Chapter 5 Nuclear Chemistry - Key Concepts and Study Notes

Radioisotopes and Nuclear Radiation

• A radioisotope has an unstable nucleus and emits radiation to become more stable.
• An unstable nucleus is radioactive.
• Radioisotopes can be one or more isotopes of an element and include the mass number in their name.
• Radiation is emitted as small energetic particles or photons as the nucleus seeks stability.

Types of Radiation

• Alpha particles: identical to a helium nucleus; symbol: ^{4}_{2}{
  m He}; mass number 4; charge +2.
• Beta particles: high-energy electrons; symbol: $^{0}_{-1}e$ (beta minus); mass number 0; charge −1.
• Positrons: positive electrons; symbol: $^{0}_{+1}e^{+}$; mass number 0; charge +1.
• Gamma rays: pure energy; symbol: ^{0}_{0}{
  m gamma}; mass number 0; charge 0.

Some Forms of Radiation (Table 5.2)

• Alpha particle: A = 4; Z = 2; radiation type α; symbol of particle: ^{4}_{2}{
  m He}.
• Beta particle: A = 0; Z = −1; radiation type β−; symbol: $^{0}_{-1}e$.
• Positron: A = 0; Z = +1; radiation type β+; symbol: $^{0}_{+1}e^{+}$.
• Gamma ray: A = 0; Z = 0; radiation type γ; symbol: ^{0}_{0}{
  m Gamma}.
• Proton: A = 1; Z = +1; (nuclear particle with A = 1, Z = 1).
• Neutron: A = 1; Z = 0; (nuclear particle).

Learning Check 1 (Mass number and charge)

• A. Alpha particle: A = $4$, Z = $2$.
• B. Positron: A = $0$, Z = $+1$.
• C. Beta particle: A = $0$, Z = $-1$.
• D. Neutron: A = $1$, Z = $0$.
• E. Gamma ray: A = $0$, Z = $0$.

Biological Effects of Radiation

• Ionizing radiation strikes molecules and damages cells, especially:
  • rapidly dividing cells in bone marrow, skin, and reproductive organs,
  • cancer cells (cancer cells are highly sensitive to radiation),
  • surrounding normal tissue can be damaged if exposed to high doses.
• Radiation can contribute to malignant tumors, leukemia, anemia, and genetic mutations.
• High doses are used to destroy cancer cells, but surrounding tissue is a consideration.

Radiation Protection (1 of 2)

• Protection strategies include:
  • Alpha: paper and clothing as shielding.
  • Beta: lab coat or gloves.
  • Gamma: lead shield or thick concrete wall.
  • Limit time spent near a radioactive source.
  • Increase the distance from the source.

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

• Alpha particle: travels 2–4 cm in air; tissue depth ~0.05 mm; shielding: paper, clothing; typical source: Radium-226.
• Beta particle: travels 200–300 cm in air; tissue depth ~4–5 mm; shielding: heavy clothing, lab coats, gloves; source: Carbon-14.
• Gamma ray: travels ~500 m in air; tissue depth ≥50 cm; shielding: lead, thick concrete; typical source: Technetium-99m.

Learning Check 2 (Radiation shielding)

• Identify shielding for each type: A. heavy clothing (beta or gamma), B. paper (alpha), C. lead (gamma), D. thick concrete (gamma).

Radioactive Decay and Nuclear Equations

• Radioactive decay is the process by which an unstable nucleus spontaneously breaks down by emitting radiation.
• Nuclear equations are written to balance both mass numbers and atomic numbers.
• In a balanced nuclear equation:
  • Mass number sum on both sides is equal: $ext{Mass on left} = ext{Mass on right},$
  • Atomic number sum on both sides is equal: $ext{Z sum on left} = ext{Z sum on right}.$
• Example concept: Alpha decay reduces A by 4 and Z by 2: ^{A}{Z}X ightarrow ^{A-4}{Z-2}Y + ^{4}_{2}{
  m He}.
• Example concept: Beta decay increases Z by 1 (neutron → proton + beta particle): ^{A}{Z}X ightarrow ^{A}{Z+1}Y + ^{0}_{-1}e.
• Example concept: Positron emission decreases Z by 1 (proton → neutron + positron): ^{A}{Z}X ightarrow ^{A}{Z-1}Y + ^{0}_{+1}e^{+}.
• Gamma emission occurs with no change in A or Z; energy is released as a gamma photon: ^{A}{Z}X ightarrow ^{A}{Z}X' + ^{0}_{0}{
  m Gamma}.

Alpha Decay (Example)

• Americium-241 alpha decay:
  • Reactant: ^{241}_{95}{
    m Am}
  • Product: ^{237}_{93}{
    m Np}
  • Emitted particle: ^{4}_{2}{
    m He}
  • Equation: ^{241}{95}{ m Am} ightarrow ^{237}{93}{
    m Np} + ^{4}_{2}{
    m He}.

Beta Decay (Example)

• Yttrium-90 beta decay:
  • Reactant: ^{90}_{39}{
    m Y}
  • Product: ^{90}_{40}{
    m Zr}
  • Emitted particle: $^{0}_{-1}e$
  • Equation: ^{90}{39}{ m Y} ightarrow ^{90}{40}{
    m Zr} + ^{0}_{-1}e.

Positron Emission (Example)

• Example general form:
  • Reactant: $^{A}_{Z}X$
  • Product: $^{A}_{Z-1}Y$
  • Emitted particle: $^{0}_{+1}e^{+}$
  • Equation: ^{A}{Z}X ightarrow ^{A}{Z-1}Y + ^{0}_{+1}e^{+}.

Gamma Emission (Example)

• Isomeric gamma emission (no change in A or Z):
  • Reactant: ^{A}_{Z}X^{}(m) may revert to ground state:
  • Equation: ^{A}{Z}X^{m} ightarrow ^{A}{Z}X + ^{0}_{0}{
    m Gamma}.
• Example using technetium-99m:
  • ^{99m}{43}{ m Tc} ightarrow ^{99}{43}{
    m Tc} + ^{0}_{0}{
    m Gamma}.

Bombardment Reaction (Learning Check 2 Answer)

• Bombardment: nickel-58 by a proton producing a radioactive isotope and an alpha particle:
• Reaction: ^{58}{28}{ m Ni} + ^{1}{1}{
  m p}
  ightarrow ^{55}{27}{ m Co} + ^{4}{2}{
  m He}.
• Check: A conserved total mass: 58+1 = 59; Right: 55+4 = 59. Charge: 28+1 = 29; Right: 27+2 = 29.

Measuring Radiation Damage and Dosimetry

• The rem (radiation equivalent in humans) concept: measures damage by different radiation types (alpha, beta, gamma).
• Alpha particles: do not penetrate skin but can cause damage if ingested or inhaled.
• High-energy radiation (beta, protons, neutrons): travel into tissue and cause damage.
• Gamma rays: penetrate long distances in tissue; significant external exposure risk.

Dosimeters

• Worn by personnel working with radiation.
• Dosimeters detect exposure due to X-rays, gamma rays, and beta particles.

Half-Life and Decay Curves

• Half-life definition: the time required for the activity to decrease to half its original value.
• A decay curve illustrates how a radioactive sample decays over time (half-life intervals).
• Example: Iodine-131 has a half-life of $t_{1/2} = 8.0 ext{ days}.$ The decay curve would show half of the sample remaining after each 8-day interval.
• Example: Strontium-90 has a half-life of $38.1 ext{ years}.$ If a 36 mg sample is held for 114.3 years, after 3 half-lives (114.3 ÷ 38.1 = 3) the remaining mass is $36 imes \bigl( frac{1}{2}\bigr)^{3} = 4.5 ext{ mg}.$
• Common decay relations include:
  • $N(t) = N<em>0 \biggl(\frac{1}{2}\biggr)^{\frac{t}{t</em>{1/2}}},$
  • N(t) = N0 e^{-bb t}, where t{1/2} = rac{ 2}{ bb} (decay constant) is often used to relate to the half-life via t_{1/2} = rac{ 2}{
    }.

Medical Applications of Radioisotopes (Table 5.8)

• Au-198: $t_{1/2} = 2.7 ext{ days}$; Radiation: Beta; Applications: liver imaging; treatment of abdominal carcinoma.
• Ce-141: $t_{1/2} = 32.5 ext{ days}$; Radiation: Beta; Applications: GI tract diagnosis; measuring blood flow to the heart.
• Cs-131: $t_{1/2} = 9.7 ext{ days}$; Radiation: Gamma; Application: prostate brachytherapy.
• F-18: $t_{1/2} = 110 ext{ minutes}$; Radiation: Positron; Application: Positron emission tomography (PET).
• Ga-67: $t_{1/2} = 78 ext{ hours}$; Radiation: Gamma; Applications: abdominal imaging; tumor detection.
• Ga-68: $t_{1/2} = 68 ext{ minutes}$; Radiation: Gamma; Applications: detection of pancreatic cancer.
• I-123: $t_{1/2} = 13.2 ext{ hours}$; Radiation: Gamma; Applications: treatment of thyroid, brain, and prostate cancer.
• I-131: $t_{1/2} = 8.0 ext{ days}$; Radiation: Beta; Applications: treatment of Graves’ disease, goiter, hyperthyroidism, thyroid and prostate cancer.
• Ir-192: $t_{1/2} = 74 ext{ days}$; Radiation: Gamma; Applications: treatment of breast and prostate cancer.
• P-32: $t_{1/2} = 14.3 ext{ days}$; Radiation: Beta; Applications: treatment of leukemia, excess red blood cells, and pancreatic cancer.
• Pd-103: $t_{1/2} = 17 ext{ days}$; Radiation: Gamma; Applications: prostate brachytherapy.
• Sm-153: $t_{1/2} = 46 ext{ hours}$; Radiation: Beta; Applications: treatment of bone cancer.
• Sr-85: $t_{1/2} = 65 ext{ days}$; Radiation: Gamma; Applications: detection of bone lesions; brain scans.
• Tc-99m: $t_{1/2} = 6.0 ext{ hours}$; Radiation: Gamma; Applications: imaging of skeleton, heart muscle, brain, liver, heart, lungs, bone, spleen, kidney, thyroid; most widely used in nuclear medicine.
• Xe-133: $t_{1/2} = 5.2 ext{ days}$; Radiation: Beta; Applications: pulmonary function diagnosis.
• Y-90: $t_{1/2} = 2.7 ext{ days}$; Radiation: Beta; Applications: treatment of liver cancer.

Imaging Techniques in Nuclear Medicine

• Scans with Radioisotopes (1 of 2):
  • After administration, a gamma camera scans across the body to detect radioactivity distribution.
  • The gamma rays emitted expose a photographic plate to produce an image of an organ.
• Scans with Radioisotopes (2 of 2):
  • Thyroid scan shows accumulation of radioactive iodine-131 in the thyroid.
• Positron Emission Tomography (PET) (1 of 2):
  • PET uses positron emitters with short half-lives combined with body substances such as glucose.
  • Used to study brain function, metabolism, and blood flow; produces 3D images.
• Positron Emission Tomography (PET) (2 of 2):
  • Positrons are emitted from isotopes like carbon-11, oxygen-15, nitrogen-13, and fluorine-18.
  • Positrons annihilate with electrons to produce gamma rays detected by equipment to form images.
• Computed Tomography (CT):
  • Uses thousands of X-ray beams; computer reconstructs images based on tissue densities.
  • Produces cross-sectional images of organs.
• Magnetic Resonance Imaging (MRI):
  • Does not use X-rays; based on energy absorption when protons in hydrogen are excited by a strong magnetic field.
  • Converts absorbed energy into color images; noninvasive and provides soft-tissue contrast.

Practice Problems and Practice Content

• Recommended problems for study: 5.57, 5.61, 5.65, 5.67, 5.83, 5.89.
• Review the learning-check concepts and be able to balance simple nuclear equations, identify shielding requirements, and interpret decay curves.