Download NUCLEAR CHEMISTRY AND RADIOACTIVE DECAY

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NUCLEAR CHEMISTRY AND RADIOACTIVE DECAY
1 How long does it take a 100.00g sample of Au-l 98 to decay to 6.25g (half life = 2.69 days)?
2. How many half-lives will pass by the time a 60.0g sample of Co-60 decays to 7.50?
3. How long does it take a 180g sample of Au-198 to decay to 1/8 its original mass?
4. What fraction of a sample of N-16 remains undecayed after 43.2 seconds (half life = 7.2s) ?
5. What is the half-life of a radioactive isotope if a 500.0g sample decays to 62.5g in 24.3 hours?
6. How old is a bone if it presently contains 0.3125g of C-14, but it was estimated to have originally
contained 80.000g of C-14 (half life = 5730 yr)?
7. If you are injected with 1.0000 mg of Tc-99, how long will it take for the sample to decay to 1/64
of its original mass (half life = 6.02 hrs) ?
8. What is the half-live of a radioactive isotope if it takes 6.2 days for a 72g sample to decay to
18g?
9. Cs-137 is produced as a waste product in nuclear fission reactors. What fraction remains
undecayed after 241.84 years (half life = 30.23 yrs)?
10. How many half-lives of K-37 will pass after 6.15 seconds (half life = 1.23 s)?
11. What fraction of Pu-239 (an artificially produced isotope used as a fuel in some nuclear fission
reactors) remains undecayed after 219,600 years (half life = 24,000 yrs)?
12. If a 700.00g sample of I-131 decays to 43.75g, how much time has passed (half life = 8.01
days)?
13. How long will it take a 3.5g sample of Fr-220 to decay so that only 1/4 of the original amount
of Fr-220 remains (half life = 27.4 s)?
14. What is the half-life of a radioisotope if 1/16 of it remains undecayed after 26.4 days?
15. H-3 (tritium) is an artificially produce radioisotope used in some nuclear reactions. How much
of a 1.000 kg sample remains undecayed after 85.82 years (half life = 12.2 yrs)?
16. Co-60 is used in some cancer radiation therapies. What fraction of a sample of Co-60 will
remain undecayed after 5.26 years (half life = 5.26 yrs)?
17. Sr-90 is a common waste product of nuclear fission reactors. How many half-lives of Sr-90 will
pass after 144 years (half life = 28.8 yrs)?
Alpha decay can most simply be described like this:
1) The nucleus of an atom splits into two parts.
2) One of these parts (the alpha particle) goes zooming off into space.
3) The nucleus left behind has its atomic number reduced by 2 and its mass number reduced by 4
(that is, by 2 protons and 2 neutrons).
There are other points, but the three above are enough for this class. Here is a typical alpha decay
equation:
Notice several things about it:
1) The atom on the left side is the one that splits into two pieces.
2) One of the two atoms on the right is ALWAYS an alpha particle.
3) The other atom on the right ALWAYS goes down by two in the atomic number and four in the
mass number.
Write the alpha decay equations for these five nuclides
Beta decay is somewhat more complex than alpha decay is. These points present a simplified view
of what beta decay actually is:
1) A neutron inside the nucleus of an atom breaks down, changing into a proton.
2) It emits an electron and an anti-neutrino (see below) which go zooming off into space.
3) The atomic number goes UP by one and mass number remains unchanged.
Here is an example of a beta decay equation:
Some points to be made about the equation:
1) The nuclide that decays is the one on the left-hand side of the equation.
2) The order of the nuclides on the right-hand side can be in any order.
3) The way it is written above is the usual way.
4) The mass number and atomic number of the antineutrino are zero and the bar above the symbol
indicates it is an anti-particle.
5) The neutrino symbol is the Greek letter "nu."
Write out the full beta decay equation:
**A Brief Note on the Antineutrino:
As beta decay was studied over the years following 1899, it was found that the same exact beta decay produced an electron with variable energies.
For example, let us study Li-8 becoming Be-8. Each atom of Li-8 produces an electron and the theory says all the electrons should have the same
energy.
This was not the case.
The electrons were coming out with any old value they pleased up to a maximun value, characteristic of each specific decay.
To make a long story short, Wolfgang Pauli (in about 1930 or so) suggested the energy was being split randomly between two particles - the electron
and an unknown light particle that was escaping detection. Enrico Fermi suggested the name "neutrino," which was Italian for "little neutral one."
The neutrino itself was not detected until 1956 and the discoverers informed Pauli just a few months before his death due to cancer. Later on, it was
discovered that it was an antineutrino that was produced in beta decay.
Positron decay is like a mirror image of beta decay. These points present a simplified view of what
positron decay actually is:
1) Something inside the nucleus of an atom breaks down, which causes a proton to become a
neutron.
2) It emits a positron and a neutrino which go zooming off into space.
3) The atomic number goes DOWN by one and mass number remains unchanged.
Here is an example of a positron decay equation:
Some points to be made about the equation:
1) The nuclide that decays is the one on the left-hand side of the equation.
2) The order of the nuclides on the right-hand side can be in any order.
3) The way it is written above is the usual way.
4) The mass number and atomic number of the neutrino are zero.
5) The neutrino symbol is the Greek letter "nu."
Write out the full positron decay equation:
Electron capture is not like any other decay - alpha, beta, or position. All other decays shoot
something out of the nucleus. In electron capture, something ENTERS the nucleus. These points
present a simplified view of what electron capture is:
1) An electron from the closest energy level falls into the nucleus, which causes a proton to become
a neutron.
2) A neutrino is emitted from the nucleus.
3) Another electron falls into the empty energy level and so on causing a cascade of electrons
falling. One free electron, moving about in space, falls into the outermost empty level.
4) The atomic number goes DOWN by one and mass number remains unchanged.
Here is an example of a electron capture equation:
Write out the full electron capture equation: