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Unit 8 - Chpt 19 - Nuclear Chemistry
• Radioactive Decay• Nuclear Transformations• Stability of nucleus• Uses of radiation• Fission and Fusion• Effects of radiation
• HW set1: Chpt 19 - pg. 901-905 # 12, 14, 16, 20, 22, 26, 29, 34, 40, 42 - Due Tues. Apr 13
Radioactivity review
• Atomic Number (Z) – number of protons
• Mass Number (A) – sum of protons and neutrons
XAZ
Radioactive Decay - Nucleus undergoes decomposition to form a different nucleus.
Nuclear Stability - review• Nuclides with 84 or more protons are
unstable.• Light nuclides are stable when Z equals A – Z (neutron/proton ratio is 1).
• For heavier elements the neutron/proton ratio required for stability is greater than 1 and increases with Z.
• Certain combinations of protons and neutrons seem to confer special stability. Even numbers of protons and neutrons are more
often stable than those with odd numbers.
Nuclear Stability - review
• Certain specific numbers of protons or neutrons produce especially stable nuclides.
• 2, 8, 20, 28, 50, 82, and 126
Decay Processes
Decay Series (Series of Alpha and Beta Decays)
Concept checkWhich decay process are the following?
electron capture
positron
alpha particle
beta particle
68 0 6831 1 30a) Ga + e Zn− →
62 0 6229 1 28b) Cu e + Ni+→
212 4 20887 2 85c) Fr He + At→
129 0 12951 1 52d) Sb e + Te−→
If the atomic number is not provided, you need to look it up in the periodic table i.e. 129Sb find Sb put in 51
Rate of decay
Rate = kN• The rate of decay is proportional to
the number of nuclides. This represents a first-order process.
• Half-life - Time required for the number of nuclides to reach half the original value.
( )1/ 2 =
ln 2 0.693 = t
k k
Rate problem
We don’t have to have an exact number of half-lives anymore, now that we know the rate is 1st order.
A first order reaction is 35% complete at the end of 55 minutes. What is the value of k?HINT: go back to kinetics for 1st order
Nuclear Transformation
• The change of one element into another.
• Math is same as radioactive decay can solve for what may be missing
27 4 30 113 2 15 0Al + He P + n→
249 18 263 198 8 106 0Cf + O Sg + 4 n→
How to make collisions - new elements
Schematic diagram of a cyclotron
Carbon-14 dating
• Used to date wood and cloth artifacts.
• Based on carbon–14 to carbon–12 ratio.
• Half-life 5730years
Radiotracer Applications• Radioactive nuclides that are
introduced into organisms in food or drugs and whose pathways can be traced by monitoring their radioactivity.
Energy Stability of Nucleus
• When a system gains or loses energy it also gains or loses a quantity of mass.
E = mc2
m = mass defectE = change in energy
• If E is negative (exothermic), mass is lost from the system.
Mass Defect
• Calculating the mass defect for : Since atomic masses include the masses of the
electrons, we must account for the electron mass.
• nucleus is “synthesized” from 2 protons and two neutrons.
4 42 24.0026 = mass of He atom = mass of He nucleus + 2 em
1 11 11.0078 = mass of H atom = mass of H nucleus + em
42He
( ) ( ) ( )[ ] 2 1.0078 + 2 1.0087 = 4.0026 2 e e− −Δ −m m m
= 0.0304 amu −m
42He
Missing mass in binding energy
Binding Energy• The energy required
to decompose the nucleus into its components.
• Iron-56 is the most stable nucleus and has a binding energy of 8.97 MeV per nucleon.
• 931.5 MeV per amu
Fission or Fusion
• Fusion – Combining two light nuclei to form a heavier, more stable nucleus.
• Fission – Splitting a heavy nucleus into two nuclei with smaller mass numbers. (A self-sustaining fission process is called a chain reaction.)
1 235 142 91 10 92 56 36 0n + U Ba + Kr + 3 n→
Fission or Fusion?
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