Nuclear Chemistry: Comprehensive Study Notes

Radioisotopes

• A radioisotope has an unstable nucleus and emits radiation.
• An unstable nucleus is radioactive, meaning it spontaneously emits small particles of energy called radiation to become more stable.
• A radioisotope can be one or more isotopes of an element.
• The mass number is included in its name.

Types of Radiation Emitted

• Radioisotopes emit:
  • Alpha particles, identical to a helium nucleus: $^4_2\mathrm{He}$
  • Beta particles, high-energy electrons: $^0_{-1}e$
  • Positrons: $^0_{+1}e$
  • Gamma rays: $^0_0\gamma$ (pure energy)

Some Forms of Radiation (Table 5.2)

• Alpha particle: symbol $^4_2\mathrm{He}$, mass number 4, charge +2.
• Beta particle: symbol $^0_{-1}e$, mass number 0, charge −1.
• Positron: symbol $^0_{+1}e$, mass number 0, charge +1.
• Gamma ray: symbol $^0_0\gamma$, mass number 0, charge 0.
• Proton: symbol $^1_1\mathrm{H}$, mass number 1, charge +1.
• Neutron: symbol $^1_0n$, mass number 1, charge 0.

Learning Check 1 (mass number and charge)

• A. Alpha particle: mass number 4, charge +2.
• B. Positron: mass number 0, charge +1.
• C. Beta particle: mass number 0, charge −1.
• D. Neutron: mass number 1, charge 0.
• E. Gamma ray: mass number 0, charge 0.

Biological Effects of Radiation

• Ionizing radiation strikes molecules in its path.
• Most sensitive cells: rapidly dividing cells in bone marrow, skin, reproductive organs, and cancer cells.
• Cancer cells are highly sensitive; large doses destroy them.
• Normal tissue around cancer cells divides more slowly and suffers less damage.
• Radiation may cause malignant tumors, leukemia, anemia, and genetic mutations.

Radiation Protection (1 of 2)

• Protection requires:
  • Paper and clothing for alpha particles.
  • Lab coat or gloves for beta particles.
  • Lead shield or thick concrete wall for gamma rays.
  • Limiting time spent near a radioactive source.
  • Increasing distance from the source.

Radiation Protection (2 of 2) — Table 5.3 Properties of Radiation and Shielding

• Alpha particle: Travel distance in air 2–4 cm; Tissue depth 0.05 mm; Shielding: Paper, clothing; Typical source: Radium-226.
• Beta particle: Travel distance in air 200–300 cm; Tissue depth 4–5 mm; Shielding: Heavy clothing, lab coats, gloves; Typical source: Carbon-14.
• Gamma ray: Travel distance in air ~500 m; Tissue depth 50 cm or more; Shielding: Lead, thick concrete; Typical source: Technetium-99m.

Learning Check 2 (shielding by type)

• A. Heavy clothing protects against: Beta radiation.
• B. Paper protects against: Alpha radiation.
• C. Lead protects against: Gamma radiation.
• D. Thick concrete protects against: Gamma radiation.

Radioactive Decay

• Radioactive decay is a process where an unstable nucleus spontaneously breaks down by emitting radiation.
• Described by writing a nuclear equation: radioactive nucleus → new nucleus + radiation (α, β, β, γ).
• Example form: $^{A}<em>{Z}X \rightarrow ^{A-\Delta A}</em>{Z-\Delta Z}Y + \text{radiation}$

Nuclear Equations

• In a balanced nuclear equation, the sum of the mass numbers on both sides must be the same.
• The sum of the atomic numbers on both sides must be the same.
• Example (illustrative): Mass number sum: $^{251}<em>{98}\mathrm{Cf} + ^{4}</em>{2}\mathrm{He} \rightarrow ^{247}<em>{96}\mathrm{Cm} + ^{4}</em>{2}\mathrm{He}$ (conceptual)
• Atomic number sum: $98 = 98$

Alpha Decay

• When a radioactive nucleus emits an alpha particle, the resulting nucleus has a mass number decreased by 4 and atomic number decreased by 2.
• Equation form: $^{A}<em>{Z}X \rightarrow ^{A-4}</em>{Z-2}Y + ^{4}_{2}\mathrm{He}$

Writing an Equation for Alpha Decay

• Example: Americium-241 (used in smoke detectors):
  $^{241}<em>{95}\mathrm{Am} \rightarrow ^{237}</em>{93}\mathrm{Np} + ^{4}_{2}\mathrm{He}$

Beta Decay

• Superscript $^0_{-1}e$ (beta particle) is emitted when a neutron in the nucleus breaks down to form a proton and a beta particle.
• Atomic number increases by 1; mass number unchanged.
• Equation form: $^{A}<em>{Z}X \rightarrow ^{A}</em>{Z+1}Y + ^0_{-1}e$

Writing an Equation for Beta Decay

• Example: Yttrium-90 decay:
  $^{90}<em>{39}\mathrm{Y} \rightarrow ^{90}</em>{40}\mathrm{Zr} + ^0_{-1}e$

Positron Emission

• In positron emission, a proton is converted to a neutron and a positron is emitted.
• Mass number unchanged; atomic number decreases by 1.
• Equation form: $^{A}<em>{Z}X \rightarrow ^{A}</em>{Z-1}Y + ^0_{+1}e$

Gamma Emission

• Gamma emission involves energy being emitted from an unstable nucleus; the mass number and atomic number of the new nucleus are the same.
• Equation form: $^{A}<em>{Z}X \rightarrow ^{A}</em>{Z}X + ^0_{0}\gamma$
• Example: $^{99m}<em>{43}\mathrm{Tc} \rightarrow ^{99}</em>{43}\mathrm{Tc} + ^0_{0}\gamma$

Learning Check 2 (nuclear equation) — Nickel-58 bombardment

• Write the balanced nuclear equation for the bombardment of nickel-58 by a proton that produces a radioactive isotope and an alpha particle:
  $^{58}<em>{28}\mathrm{Ni} + ^{1}</em>{1}\mathrm{p} \rightarrow ^{55}<em>{27}\mathrm{Co} + ^{4}</em>{2}\mathrm{He}$

Measuring Radiation Damage

• The rem (radiation equivalent in humans) measures:
  • Alpha particles: do not penetrate skin; if ingested, cause extensive tissue damage.
  • High-energy radiation (beta particles, high-energy protons, neutrons): travel into tissue and cause damage.
  • Gamma rays: travel long distances through body tissue and are damaging.

Dosimeters

• People who work in radiation laboratories wear dosimeters attached to clothing.
• Dosimeters detect exposure from X-rays, gamma rays, and beta particles.

Half-Life of a Radioisotope

• The half-life is the time required for the radiation level (activity) to decrease to one-half of its original value.

Decay Curve

• A decay curve shows how a radioactive isotope decays over time.
• Example: Iodine-131 (half-life = 8 days): after one half-life, 1/2 remains; after two half-lives, 1/4 remains, etc.

Using Half-Lives of a Radioisotope

• Example: Strontium-90 has a half-life of $38.1\ \text{y}$.
• If a sample contains $36\ \text{mg}$ of Sr-90, remaining after $114.3\ \text{y}$ is:
  • Number of half-lives, $n = \dfrac{114.3}{38.1} = 3$
  • Remaining mass: $36\ \text{mg} \times (\tfrac{1}{2})^{3} = 4.5\ \text{mg}$

Learning Check I

• Iodine-123, used in treatment of thyroid, brain, and prostate cancer, has a half-life of $13.2\ \text{h}$. If a sample of $64\ \text{mg}$ is left after $26.4\ \text{h}$, how much remains?
  • After 1 half-life: 32 mg; after 2 half-lives: 16 mg.
  • Answer: $16\ \text{mg}$

Medical Applications of Radioisotopes

• Radioisotopes with short half-lives are used in nuclear medicine because:
  • The body's cells do not distinguish between nonradioactive and radioactive atoms.
  • Once incorporated, radioactive atoms are detected because they emit radiation, giving an image of an organ.

Radioisotopes in Medicine (1 of 2) — Table 5.8 (selected entries)

• Au-198: half-life 2.7 days; radiation Beta; liver imaging; treatment of abdominal carcinoma.
• Ce-141: half-life 32.5 days; Beta; GI tract diagnosis; measuring blood flow to the heart.
• Cs-131: half-life 9.7 days; Gamma; Prostate brachytherapy.
• F-18: half-life 110 minutes; Positron; PET.
• Ga-67: half-life 78 hours; Gamma; abdominal imaging; tumor detection.
• Ga-68: half-life 68 minutes; Gamma; detection of pancreatic cancer.
• I-123: half-life 13.2 hours; Gamma; treatment of thyroid, brain, and prostate cancer.
• I-131: half-life 8.0 days; Beta; treatment of Graves’ disease, goiter, hyperthyroidism, thyroid and prostate cancer.

Radioisotopes in Medicine (2 of 2) — Table 5.8 (continued)

• Ir-192: half-life 74 days; Gamma; treatment of breast and prostate cancer.
• P-32: half-life 14.3 days; Beta; treatment of leukemia, excess red blood cells, and pancreatic cancer.
• Pd-103: half-life 17 days; Gamma; prostate brachytherapy.
• Sm-153: half-life 46 hours; Beta; treatment of bone cancer.
• Sr-85: half-life 65 days; Gamma; detection of bone lesions; brain scans.
• Tc-99m: half-life 6.0 hours; Gamma; imaging of skeleton and heart muscle, brain, liver, heart, lungs, bone, spleen, kidney, thyroid; most widely used in nuclear medicine.
• Xe-133: half-life 5.2 days; Beta; pulmonary function diagnosis.
• Y-90: half-life 2.7 days; Beta; treatment of liver cancer.

Learning Check 1 (nuclear medicine usage)

• Which of the following radioisotopes are most likely to be used in nuclear medicine?
  • A. K-40
  • B. K-42 (half-life 12 h)
  • C. I-131 (half-life 8 days)
• Correct: C. I-131 (and many others with short half-lives, like K-42, are not as typical as well-known diagnostically or therapeutically used isotopes like I-131).

Scans with Radioisotopes (1 of 2)

• After administration, the scanner moves slowly across the body area where the organ containing the radioisotope is located.
• The technologist determines the level and location of radioactivity emitted by the radioisotope.
• The gamma rays emitted are used to expose a photographic plate, producing a scan of the organ.

Scans with Radioisotopes (2 of 2)

• A thyroid scan shows accumulation of radioactive iodine-131 in the thyroid.

Positron Emission Tomography (PET) (1 of 2)

• In PET, positron emitters with short half-lives are combined with body substances such as glucose.
• Positron emission is used to study brain function, metabolism, and blood flow.
• These PET scans can show a normal brain on one side and a brain affected by diseases (e.g., Alzheimer’s) on the other side.

Positron Emission Tomography (PET) (2 of 2)

• Positrons are emitted from positron emitters such as carbon-11, oxygen-15, nitrogen-13, and fluorine-18.
• Positrons combine with electrons to produce gamma rays, which are detected by computerized equipment to create a three-dimensional image of an organ.
• Example representation: $^{18}<em>{9}\mathrm{F} \rightarrow ^{18}</em>{8}\mathrm{O} + e^{+}$ (illustrative of positron emitter behavior; actual PET tracers use F-18 among others)

Computed Tomography (CT)

• CT uses computer-monitored absorption of 30,000 X-ray beams directed at successive layers of the target organ.
• Differences in absorption, based on tissue densities, provide a series of images of the organ.
• Example: ACT scan shows a tumor in the brain (yellow).

Magnetic Resonance Imaging (MRI)

• MRI does not involve X-ray radiation.
• It is the least invasive imaging method available.
• MRI is based on the absorption of energy when protons in hydrogen atoms are excited by a strong magnetic field.
• The absorbed energy is converted to color images of the body.
• An MRI scan provides images of the heart and lungs.

Recommended Problems

• 5.57, 5.61, 5.65, 5.67, 5.57, 5.83, 5.89