Nuclear Chemistry Notes
Nuclear Chemistry
Introduction to Nuclear Chemistry
Nuclear chemistry involves changes in the nucleus of an atom.
It has applications in power generation, industry, medicine, and research.
Key questions to consider:
What makes nuclear power unique?
How are nuclear reactions different from ordinary chemical reactions?
Is nuclear chemistry a natural or human-made phenomenon?
What are the uses and dangers of nuclear chemistry?
Stability of Nuclei
Nuclei are composed of protons and neutrons (except hydrogen, which has only one proton).
Most nuclei are stable and fall within the "belt of stability."
Stability is determined by the neutron-to-proton ratio.
Nuclei with atomic numbers greater than 83 are unstable due to the neutron-to-proton ratio.
Unstable isotopes are called radioisotopes.
Radioisotopes decay in a series of steps to produce a stable nucleus.
During decay, radiation is emitted in the form of alpha particles, beta particles, positrons, and/or gamma radiation.
Alpha particle: Helium nucleus ( ^{4}_{2}He or \alpha), composed of two protons and two neutrons.
Beta particle: (\beta^{-}) An electron emitted from the nucleus
Positron: (\beta^{+}) Identical to an electron but with a positive charge.
Gamma rays: (\gamma) High-energy electromagnetic radiation similar to X-rays.
Types of Radioactive Emissions
Alpha particles are positively charged and attracted to a negative plate in an electric field.
Beta particles are negatively charged and attracted to a positive plate in an electric field.
Gamma rays have no charge and are undeflected in an electric field.
Alpha particles are deflected less than beta particles due to their larger mass.
Radiation can cause ionization of normal tissue, leading to cell death or mutations.
Mutations in sperm or egg cells can be transmitted to future generations.
Common Forms of Radiation:
Alpha:
Mass: 4 amu
Charge: +2
Symbol: ^{4}_{2}He, \alpha
Penetrating Power: Low
Beta:
Mass: 0 amu I'm
Charge: -1
Symbol: ^{0}_{-1}e, \beta^{-}
Penetrating Power: Moderate
Positron:
Mass: 0 amu
Charge: +1
Symbol: ^{0}_{+1}e, \beta^{+}
Penetrating Power: Moderate
Gamma:
Mass: 0 amu
Charge: None
Symbol: \gamma
Penetrating Power: High
Alpha Decay
Alpha emission is common in heavy nuclei, especially those with atomic numbers > 82.
During alpha decay:
Atomic number decreases by 2.
Number of protons decreases by 2.
Number of neutrons decreases by 2.
Mass number decreases by 4.
Example: Radium-226 decays into Radon-222:
^{226}{88}Ra \rightarrow ^{222}{86}Rn + ^{4}_{2}He
Alpha decay is a transmutation because the atomic number changes, creating a different element.
Beta Decay
Beta decay involves the emission of a beta particle (electron) during the conversion of a neutron to a proton.
Equation: ^{1}{0}n \rightarrow ^{1}{1}p + ^{0}_{-1}e
During beta decay:
Atomic number increases by 1.
Number of protons increases by 1.
Number of neutrons decreases by 1.
Mass number remains the same.
Example: Lead-214 decays into Bismuth-214:
^{214}{82}Pb \rightarrow ^{214}{83}Bi + ^{0}_{-1}e
Positron Emission
Positron emission involves the production of a positron during the conversion of a proton to a neutron.
Equation: ^{1}{1}p \rightarrow ^{1}{0}n + ^{0}_{+1}e
During positron emission:
Atomic number decreases by 1.
Number of protons decreases by 1.
Number of neutrons increases by 1.
Mass number remains the same.
Example: Potassium-37 decays into Argon-37:
^{37}{19}K \rightarrow ^{37}{18}Ar + ^{0}_{+1}e
K-Capture
K-capture is an alternative process that produces the same result as positron emission.
It involves the capture of a low-energy electron by a radioactive nucleus.
Example: ^{37}{19}K + ^{0}{-1}e \rightarrow ^{37}_{18}Ar
Nuclear Equations
Nuclear reactions can be represented by equations.
Mass and charge must be balanced on both sides of the equation.
Example: ^{14}{7}N + ^{4}{2}He \rightarrow ^{17}{8}O + ^{1}{1}H
Sum of charges on both sides: 7 + 2 = 8 + 1 = 9
Sum of mass numbers on both sides: 14 + 4 = 17 + 1 = 18
Missing particles in a nuclear equation can be identified by using the principle of conservation of charge and mass number.
Transmutations
Transmutation is the conversion of one element into another.
Natural transmutation occurs spontaneously (e.g., alpha, beta, positron decay).
Artificial transmutation is induced by bombarding a nucleus with high-energy particles.
Types of Artificial Transmutations:
Collision of a charged particle with a target nucleus.
Requires sufficient energy to overcome repulsive forces between positively charged objects.
Accelerators like cyclotrons and synchrotrons are used to increase the kinetic energy of charged particles.
Collision of a neutron with a target nucleus.
Neutrons are obtained from nuclear reactors.
Neutrons are not repelled by the nucleus and can be captured by the strong nuclear force.
Used to prepare radioactive nuclei from stable nuclei.
Examples:
^{238}{92}U + ^{1}{0}n \rightarrow ^{239}_{92}U
^{59}{27}Co + ^{1}{0}n \rightarrow ^{60}_{27}Co
^{32}{16}S + ^{1}{0}n \rightarrow ^{32}{15}P + ^{1}{1}H
Distinguishing Between Natural and Artificial Transmutation:
Natural transmutation: A single nucleus undergoes decay.
Artificial transmutation: Two reactants - a fast-moving particle and a target material.
Fission and Fusion
Fission: Splitting a heavy nucleus into lighter nuclei.
Fusion: Combining light nuclei to produce a heavier nucleus.
Conversion of Matter to Energy
In both fission and fusion, the total mass of the products is less than the total mass of the reactants.
This loss of mass is converted into energy, according to Einstein's equation:
E = mc^2
Where:
E = energy
m = mass
c = speed of light (3.00 \times 10^8 m/s)
A small amount of matter converted into energy produces an enormous amount of energy.
Nuclear reactions release far greater energy than ordinary chemical reactions.
Example: Conversion of 1.00 g of matter into energy yields 9.00 \times 10^{13} J.
Burning 1.00 g of methane yields 5.56 \times 10^4 J.
Mass defect: The difference between the mass of a nucleus and the sum of the masses of its individual protons and neutrons. This "missing" mass has been converted into energy.
Fission Reactions
Fission begins with the capture of a neutron by a heavy nucleus (e.g., Uranium-235 or Plutonium-239).
The resulting nucleus is unstable and splits, releasing two middle-weight nuclei, one or more neutrons, and a large amount of energy.
Example:
^{1}{0}n + ^{235}{92}U \rightarrow ^{142}{56}Ba + ^{91}{36}Kr + 3 ^{1}_{0}n + energy
Fusion Reactions
Fusion involves combining light nuclei to form heavier ones.
The most common example is in the sun, where hydrogen nuclei fuse to produce helium.
Requires extremely high temperatures and pressures.
Example sequence:
^{1}{1}H + ^{1}{1}H \rightarrow ^{2}{1}H + ^{0}{+1}e
^{1}{1}H + ^{2}{1}H \rightarrow ^{3}_{2}He
^{3}{2}He + ^{3}{2}He \rightarrow ^{4}{2}He + 2 ^{1}{1}H
^{3}{2}He + ^{1}{1}H \rightarrow ^{4}{2}He + ^{0}{+1}e
Fusion on Earth is difficult to achieve due to the extreme conditions required.
Advantage: Fusion products are not highly radioactive (unlike fission products).
Half-Life
Half-life: The time it takes for half of the atoms in a given sample of an element to decay.
Radioactive substances decay at a constant rate, independent of temperature, pressure, or concentration.
Decay is a random event; impossible to predict when a specific nucleus will decay.
Each isotope has its own half-life.
Shorter half-life indicates a less stable isotope.
The fraction remaining after a given number of half-lives is calculated as:
Fraction Remaining = (\frac{1}{2})^n
where n = number of half-lives.
The number of half-lives is calculated by dividing the total time by the half-life of the isotope.
Number of Half-lives = \frac{time elapsed (t)}{half-life (T)}
Uses and Dangers of Radioisotopes
Radioisotopes have applications in industry, medicine, and research.
Potential dangers due to the harm from radiation.
Dating
Carbon-14 is used for dating previously living materials.
Living organisms incorporate C-14 from the atmosphere.
After death, C-14 decays, and its concentration decreases over time.
Half-life of C-14 is 5730 years.
A reading of 7 dpm/g carbon indicates the remains are about 5700 years old.
After about four half-lives, C-14 dating becomes ineffective.
Uranium-238 decays through a series of steps to form stable Lead-206.
The ratio of U-238/Pb-206 is used to date rocks and geological formations.
Chemical Tracers
Radioisotopes can be used to follow the path of a material in a system.
Radioactive P-31 is used to trace phosphorus uptake in plants.
C-14 is used to map the path of carbon in metabolic processes.
Industrial Applications
Radiation is used to measure the thickness of materials (plastic wrap, aluminum foil).
Used to test the strength of welds.
Medical Applications
I-131 is used to detect and treat thyroid conditions.
Cobalt-60 emits gamma radiation to kill cancerous tumors.
Gamma radiation is used to irradiate foods to kill bacteria.
Technetium-99 is absorbed by cancerous cells for tumor detection.
Radioisotopes used for diagnostics should have short half-lives and be quickly eliminated from the body.
Radiation Risks
Radioisotopes can damage normal tissue and cause mutations.
High doses of radiation can cause illness and death.
Nuclear power plants produce radioactive waste products with long half-lives.
There is a risk of nuclear accidents that release radioactivity into the environment.