Nuclear Chemistry

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Nuclear Chemistry

• Just the basics.
• By
• J.M.Soltmann

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What is Nuclear Chemistry

• As its name implies, nuclear chemistry is the
  study of the nucleus and reactions between
  nuclei.
• Remember that virtually all of the mass of an
  atom resides in the nucleus, as does all of the
  positive charge.
• Nuclear energy is a much greater form of energy
  than bond energy.

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Radioactivity

• While most nuclei are stable, many nuclei are
  unstable and spontaneously emit particles and
  electromagnetic radiation.
• These nuclei are refered to as radionuclides.

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Nuclear Equations

• In a nuclear equation, mass numbers and atomic
  numbers are balanced instead of elements.
• The example here to the right depicts a
  radioactive decay specifically an alpha decay.
• The helium ion is called an alpha particle.

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3 Common types of Radioactive Decay

• Alpha decay
• Beta decay - a ß- particle is a subatomic nuclear
  particle essentially equivalent to an electron
  and a ß particle is a positively charge
  electron, called a positron.
• Gamma decay - high energy photons are emitted
  which have virtually no mass nor charge.

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Nuclear electrons?

• Modern theory has shown that a neutron is
  actually comprised of a proton and an electron.
• So, if a nucleus emits an electron, it has really
  transformed a neutron into a proton.
• Also, if a nucleus absorbs an electron, it will
  convert a proton into a neutron.

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Common Particles in Nuclear Reactions

• Neutrons (10n)
• Protons (11p or 11H)
• Electrons (0-1e)
• Alpha Particles (42He or 42?)
• Beta- Particles (0-1e or 0-1?)
• Gamma (00?) - Gamma radiation consists of
  high-energy photons, with a mass far too little
  for consideration.
• Positron (01e) - A positron is a positively
  charged electron. It has the mass of an electron
  but a positive charge.

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Differentiating the Radiations

• Alpha emissions are the heaviest and thus have
  the least penetrating power.
• Beta emissions have masses much smaller than
  protons or neutrons, so they have more
  penetrating power. In terms of penetrating
  power, ?100? .
• Gamma emissions have essentially no mass, so they
  are the most powerful. In terms of penetrating
  power. ? 100? .

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Try this

• Write a nuclear equation for the process when
  mercury-201 undergoes electron capture.

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To answer this question

• First we have to understand what mercury-201 is.
  Since mercury is always atomic number 80, this
  isotope is 20180Hg.
• Since we are capturing an electron, the electron
  must be a reactant.
• Now we add up mass numbers and atomic numbers.
  (201 0 201 and 80 -1 79).
• Element 79 is gold, so the answer is
• 20180Hg 0-1e --gt20179Au

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Try another

• Thorium-231 decays into protactinium-231.
• What is the balanced equation?
• What other particle(s) is/are involved in the
  reaction?

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The answers are

• 23190Th --gt 23191Pa 0-1e
• The extra particle is an electron, but because it
  is being emitted, it would be called a Beta
  emission.

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Nuclear Transformations

• The first manmade conversion of one nucleus into
  another was performed by Sir Ernest Rutherford
  (1919).
• Rutherford bombarded a nitrogen-14 atom with
  alpha particles to produce an oxygen-17 atom plus
  a proton.
• 147N 42He --gt 178O 11H
• The shorthand version of this reaction is
• 147N(?,p)178O WHY???

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Now try this one

• Write the balanced nuclear equation for the
  process noted by the shorthand
• 2713Al(n,?)2411Na

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Now try this one

• Write the balanced nuclear equation for the
  process noted by the shorthand
• 2713Al(n,?)2411Na
• 2713Al 10n --gt 2411Na 42He

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Nuclear Stability

• Why are some nuclei more stable than others?
• To be honest, there are several factors, most of
  which are beyond the scope of this course.
• However, there are a few easy to see indications
  of nuclear stability.

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Did you ever wonder?

• We know that like charges repel each other, yet a
  nucleus can have dozens of positively charged
  protons held together. Why?
• Neutrons are a major reason. All nuclei with 2
  or more protons have neutrons. The neutrons and
  the protons meld by a force of nature, different
  than gravity or electromagnetism, called the
  strong (nuclear) force.
• Because of the way this force binds the protons
  and neutrons together, the ratio of protons to
  neutrons is an issue.

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Smaller Atoms vs Bigger Atoms

• In smaller atoms, most stable atoms have neutron
  to proton ratios of about 1.00.
• As isotopes increase in atomic number, most
  stable isotopes have increasingly larger ratios
  of neutrons to protons.
• To our knowledge, any isotope with an atomic
  number greater than or equal to 84 would be
  radioactive.

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Some stability trends

• Of the 265 known stable isotopes
• 157 of them have even numbers of protons and
  neutrons.
• 53 of them have an even number of protons but an
  odd number of neutrons.
• 50 of them have an odd number of protons but an
  even number of neutrons.
• Only 5 of them have odd numbers of both protons
  and neutrons.

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Magic Numbers

• For some reason, nuclei with 2,8,20,28,50 or 82
  protons and/or 2,8,20,28,50,82, or 126 neutrons
  are generally more stable than isotopes without
  these numbers.
• When we think of substances that shield
  radiation, we tend to think of lead. The most
  common isotope of lead is 20882Pb that means it
  has 82 protons and 126 neutrons.

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Decays and Half-lifes

• When a radioactive substance decays, the amount
  of that particular isotope will decrease.
• We call the rate of decay the half-life, because
  it is the time needed for exactly 1/2 of the
  isotope to decay.

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More on Half-life

• If we examine the graph to the right, we see that
  we started with 50 g of the isotope. Each
  subsequent point represents half of the previous
  mass (50 to 25 to 12.5 to 6.25 to 3.125 to 1.5625
  to .78125).
• Each point is approximately 24 days apart The
  half-life for this substance is 24 days.

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Calculations with half-life

• Although it is possible to determine the amount
  remaining of a radioisotope using natural logs
  ln(Nt/N0)-kt we do not need to do this.
• We only work with whole number increments of the
  half-life.

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For example

• The half-life of an isotope is 8 days. If we
  start with 100 grams of the isotope, how much is
  present in 32 days?
• 32 days/(8 days/half-life) 4 half-lives.
• Each half-life divides the previous mass in half.
  
• 100g/2 50g/2 25g/2 12.5g/2 6.25g
• There would be 6.25 g of that isotope left.

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You try one

• The half-life of Bismuth-211 is 185 years. How
  much time would it take for a 360 g sample to
  decay to 11.25 g?

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You try one

• The half-life of Bismuth-211 is 185 years. How
  much time would it take for a 360 g sample to
  decay to 11.25 g?
• 360g/2180g/290g/245g/222.5g/211.25g.
• That is 5 half-lives.
• 5 half-lives185 years/half-life 925 years.

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Fusion vs. Fission

• Fission and Fusion are two types of highly
  exothermic nuclear reactions, different than the
  decays covered earlier.
• Fusion means to bring two smaller nuclei together
  to make a larger nucleus.
• 136C 136C --gt 2511Na 11p
• Fission means to break a larger nucleus into 2 or
  more smaller nuclei.
• 23592U 10n --gt 13752Te 9740Zr 2 10n

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How much Energy are we talking about?

• In fusion and fission, a tiny, almost meaningless
  mass of each affected nucleus is converted to
  energy.
• Einstein theorized that the amount of energy was
  dependent on the mass lost and the square of the
  speed of light. E mc2.

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So, how much energy is that?

• Well if each uranium atom in a given fission
  process loses the mass of one electron
  (9.11x10-31 kg)
• E mc2
• E (9.11x10-31 kg) (3.0x108 m/s)2
• E 8.2x10-14 J

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But that seems like a small number!

• 8.2x10-14 J is a small amount, but that was for
  just one atom or uranium. If we had 1 mg of
  uranium (about the mass of a cystal of salt),
  that would contain roughly 2.5 x 1018 atoms of
  uranium.
• 8.2x10-14 J/atom 2.5 x 1018 atoms 2.1x105 J
• That is almost enough energy to handle the
  electrical needs of this school for a day - from
  a tiny starting mass.

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Think about this

• If we could convert the .25 kg mass of a banana
  peel (using our Mr. Fusion power supply) into
  pure energy,
• E mc2
• E (.25 kg)(3.0x108 m/s)2
• E 2.25x1016 J
• Thats enough energy to run New York City for a
  year (with enough energy left over to go Back to
  the Future)!