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Nuclear Chemistry

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Title: Nuclear Chemistry


1
Nuclear Chemistry
  • Just the basics.
  • By
  • J.M.Soltmann

2
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.

3
Radioactivity
  • While most nuclei are stable, many nuclei are
    unstable and spontaneously emit particles and
    electromagnetic radiation.
  • These nuclei are refered to as radionuclides.

4
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.

5
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.

6
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.

7
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.

8
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? .

9
Try this
  • Write a nuclear equation for the process when
    mercury-201 undergoes electron capture.

10
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

11
Try another
  • Thorium-231 decays into protactinium-231.
  • What is the balanced equation?
  • What other particle(s) is/are involved in the
    reaction?

12
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.

13
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???

14
Now try this one
  • Write the balanced nuclear equation for the
    process noted by the shorthand
  • 2713Al(n,?)2411Na

15
Now try this one
  • Write the balanced nuclear equation for the
    process noted by the shorthand
  • 2713Al(n,?)2411Na
  • 2713Al 10n --gt 2411Na 42He

16
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.

17
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.

18
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.

19
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.

20
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.

21
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.

22
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.

23
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.

24
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.

25
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?

26
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.

27
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

28
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.

29
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

30
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.

31
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)!
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