nuclear chemistry. the nucleus remember that the nucleus is comprised of protons and neutrons. the...
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Nuclear Chemistry
The Nucleus
• Remember that the nucleus is comprised of protons and neutrons.
• The number of protons is the atomic number.• The number of protons and neutrons together is
the mass of the atom.
Isotopes
• Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.
• There are three naturally occurring isotopes of uranium:– Uranium-234– Uranium-235– Uranium-238
Stable NucleiThe shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.
http://phet.colorado.edu/en/simulation/isotopes-and-atomic-mass
Most nuclei are stable.
It is the ratio of neutrons to protons that determines the stability of a given nucleus.
Radioactivity
• It is not uncommon for some nuclei to be unstable, or radioactive.
• There are no stable nuclei with an atomic number greater than 83.
• Radioisotopes = isotopes that are unstable and thus radioactive
• There are several ways radionuclides can decay into a different nuclide.
Radioactive Series
• Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.
• They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).
• Transmutation = the reaction by which the atomic nucleus of one element is changed into the nucleus of a different element
pHET simulations of alpha decay of Polonium-211 to form Lead-207 and of
Beta decay of Hydrogen-3 to Helium-3
• http://phet.colorado.edu/en/simulation/alpha-decay
• http://phet.colorado.edu/en/simulation/beta-decay
Types of Radioactive Decay Alpha Decay
= Loss of an -particle (a helium nucleus)
He42
U238
92 Th234
90 He42+
CorrectionAtomic # decreases by 2# of protons decreases by 2# of neutrons decreases by 2Mass # decreases by 4
Types of Radioactive Decay Beta Decay
= Loss of a -particle (a high energy electron)
0
−1 e0
−1or
I131
53 Xe131
54 + e0
−1
Atomic # increases by 1# of protons increases by 1# of neutrons decreases by 1Mass # remains the same
Types of Radioactive Decay Positron Emission
= Loss of a positron (a particle that has the same mass as but opposite charge than an electron)
e01
C11
6 B11
5 + e01
Atomic # decreases by 1# of protons decreases by 1# of neutrons increases by 1Mass # remains the same
Types of Radioactive Decay Gamma Emission
= Loss of a -ray (a photon of high-energy light that has no mass or charge & that almost always accompanies the loss of a nuclear particle; often not shown when writing nuclear equations)
00
Artificial Transmutation= done by bombarding the nucleus with high-energy particles (such
as a neutron or alpha particle), causing transmutation
4020Ca + _____ -----> 40
19K + 11H
9642Mo + 2
1H -----> 10n + _____
**Natural transmutation has a single nucleus undergoing change, while artificial transmutation will have two reactants (fast moving particle & target nuclei.**
Nuclear Fission• Nuclear fission is the type of reaction carried out in
nuclear reactors.• = splitting of large nuclei into middle weight nuclei and
neutrons
Nuclear Fission
• Bombardment of the radioactive nuclide with a neutron starts the process.
• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
• http://phet.colorado.edu/en/simulation/nuclear-fission
• This process continues in what we call a nuclear chain reaction.
• = the combining of light nuclei into a heavier nucleus
• 21H + 2
1H 42He + energy
• Two small, positively-charged nuclei smash together at high temperatures and pressures to form one larger nucleus.
Nuclear Fusion
Energy changes in Nuclear Reactions E =mc2
• Einstein E =mc2
• mass defect For nuclear reactions
• E = energy in Joules (J = kg•m2/s2)
• m = mass in kg• C = speed of light • (2.9979 x 108 m/s)
Half-Life
= the time it takes for half of the atoms in a given sample of an element to decay
- Each isotope has its own half-life; the more unstable, the shorter the half-life.
- Table T Equations:
fraction remaining = (1/2)(t/T)
# of half-lives remaining = t/T
Key: t = total time elapsed T = half-life
PhET simulation of decay and half-life
• http://phet.colorado.edu/en/simulation/radioactive-dating-game
Sample Half-Life Question 1AMost chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. (a) What fraction of a sample of Cr-51 will remain after 168 days?
Step 1: Determine how many half-lives elapse during 168 days.
Step 2: Calculate the fraction remaining.
Sample Half-Life Question 1B
(b) If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days?
Step 1: Calculate the mass remaining:
mass remaining = fraction remaining X original mass
(Note: Mass remaining can also be calculated by dividing the current mass by 2 at the end of each
half-life.)
Sample Half-Life Question 2
How much was present originally in a sample of Cr-51 if 0.75gremains after 168 days?
Step 1: Determine how many half-lives elapsed during 168 days.
Step 2: Multiply the remaining amount by a factor of 2 for each half-life.
Some practical uses of Radioisotopes (dating, chemical tracers, industrial applications, medical
applications, nuclear power plants)
Medical Uses• 60Co (cobalt-60) used in
cancer treatments and used to kill bacteria in food products
• 226Ra (Radium-226) used in Cancer treatment
• 131I diagnosis and treatment of thyroid disorders
• 11C Positron emission tomography (PET scans)
Other Uses• 14C archaeological dating (of
once living things) and radiolabelled organic compounds
• 238U archaeological dating (U-238 to Pb-206 ratio)
• 241Am (Americium-241) smoke detectors
• 235U nuclear reactors and weapons