Mass number = number of nucleons (protons and neutrons) in nucleus
Nuclear reactions
Represented by balanced equations

6. Types of radioactive decay
Alpha emission
Beta emission
Gamma decay

7. Radioactive decay series
One radioactive nucleus decays to a 2nd, which decays to a 3rd, which…
Three naturally occurring series
Thorium-232 to lead-208
Uranium-235 to lead-207
Uranium-238 to lead-206

8. Writing Nuclear Equations
Write what you start with on the left side of the arrow
Using conservation of mass and atomic #, determine what the new nucleus will be

9. Things to remember: Alpha decay gives off a He nucleus
Beta decay gives off a high energy e-

10. Practice Thorium-232 undergoes Alpha decay
Radium 228 undergoes Beta decay

11. Radioactive Decay Rates
It is impossible to predict the exact moment a nucleus will decay
We CAN predict the time for half the nuclei in a sample to decay

12. Half-life Time required for 1/2 of a radioactive sample to decay
Example: 1 kg of an unstable isotope with a one-day half-life
After 1 day:
After 2 days:
After 3 days:
U-238 decay series: wide half-life variation

13. Calculating Half Lives
Radium-226 has a half life of 1599 years. How long would it take seven-eighths of a radium-226 sample to decay?
Step 1- List the given and unknown values

14. Calculating Half Lives
Step 2- Calculate the fraction of radioactive sample remaining
Step 3- Calculate the number of half lives

15. Calculating Half-Lives
Radium-226 has a half life of 1599 years. How long would it take seven-eighths of a radium-226 sample to decay?
Step 4- Calculate the total time required for the radioactive decay

16. Nuclear energy There is an overall loss of mass in nuclear reactions
Why?
Interconversion of mass and energy
Mass defect

17. Nuclear Energy Binding energy Energy out?
Energy required to break a nucleus into individual protons and neutrons
Energy out?
E=mc2
E= energy in J
M= mass defect in kg
C= speed of light
3.0e8 m/s

18. Nuclear fission Chain reactions Critical mass
Possible when one reaction can lead to others
One neutron in, two or more out
Critical mass

19. Many possible fission fragments

20. Nuclear fusion Less massive nuclei forming more massive nuclei
Requirements for fusion
High temperature
Sufficient confinement time

21. Nuclear power plants Rely on controlled fission chain reactions
Steel vessel contains fuel rods and control rods
Full plant very intricate
Containment and auxiliary buildings necessary
Spent fuel rods
Contain fissionable materials U-235, Pu-239
Disposal issues not settled

23. Source of nuclear energy
Ultimately connected to origins of the Universe and the life cycles of stars
Big Bang theory
Incredibly hot, dense primordial plasma cools, creating protons and neutrons
Continued cooling leads to hydrogen atoms which collapse gravitationally into 1st generation stars
Stellar evolution
Interior temperatures and densities suitable for fusion of heavy elements beyond hydrogen and helium
Certain massive stars explode in supernovae, spreading heavy elements (some radioactive)
Ultimate source: gravitational attraction!