Nuclear Chemistry and Radioactivity

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The spontaneous emission of radiation from an unstable atomic nucleus as it decays toward a more stable configuration. The rate of decay is not affected by temperature, pressure, or chemical state.

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Emission of an alpha particle = 2 protons + 2 neutrons (identical to a helium-4 nucleus, ⁴₂He). Nucleus loses 2 protons and 2 neutrons. Low penetration — stopped by paper or skin. Highly ionizing. Most dangerous if inhaled/ingested.

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Emission of a beta particle = a fast-moving electron (β⁻) from the nucleus when a neutron converts to a proton: n → p + e⁻ + antineutrino. Nucleus gains 1 proton. Penetrates skin; stopped by aluminium foil.

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High-energy electromagnetic radiation emitted from the nucleus, often following alpha or beta decay. No change in atomic number or mass number. Highly penetrating — requires thick lead or concrete shielding. Least ionizing.

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The sum of mass numbers (A, top) and the sum of atomic numbers (Z, bottom) must be equal on both sides of the equation. Example: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. (238=234+4 ✓ and 92=90+2 ✓)

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The time taken for half the radioactive atoms in a sample to decay. It is constant for a given isotope. After n half-lives: N = N₀ × (½)ⁿ or m = m₀ × (½)^(t/t½).

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N(t) = N₀ e^(−λt) where N₀ = initial number of atoms, λ = decay constant (s⁻¹), t = time. The decay constant λ = ln2 / t½ = 0.693 / t½. First-order kinetics — same mathematics as first-order chemical reactions.

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Uses the known half-life of ¹⁴C (5,730 years) to date organic materials up to ~50,000 years old. Living organisms maintain a constant ¹⁴C/¹²C ratio. After death, ¹⁴C decays without replacement. Measuring the ratio determines the age.

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The splitting of a heavy nucleus (e.g., ²³⁵U or ²³⁹Pu) into two smaller nuclei when struck by a neutron, releasing a large amount of energy and 2–3 more neutrons. These neutrons can trigger further fissions — a chain reaction.

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A chain reaction occurs when neutrons from one fission event cause additional fissions. Critical mass: The minimum mass of fissile material needed to sustain a self-sustaining chain reaction. Above critical mass → uncontrolled reaction (nuclear explosion).

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The combination of two light nuclei (e.g., deuterium and tritium, isotopes of hydrogen) to form a heavier nucleus, releasing enormous energy. Powers the Sun and hydrogen bombs. Requires extremely high temperatures (~10⁷–10⁸ K) to overcome electrostatic repulsion.

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Mass defect: The difference between the mass of a nucleus and the sum of masses of its constituent protons and neutrons. Binding energy: Energy equivalent of mass defect (E=mc²). Higher binding energy per nucleon = more stable nucleus. Iron-56 has the highest binding energy per nucleon.

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Albert Einstein's mass-energy equivalence equation: energy (E) equals mass (m) times the speed of light squared (c² = 9×10¹⁶ m²/s²). In nuclear reactions, small mass changes release enormous energy. Used to calculate binding energy from mass defect.

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Atoms of the same element with the same number of protons but different numbers of neutrons — and therefore different mass numbers. Example: ¹²C (6p, 6n), ¹³C (6p, 7n), ¹⁴C (6p, 8n). Some isotopes are stable; others are radioactive.

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ᴬzX where A = mass number (protons + neutrons), Z = atomic number (protons), X = element symbol. Example: ²³⁵₉₂U = Uranium with 92 protons, 235 total nucleons, and 143 neutrons.

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Alpha (α): Stopped by paper, skin, or a few cm of air. Beta (β): Stopped by a few mm of aluminium. Gamma (γ): Requires several cm of lead or meters of concrete. Ionizing power is the reverse order: α > β > γ.

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A proton converts to a neutron: p → n + e⁺ + neutrino. A positron (β⁺) — the antimatter equivalent of an electron — is emitted. Atomic number decreases by 1. Used in PET (Positron Emission Tomography) scans.

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A nucleus captures an inner-shell electron, converting a proton to a neutron: p + e⁻ → n + neutrino. Atomic number decreases by 1. Competes with positron emission for proton-rich nuclei.

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A controlled nuclear fission device. Fuel: Enriched uranium (²³⁵U). Moderator (e.g., water, graphite): Slows neutrons to sustain chain reaction. Control rods (e.g., boron, cadmium): Absorb neutrons to control reaction rate. Coolant: Transfers heat to generate steam and electricity.

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Byproducts of nuclear reactions that remain radioactive for varying periods. Low-level waste: Clothing, tools — short-lived. Intermediate-level waste: Reactor components. High-level waste: Spent fuel rods — remain dangerous for thousands of years; stored in deep geological repositories.

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PET scan: ¹⁸F-fluorodeoxyglucose detects cancer. MRI contrast: Gadolinium. Thyroid treatment: ¹³¹I (iodine-131). Bone scans: ⁹⁹ᵐTc (technetium-99m — most used medical isotope). Cancer radiotherapy: High-energy γ radiation from cobalt-60.

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A graph of neutron number (N) vs proton number (Z) showing which nuclei are stable. Stable nuclei lie in the "valley." Above the valley (too many neutrons): β⁻ decay. Below (too few neutrons): β⁺ decay or electron capture. Very heavy nuclei: α decay.

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Natural uranium is ~99.3% ²³⁸U and only ~0.7% fissile ²³⁵U. Enrichment increases the ²³⁵U proportion. Reactor-grade: 3–5% ²³⁵U. Weapons-grade: >90% ²³⁵U. Done by gaseous diffusion or centrifuge separation.

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An instrument used to detect and measure ionizing radiation (α, β, γ). Radiation ionizes gas inside the tube, creating a current pulse that is counted. Measures activity in counts per second (cps) or Becquerels (Bq).

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Becquerel (Bq): 1 decay per second (activity). Gray (Gy): Energy absorbed per kg of tissue (absorbed dose). Sievert (Sv): Dose equivalent — accounts for biological damage potential of different radiation types. 1 Sv = 1 Gy × radiation weighting factor.