Nuclear Chemistry Notes
Nuclear Chemistry
Intro to Nuclear Chemistry (05/27)
Nuclear Equations
Nuclear Chemistry
Nuclear Chemistry Energy
Chemical vs. Nuclear Energy:
Chemical energy is associated with making and breaking chemical bonds.
Nuclear energy is enormous in comparison.
Nuclear energy is due to changes in the nucleus of atoms, transforming them into different atoms.
13% of worldwide energy usage is derived from nuclear energy.
Worldwide Energy Consumption
Worldwide:
Oil: 3%
Other: 5%
Nuclear: 13%
Hydroelectric: 16%
Coal: 41%
Gas: 22%
France:
Nuclear: 77%
Gas: 2%
Hydroelectric: 1%
Oil: 1%
Other: 2%
Coal: 17%
United States:
Gas: 30%
Coal: 37%
Nuclear: 19%
Hydroelectric and oil: 2%
Other: 11%
China:
Coal: 79%
Other: 6%
Hydroelectric: 7%
Nuclear: 4%
Gas: 5%
Nuclear Chemistry Applications
Energy uses
Reaction mechanisms
Medical imaging
Medicine
Dating historical artifacts
Agriculture
Nuclear weapons
Radiation
What is Radiation?
Electromagnetic (EM) radiation - energy (waves)
Nuclear radiation - originates in the nucleus (unstable/radioactive isotopes)
Source of Radiation:
EM radiation carries energy in waves.
Nuclear radiation originates from unstable, radioactive isotopes within the nucleus.
Ionizing Radiation (unsafe):
Nuclear radiation + high energy EM radiation
The Nucleus
Composed of two nucleons: protons and neutrons.
Atomic number: number of protons.
Mass number: combined number of protons and neutrons, effectively representing the atom's mass.
Isotopes
Atoms of the same element can have different masses due to varying numbers of neutrons.
Examples of naturally occurring uranium isotopes:
Uranium-234
Uranium-235
Uranium-238
Radioactivity
Some nuclides of an element are unstable, or radioactive; these are referred to as radionuclides.
Atoms containing these unstable nuclides are called radioisotopes.
Radionuclides decay into different nuclides through several processes.
Types of Radioactive Decay
Alpha decay
Beta decay
Gamma emission
Positron emission
Electron capture
Nuclear Equations
Chemical equations: atoms and charges must balance.
Nuclear equations: atomic number and mass number must balance.
Balancing charge (atomic number) and mass (mass number) on an atomic scale.
Radioactive properties of the nucleus are independent of the atom's chemical state; the chemical form (element or compound) is not a concern.
Types of Radioactive Decay (Table 21.3)
Type | Nuclear Equation | Change in Atomic Number | Change in Mass Number |
|---|---|---|---|
Alpha decay | ^{A}{Z}X \rightarrow ^{A-4}{Z-2}Y + ^4_2He | -2 | -4 |
Beta emission | ^{A}{Z}X \rightarrow ^{A}{Z+1}Y + ^0_{-1}e | +1 | Unchanged |
Positron emission | ^{A}{Z}X \rightarrow ^{A}{Z-1}Y + ^0_{+1}e | -1 | Unchanged |
Electron capture | ^{A}{Z}X + ^0{-1}e \rightarrow ^{A}_{Z-1}Y (The electron captured comes from the electron cloud surrounding the nucleus) | -1 | Unchanged |
Alpha Radiation - Alpha Decay
Loss of an \alpha-particle (a helium nucleus).
Causes the atomic number to decrease by 2 and the mass number to decrease by 4.
Example: \^{238}{92}U \rightarrow \^{234}{90}Th + \^{4}_{2}He
Beta Radiation - Beta Decay
Loss of a \beta-particle (a high energy electron).
Causes the atomic number to increase by 1.
\beta particle is represented as ^0_{-1}e
Examples:
\^{131}{53}I \rightarrow \^{131}{54}Xe + \^{0}_{-1}e
\^{1}{0}n \rightarrow \^{1}{1}p + \^{0}_{-1}e
Gamma Radiation
Loss of a \gamma-ray (high-energy radiation that accompanies other radioactive emission).
Represents energy lost when unstable nuclei reorganize to a more stable arrangement.
\gamma-ray is represented as ^0_0\gamma
Positron Emission
Loss of a positron (a particle with the same mass as, but opposite charge to, an electron).
Causes the atomic number to decrease by 1.
Example: \^{11}{6}C \rightarrow \^{11}{5}B + \^{0}_{1}e
Electron Capture
Addition of an electron to a proton in the nucleus.
Results in a proton transforming into a neutron.
Causes the atomic number to decrease by 1.
Example: \^{1}{1}p + \^{0}{-1}e \rightarrow \^{1}_{0}n
Practice Problems (Radioactive Decay)
Problem 1: What product is formed when radium-226 (Ra) undergoes alpha decay?
Radon-222
Problem 2: What product is formed when:
Mercury-201 (Hg) undergoes electron capture? Gold-201 (Au-201)
Thorium-231 (Th) decays into protactinium-231 (Pa)? Beta decay (beta particle/electron)
Oxygen-15 undergoes positron emission? Nitrogen-15 (N-15) and positron
Problem 3: What happens to mass # and atomic # during:
Alpha decay: mass # decreases by 4, atomic # decreases by 2
Beta decay: mass # stays the same, atomic # increases by 1
Problem 4: Analyze the mass numbers of both alpha and beta decay nuclear equations. Which of the following statements are true?
c. The sum of the mass numbers on the reactant side always equals the sum of the mass numbers on the products side
Problem 5: Complete the following:
a. \^{42}{19}K \rightarrow \^{42}{20}Ca + \^{0}_{-1}e
b. \^{158}{74}W \rightarrow \^{154}{72}Hf + \^{4}_{2}He
Problem 6: Complete the following Uranium decay series (hint - the “product” becomes the new “reactant” for the next rxn)
Nuclear Stability (05/28)
Any atom with more than one proton (anything but H) will have repulsions between the protons in the nucleus.
Strong nuclear force helps keep the nucleus together.
Neutrons play a key role stabilizing the nucleus, so the ratio of neutrons to protons is an important factor.
Neutron-Proton Ratios
For smaller nuclei (Z \leq 20) stable nuclei have a neutron-to-proton ratio close to 1:1
As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
Stable Nuclei
The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability. This is essentially the stable neutron to proton ratio.
Notice that it increases as # protons increases
Nuclei above this belt have too many neutrons.
What type of nuclear decay do you think this nuclei would undergo?
need to decrease neutrons OR increase protons
beta decay
Nuclei below the belt have too many protons.
What type of nuclear decay do you think this nuclei would undergo?
need to increase neutrons, decrease protons
positron emission (or electron capture)
There are no stable nuclei with an atomic number greater than 83.
These nuclei tend to decay by alpha emission.
Practice Problems (Nuclear Stability)
Problem 1: Predict: would the nucleus of a carbon atom be stable if it had 6 protons and 10 neutrons? Why or why not?
Likely unstable because for small nuclei (Z < 20), the proton:neutron ratio is not 1:1.
Problem 2: Why do you think an alpha particle is the most dangerous form of radiation, but also the most harmless?
Alpha particles are much larger/heavier than other products of nuclear decay, so they can’t penetrate skin, but if ingested they can easily be absorbed by cells and cause cellular damage.
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).
Different isotopes of the same element will result in different decay series.
U-238 will produce Pb-206
U-235 will produce Pb-207.
Some Trends
Alpha decay: above atomic #83
Beta decay: above belt of stability (n:p ratio too high)
Gamma emission: high energy radiation
Positron emission: below belt of stability (n:p ratio too low)
The type of radioactive decay that a particular radionuclide undergoes largely depends on how its neutron-to-proton ratio compares with those of nearby nuclei that lie within the belt of stability.
Nuclei above the belt of stability (high neutron-to-proton ratios).
These neutron-rich nuclei can lower their ratio and move toward the belt of stability by emitting a beta particle because beta emission decreases the number of neutrons and increases the number of protons (Equation \ 21.3).
Nuclei below the belt of stability (low neutron-to-proton ratios).
These proton-rich nuclei can increase their ratio and move closer to the belt of stability by either positron emission or electron capture because both decays increase the number of neutrons and decrease the number of protons (Equations \ 21.5 and 21.7).
Positron emission is more common among lighter nuclei.
Electron capture becomes increasingly common as the nuclear charge increases.
Nuclei with atomic numbers \geq 84.
These heavy nuclei tend to undergo alpha emission, which decreases both the number of neutrons and the number of protons by two, moving the nucleus diagonally toward the belt of stability.
Practice Problems (Radioactive Series)
Problem 3: Aluminum-27 absorbed neutron and resulted nucleus decays by emitting \alpha-particle. What nuclide is produced as a result?
Forms aluminum-28, then undergoes alpha decay to sodium-24 (Na-24), \^{24}_{11}Na
Decay (Kinetics) & Half Life (05/29)
Practice Problems (Half-Life)
Problem 1: The half life of cobalt-60 is 5.27 years. How much of a 1.00 mg sample of cobalt is left after 15.81 years?
15.81 / 5.27 = 3 half lives
1.00 → 0.50 → 0.25 → 0.125 mg
Problem 2: Strontium-90 has a half-life of 28 years. Starting with 512 mg of this isotope, how much would remain after 112 years?
112/28 = 4 half lives
512 → 256 → 128 → 64 → 32 mg
Kinetics of Radioactive Decay
Radioactive decay is a first-order process.
The kinetics of such a process obey the equation: \ln{\frac{Nt}{N0}} = -kt
The half-life of such a process is: t_{1/2} = \frac{0.693}{k}
Comparing the amount of a radioactive nuclide present at a given point in time with the amount normally present, one can find the age of an object.
Half life is the time required for \frac{1}{2} of a radionuclide sample to decay
Radiometric Dating
Applying first-order kinetics and half-life information allows dating objects using a “nuclear clock.”
Carbon dating:
The half-life of C-14 is 5700 years.
Limited to objects up to about 50,000 years old because after this it is too little radioactivity left to measure.
Other isotopes can be used (U-238:Pb-206 in rock).
Radiocarbon Dating Process:
Cosmic rays (largely protons) enter the atmosphere and collide with atoms, creating neutrons
^{14}N + \^{1}{0}n \rightarrow \^{14}C + \^{1}{1}P
Nitrogen atoms capture a neutron and emit a proton, forming 14C
^{14}C atoms are incorporated in CO2, which is taken up by plants
^{14}C + O2 \rightarrow ^{14}CO2
Animals and people take in 14C by eating plants
Once an organism dies, intake of 14C ceases and its concentration decreases through beta emission to form 14N
^{14}C \rightarrow \^{14}N + \^{-1}_{0}e
Practice Problems (Radiometric Dating)
Problem 3: Uranium-238 decays into lead-206 over time through a series of radioactive decays. The half-life of uranium-238 is approximately 4.5 billion years. Suppose a rock sample originally contained 100% uranium-238 and no lead-206. Today, the sample contains 25% uranium-238 and 75% lead-206. How old is the rock?
Half life = 4.5 billion years
After 2 half lives: 100% → 50% → 25% uranium remains
2 half lives = 2 * 4.5 billion years = 9 billion years
Problem 4: If we start with 1.000 g of strontium-90, 0.953 g will remain after 2.00 years. What is the half life?
Recall: \ln[A] = -kt + \ln[A]0 and t{1/2} = \frac{0.693}{k}
\ln(0.953) = -k(2 \text{ years}) + \ln(1.000)
k = 0.024 / \text{year}
t_{1/2} = \frac{0.693}{k} = \frac{0.693}{0.024} = 28.7 \text{ years}
How much will remain after 5.00 years?
t_{1/2} = 28.7 \text{ years}
\ln(x) = -(0.024/\text{year})(5 \text{ years}) + \ln(1.000)
\ln(x) = -0.12
x = 0.887 \text{ grams}
Other Applications
Age of the Earth:
Zircon rock forms with U but never Pb - if you crack it open and find any Pb, it must have formed from U-238 decay.
Knowing half-lives and how much Pb you find enabled Clair Patterson to date the age of the earth (~1956).
Authenticating Old Art:
Looking at Carbon-14 to Carbon-12 ratios in a piece and using the C-14 half-life, you can date a piece back to 50,000 years.
Practice Problems (Half-Life)
Problem 5:
The T_{1/2} of Zn-71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed?
3 half lives so 100.0 → 50.0 → 25.0 → 12.5 g
Fermium-253 has a T_{1/2} of 0.334 seconds. A radioactive sample is considered to be completely decayed after 10 half-lives. How much time will elapse for this sample to be considered “gone”?
10 half lives * 0.334 sec = 3.34 seconds
Nuclear Waste
Video Thinking Questions:
Is it ethically justifiable to store radioactive waste deep underground (etc) given the potential risks to future generations?
Where do you think is the best place for us to store nuclear/radioactive waste? Why?
How do you think advancements in chemistry and engineering could contribute to the development of safer, more effective storage solutions?
How can chemistry education be used to improve public understanding and acceptance of radioactive waste storage solutions?
Energy, Fission & Fusion, Uses for Nuclear (05/30)
Energy in Nuclear Reactions
There is a tremendous amount of energy stored in nuclei.
Einstein’s famous equation, E = mc^2, relates directly to the calculation of this energy.
If a system loses mass, even a small amount, a tremendous amount of energy is released.
Mass Defect
Where does this energy come from?
The masses of nuclei are always less than those of the individual parts.
This mass difference is called the mass defect.
The energy needed to separate a nucleus into its nucleons is called the nuclear binding energy.
Effects of Nuclear Binding Energy on Nuclear Processes
Dividing the binding energy by the number of nucleons gives a value that can be compared.
Heavy nuclei gain stability and give off energy when they split into two smaller nuclei. This is fission.
Lighter nuclei emit great amounts of energy by being combined in fusion.
Nuclear Fission
Commercial nuclear power plants use fission.
Heavy nuclei can split in many ways. The equations below show two ways U-235 can split after bombardment with a neutron.
Nuclear Fusion
When small atoms are combined, much energy is released.
If it were possible to easily produce energy by this method, it would be a preferred source of energy.
However, extremely high temperatures and pressures are needed to cause nuclei to fuse.
This was achieved using an atomic bomb to initiate fusion in a hydrogen bomb. Obviously, this is not an acceptable approach to producing energy.
Fission vs. Fusion
Fission:
Splitting into 2 smaller isotopes
Additional chain reactions
Lots of energy released (weapons, fission bomb, nuclear power industry)
Critical mass (minimal mass)
More controllable than fusion
Radioactive waste products
Fusion:
2 atoms slam together to form a heavier atom
Fusion powers sun + stars
Several times the energy produced from fission
Consumes more energy than it produces (for most processes); need high P + T
Need more research to make it a clean source of energy
Radiation in the Environment
We are constantly exposed to radiation.
Ionizing radiation is more harmful to living systems than non-ionizing radiation, such as radiofrequency electromagnetic radiation.
Since most living tissue is ~70% water, ionizing radiation is that which causes water to ionize.
This creates unstable, very reactive OH free radicals (uneven number of electrons- unstable), which result in much cell damage.
Mini Research Share Out
For the next 15 mins: Choose any topic/use of nuclear that interests you and do mini-research. Be ready to do a share out about the topic in a small group of 3-4 people.
Who uses it?
How does it apply to nuclear?
Still in use now?
Pros/cons
Possible Topic Ideas/Inspiration: Art forgery, medicine (cancer treatment), sterilization, energy generation, space, agriculture, industry, radiocarbon dating, waste and disposal etc. etc. anything you want! (appropriate)
06/02 - Group Assessment “fun” in class assm (1h), groups of 3 30pts (weighted 30, assm grade)
06/03 - U10 Quiz 15 MCQ, 30pts (weighted 30, assm grade) everyone takes the quiz, but it will be optional to enter into aspen for revision - focus more on case studies & earlier slides