These are notes I typed a while ago - page numbers WILL NOT correspond to your textbook.
Start off with introduction to the uses/applications of nuclear chemistry
● Biomedical – determine processes in the body using isotopes such as tritium and deuterium ● Cancer treatment – irradiation of malignant tumors
● Genetics – recall that DNA was shown to be the genetic material when it was shown that DNA was injected into bacteria by bacteriophages when an isotope of phosphorus showed up, not an isotope of sulfur.
● Dating – radioactive decay proceeds at a specific rate for different elements. This fact can be used to date items. The isotope used depends on the item being dated and the probable age… ● Generate electricity – there are several nuclear plants in the Unites States and elsewhere.
They are used to power large naval vessels as well. It is probable that nuclear energy (Fission) will become increasingly important as oil reserves run out.
Short review of previous concepts that must be understood
● Three sub-atomic particles, two of which reside in the nucleus (proton and neutron). The two in the nucleus are called nucleons.
● It is possible to have different atomic mass, but identical atomic number. Atoms with different atomic mass but identical atomic number are called isotopes.
● Example of isotopes: U-234 (234,92U), U-235 (235, 92U), U-238 (238, 92U); note that the large number refers to the mass, the smaller number here is the atomic number (written with number above the other…)
● Different isotopes have different abundances. 99.3% is U-238, 0.7% is U-235, trace is U-234.
● Example of calculation of relative atomic mass….(Find one). ● Nuclide – nucleus with specified number of protons and neutrons ● Radionuclides – nuclei that are radioactive
● Radioisotopes – atoms containing radionuclides
● Radionuclides are unstable and decay – resulting in energy and radiation. ● Nuclear Equation – shows the decay of a radionuclide.
U-238 (238,92) (4,2)He + (234,90) Th U-238 decays to helium and thorium
Note (regarding above example)
o Sum of mass # on both sides the same o Sum of atomic # same on both sides.
o Example of alpha decay, where a He particle is emitted.
● Beta: Conversion of a neutron to a proton plus electron. The emitted electron is not an orbital electron, but originates in the nucleus. Thus, atomic # increases with beta decay.
(Equations 21.2 and 21.3)
o Don’t make mistake of concluding electron is part of nucleus! - it is a product of the nuclear reaction
o Find another example here…
● Gamma: high energy photons emitted in decay. (EM of very short wavelength.) o Changes neither the mass # or the atomic #.
o Almost always with other radioactive emission because it represents energy lost when remaining nucleons reorganize into more stable arrangements.
o Do example page 833
● Positron and electron capture: Both result n the conversion of a proton to a neutron and hence a reduction in the atomic number (but not mass). Positrons are particles with same mass as electron but opposite charge. The positron is eliminated in collision with orbital electron producing gamma rays. In electron capture, an orbital cloud electron is incorporated into the nucleus.
o Do example equation from page 834.
Radioactive properties of nucleus are essentially independent of the state of chemical combination of the atom. Decay occurs at same rate regardless of whether an atom is in a compound or is “free”.
Question – See 21.1 from Brown et. al.
What product is formed when radium 226 decays by alpha decay?
226,88Ra 4,2He + (226-4,88-2)X, where X, with 86 protons must be Rn (radon)
Example problems to do…Sample exercise 21.1 and 21.2
Lecture 2 – Nuclear Chemistry: Patterns of nuclear Stability
Stability of the atom is related to the Neutron to proton ratio ● Nuclear force exists between nucleons at close distances.
● Neutrons are involved in this – the more protons there are, the more neutrons are needed to bind nucleus together.
● The number of neutrons necessary to create a stable nucleus increases more rapidly than the number of protons. (Copy figure 21.2 and table 21.1 for students)
● The belt of stability ends at Bismuth (83 protons); all elements with Atomic # > 83 are radioactive.
● Type of radioactive decay depends on (page 836) Copy figure 21.3 to explain) ● High neutron- proton ratios; emit a Beta, this decreases neutrons and increases protons ● Low neutron-proton ratio; positron emission or electron capture that results in an increase of
neutrons and decrease in protons. Positron emission is more common among lighter nuclei, electron capture becomes more common as nuclear charge increases.
● Atomic number > 83, often alpha-decay decreases both neutron and protons. Do example 21.3 (sample problem)
● Radioactive series – some can’t achieve stability in one decay…several steps are necessary. Example, Uranium, to lead. (copy figure 21.4)
● Assignment, label the steps with decay, write the nuclear equations for each and identify the type of decay.
Additional Observations re: nuclear stability
● “Magic numbers”: nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82 or 126 neutrons are generally more stable than nuclei that do not contain these numbers of nucleons.
● Nuclei with even numbers of both protons and neutrons are generally more stable. ● The shell model of nucleons, analogous to shell model for electrons, explains this…
Homework: The figure 21.4 assignment (above) and 21.13 through 21.20 Brown et. al.
Determining stability….
Determine protons and neutrons, examine whether in belt of stability See if the # protons and neutrons are “magic numbers”
See if # protons >83 Examine ratio
Predicting type of decay
Lecture 3: Nuclear transmutations
Change of identity associated with collisions w/neutron or another nucleus.
Ernest Rutherford (1914): N-14 collision with He O-17 plus proton. (See page 838) This work led to synthesizing radioisotopes in laboratories.
Using charge particles to synthesize…particles must be moving very fast to overcome electrostatic repulsion.. Particle accelerators – use strong magnets and electrostatic fields to accelerate particles to the collision point.
Using neutrons – aren’t repelled because have no charge (and thus can’t be accelerated); takes place in nuclear reactors. (illustrate, 21.9-21.11)
Transuranium elements – transmutations used to produce elements w/atomic #’s > 92.
Why are some elements found and others not? ● Related to the decay rate.
● Radioactive decay proceeds continuously, with ½ decaying in a given amount of time referred to as the ½ life of that element. Some are billions of years in length, others are milliseconds.
● The ½ life is the length of time it takes for ½ of the mass of the radioactive element to decay. ● Radioactive decay is not affected by temperature, pressure, state of chemical combination. (Copy figure 21.7)
Concept of a ½ life…
Given 50.0 grams of Au-198, that has a half-life of 2.7 days, how many grams do you have after 1 half-life? 2 half-lives? 3 half-lives? 4 half-lives? 10 half-lives…..
Make a table for the first….then, introduce the equation: Nt = N0e-kt for the last.
K is the decay constant, different for all radioisotopes! Calculate using K = 0.693/(½ life) NOTE UNITS being USED!
This equation can be used to calculate the amount remaining after any time period….example: How much Au-198 remains after 200 days?
Dating using radioactive nuclei
● (14,7)N + (1,0)n (14,6)C + (1,1)P A fraction of C-14 by neutron capture occurs in the upper atmosphere.
o Decays: (14,6)C (14,7)N + (0,-1)e; beta decay, ½ life = 5715 years.
o Assumptions: Ratio of C-12 to C-14 in atm. Has been stable for at least 50,000 years. C-12 is incorporated into compounds by living organisms at an equal rate as C-14.
o After an organism dies, the ratio will begin to change because incorporation ends and C-14 decays. Determination of the ratio can be used to determine elapsed time; but 50,000 is the upper end of accuracy.
● Decay of other elements can be used to date older (and non-organic) objects. For example, U-238 has a ½ life of 4.5 X 10^9 years (to lead-206)
½ life calculations and radioactive dating
Rate = kN, k is the decay constant and N is the radioactive nuclei. K = 0.693/t1/2 ln(Nt/N0) = -kt DO examples!
K is the decay constant Nt is the amount after t years; N0 is the original amount t1/2 is the half-life in years
Note that the ratio of the mass at any time t to the mass at t=0 OR the ratio of the activities at time t and t = 0 can be substituted for Nt/N0 in the equation.
A rock contains 0.257 mg of lead-206 for every mg of U-238. The half-life for the decay of u-238 to lead-206 is 4.5 X 109 years. How old is the rock?
All of the lead is from U-238 decay.
Plan: calculate amount of U-238 at start, determine decay constant, solve for time
1. Assume there is 1.000 mg of U-238 at present
The mount of U-238 at start is therefore 1.000 + amount that became lead. Amount that became lead = (ratio of mass numbers)(amount of lead now)
=(238/206)(0.257) = 0.297 g
So, there was 1.297 g U-238 at the start.
2. k = 0.693/half-life = 0.693/4.5 X 109 = 1.5 X 10-10/yr
3. t = -(1/k)ln(Nt/N0) = -(1/1.5 X 10-10)ln((1.000/1.297) = 1.7 X 5 X 109 years
Lecture 4: Detection of radioactivity and other subjects in classroom text… Sections 18.5 through18.9, HW Pg 605-606, 47-67 Odd (evens are XC)
Start with another example of Dating…
Second example: A wooden object from an archeological site is subjected to radiocarbon dating. The activity of the sample due to C-14 is measu red to be 11.6 disintegrations/second. The activity of a carbon sample of equal mass from fresh wood is 15.2 disintegrations/second. The half life of C-14 is 5715 years. What is the age of the archeological sample? (2230 years)
Use t = -(1/k)ln(Nt/N0) and k = 0.693/half-life. Solve for K first, k = 0.693/5715 = 1.21 X 10-4
Solve for t, using ratio of activities for Nt/N0 (11.6/15.2); t = 2234 = 2230 years
Third Example: Potassium-40 decays to argon-40 with a half-life of 1.27 X 109 years. What is
the age of a rock in which the mass ratio of Ar-40 to K-40 is 3.6? We can substitute the mass ratio for the ratio of Nt/N0,
Solve for K, k = .693/1.27 X 109 years.
Use equation t = -1/k ln(3.6) =
Uses in medicine…
There are multiple uses in medicine – treatment of cancer, and the use of Radiotracers to
diagnose conditions. For example, I-131 can be used to examine thyroid activity. Thallium 201 to diagnose heart condition after a heart attack.
Essentially, radiotracers – nuclides that are biologically active and can be introduced to an organism for later detection (in a manner similar to an X-ray – except the radiation is internal!) to study processes. See table 18.4 in text.
Nuclear energy: Two types:
● Fusion: the combining of two lighter atoms to form heavier ones.
o This occurs in the sun. It turns out that all heavy atoms were produced in larger stars – and arrived in our Earth via explosion, travel to, and accumulation in the nebula that led to the formation of the solar system.
o Occurs at very high temperatures when atoms are in close proximity.
● Fission: Splitting of the atom
o Results in lighter elements plus neutrons.
o U-235 collide with neutron Ba-141 + Kr-92 + 3 neutrons. (Pg 595) Turns out that over 200 different isotopes have been observed from the splitting of U-235… o Chain reactions; neutrons that leave collide with other atoms and can split them. o Energy is much greater than in chemical bonds.
