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Nuclear ChemistryNuclear Chemistry

• In a chemical reaction, the valence electrons are important.

• But the nuclei of elements may undergo changes as well.

• When the nuclei of elements change (either spontaneously or when forced) a nuclear reaction has occurred.

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Nuclear ChemistryNuclear Chemistry

• Nuclear reactions give off enormous amounts of energy for the very small masses involved.E = mc2

• They are also dangerous to our bodies, but they have medical applications which can save our lives.

• Unlike chemical rxns, nuclear rxns are unaffected by temperature, pressure, or by catalysts.

• Nuclear rxns have 1st-order kinetics.

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RadioactivityRadioactivity

• The nucleus contains the nucleons: protons and neutrons. So the Mass Number gives you the number of nucleons.

• How many nucleons does C-14 contain?

• You should remember the above notation as an isotopic notation for the isotope C-14.

• What are some of the other ways to write the isotopic notations for C-14?

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RadioactivityRadioactivity

• Remember isotopes? • Isotopes of an element have the same Atomic Number, Z, but have different # of neutrons.

• How else are isotopes different?• If an isotope emits radiation, then it is a radioactive isotope, or a radioisotope or a radionuclide.

• Every element has at least 1 radioisotope!

• What are the 3 common types of radiation and what are they?

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Nuclear Decay Rxns or Nuclear RxnsNuclear Decay Rxns or Nuclear Rxns

• Radionuclides aren’t stable. • They spontaneously undergo nuclear reactions and emit particles and/or electromagnetic radiation.

• This process is called nuclear decay as the nucleus is changing and decomposes.

• Eventually, the unstable radioisotope will become a stable isotope!

• Here is a table showing the common types of particles and their common notations:

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Nuclear Decay Rxns or Nuclear RxnsNuclear Decay Rxns or Nuclear Rxns

• Here are the common types of nuclear decay:

α-decay or α-emissionβ-decay or β-emissionpositron emissionelectron capturegamma emission

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Patterns of Nuclear StabilityPatterns of Nuclear Stability

• What makes one isotope stable and another radioactive?

• The neutron-to-proton ratio or the n/p ratio!!

• Look at the nucleus: lots of + protons are packed into an extremely dense, tiny volume.

• Why don’t they repel and eject out of the nucleus?

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Patterns of Nuclear StabilityPatterns of Nuclear Stability

• The neutrons help “glue” the nucleus together and keep the protons from repelling too strongly.

• This is part of the strong nuclear force, the force which holds nucleons together.

• This is particle physics: gluons are the “strong force” carrier particles, and the interaction between neutrons and protons are actually residual effects of the strong force.

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Patterns of Nuclear StabilityPatterns of Nuclear Stability

• So an atom with more than 1 proton (every element but H) MUST have neutrons to stabilize the nucleus.

• There is an optimum n/p ratio or else the nucleus is unstable.

• For atoms with Z below 20, the ideal n/p ratio is about 1.

• As the atomic number increases above 20, the ideal n/p ratio increases till it reaches about 1.5/1 with Hg-200.

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Patterns of Nuclear StabilityPatterns of Nuclear Stability

• This is the Belt of Stability, or what the ideal n/p ratio is for stable nonradioactive isotopes.

• Notice this graph stops at Z = 84. All nuclei with 84 or more protons are radioactive!

• There is another graph showing nonradioactive isotopes AND radioactive isotopes with measurable half-lives.

• Radioisotopes with measurable half-lives may be considered to be “stable” radioisotopes.

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Patterns of Nuclear StabilityPatterns of Nuclear Stability

• What can an unstable radionuclide do to decay to reach the Belt of Stability?

• If the radionuclide is above the Belt, it has too many neutrons (Yes, it can have too many neutrons, ever used too much glue?). How can it get rid of extra neutrons and lower the n/p ratio?

• If the radionuclide is below the Belt, it has too many protons. How can it lower its n/p ratio?

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Patterns of Nuclear StabilityPatterns of Nuclear Stability

• What if the radionuclide has Z > 84 and is beyond the Belt?

• These heavy elements often have to undergo many radioactive decays, or a series, in order to reach a stable isotope. These series are called radioactive series or nuclear disintegration series.

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Magic NumbersMagic Numbers

• If you study tables of stable and unstable isotopes, you might notice something strange.

• Nuclei with 2, 8, 28, 50, or 82 protons tend to be more stable.

• Nuclei with 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable.

• These are the “magic numbers” of protons and neutrons.

• Also, nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of nucleons.

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Kinetics of Nuclear Decay RxnsKinetics of Nuclear Decay Rxns

• Normal nuclear decay reactions are first order, and so are solved using the first-order equations that you already learned.

• Problems:– The half-life of Co-60 is 5.3 years. How much of a 1.000 mg sample is left after 15.9 years? How much is left after 17.0 years?

– A rock contains 0.257 mg Pb-206 per mg U-238. The half-life for the decay of U-238 to Pb-206 is 4.5x109 years. How old is this rock?

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Nuclear Binding EnergyNuclear Binding Energy

• It takes energy to bind the nucleus together (the strong force).

• The nuclear binding energy is defined as the energy required to hold or “bind” the nucleus together.

• It is also the energy required to break apart the nucleus into the nucleons.

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Nuclear Binding EnergyNuclear Binding Energy• But where does this binding energy come from?

• From MASS!E = mc2

• Units?• But where does this mass come from?• If you add up the mass of all the protons, electrons, and neutrons in an isotope and compared it to the actual isotopic mass, they AREN’T THE SAME!

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Nuclear Binding EnergyNuclear Binding Energy

• The actual mass of an isotope IS NOT equal to the sum of the parts!

• This mass difference is called the mass defect.

• It is the amount of mass which has been converted into the nuclear binding energy.

• The mass defect is usually given in amu or in kg/atom or in kg/mol.

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Nuclear Binding EnergyNuclear Binding Energy

• Nuclear binding energies are usually given in kJ/atom or kJ/nucleon or kJ/mol nucleon.

• You can see from the following slide that nuclear binding energies per nucleon don’t vary that much.

• But intermediate elements are the most stable.

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Nuclear Binding Energy per NucleonNuclear Binding Energy per Nucleon

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Energy in Nuclear ReactionsEnergy in Nuclear Reactions

• During nuclear rxns, nuclides either gain or lose mass, and so release or require energy.

• In spontaneous rxns, mass is lost so energy is released.

• This is why most nuclear rxns give off gamma rays: this is the mass which has been “lost” or converted to energy.

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Fission vs. FusionFission vs. Fusion

• During Fission, a large nuclei is split into several smaller nuclei.

• In Fusion (which is what stars do), small nuclei are fused together to make a larger one.

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Fission of U-235Fission of U-235

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Nuclear Chain ReactionNuclear Chain Reaction

Little Boy SchematicLittle Boy Schematic

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Fat Man SchematicFat Man Schematic

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Dangers of RadioactivityDangers of Radioactivity

• Although radioactivity is used in cancer therapy and in medical diagnostics, it is harmful to our cells.

• It can damage cells, damage the DNA in cells, and kill cells.

• Is there any difference between the 3 types of radiation?

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Dangers of RadioactivityDangers of Radioactivity

• There is a difference in penetration and ionization damage (but they are opposing).

• The more penetrating the radiation is, the less damaging it is.

• Alpha particles may be stopped by paper or our skin.

• Beta particles are stopped by ¼” of plexiglass.

• Gamma rays are stopped by several inches of lead or several feet of steel or concrete.

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Dangers of RadioactivityDangers of Radioactivity

• So any type of radioactivity can be dangerous.

• We also talk about the intensity of emitted radiation.

• The intensity of emitted radiation is related to:– the amount of radioactive material in a sample;

– the half-life of the radioactive isotope

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Dangers of RadioactivityDangers of Radioactivity

• How can you protect yourself when working with radioactive isotopes?

• Proper clothing• Proper lead shielding• Distance!

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Sources of RadioactivitySources of Radioactivity

• We are exposed to a variety of radioactivity in many ways.

• Do you think that your exposure is from synthetic sources (like dental X-rays, etc.) or from natural sources?

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