Types of Radioactive Decay

Radioactive decay is the spontaneous process by which an unstable atomic nucleus loses energy by emitting radiation. Nuclei become unstable when they have too many protons, too many neutrons, or simply too much internal energy. Nature drives these nuclei toward stability, and the three classical decay modes — alpha (α), beta (β), and gamma (γ) — are the primary ways this happens. Each mode emits a different type of particle or electromagnetic radiation, carries different energies, and has different penetrating power through matter. Think of an unstable nucleus like an overfilled backpack: it needs to shed weight to reach a comfortable, stable configuration. Alpha decay throws out a heavy chunk (a helium nucleus). Beta decay swaps a neutron for a proton by ejecting a fast electron (or positron). Gamma decay releases pure energy as a high-frequency photon when the nucleus drops from an excited state to a lower one — like a spring releasing tension. All three processes conserve both total charge (atomic number Z) and mass number A, which lets us write balanced nuclear equations. Understanding decay types matters far beyond the classroom. Medical imaging (PET scans), cancer radiotherapy, nuclear power, carbon-14 dating, and radiation safety all depend on knowing exactly which particles are emitted, how energetic they are, and how far they can travel. A sheet of paper stops alpha particles; beta particles need a few millimetres of aluminium; gamma rays require centimetres of lead or metres of concrete. These properties come directly from the physics of each decay process.

Key takeaways
  • Alpha (α) Decay
  • Beta-Minus (β⁻) Decay
  • Gamma (γ) Decay
  • Conservation Laws in Nuclear Equations

Alpha (α) Decay

An alpha particle is identical to a helium-4 nucleus: 2 protons and 2 neutrons, written ⁴₂He. When a heavy nucleus (typically Z > 83) undergoes alpha decay, it ejects this tightly bound cluster. The parent nucleus loses 4 in mass number and 2 in atomic number. General form: ᴬ_Z X → ᴬ⁻⁴_(Z-2) Y + ⁴₂He. Example: uranium-238 decays to thorium-234. Alpha particles carry ~4–9 MeV of kinetic energy but are stopped by a sheet of paper or a few centimetres of air because their large mass and double positive charge cause many collisions per unit length (high ionising power, low penetrating power). Alpha decay is explained quantum mechanically by tunnelling: classically the alpha particle cannot escape the nuclear potential well, but quantum mechanics gives it a small probability of tunnelling through the Coulomb barrier.

Beta-Minus (β⁻) Decay

Beta-minus decay occurs when a nucleus has too many neutrons relative to protons. A neutron converts into a proton inside the nucleus, emitting a fast electron (the beta particle, β⁻) and an electron antineutrino (ν̄_e). The mass number A stays the same; atomic number Z increases by 1. General form: ᴬ_Z X → ᴬ_(Z+1) Y + e⁻ + ν̄_e. Example: carbon-14 decays to nitrogen-14. Beta particles have a range of energies (up to several MeV) because the energy is shared randomly between the electron and antineutrino — this continuous spectrum was the key clue that led Pauli to predict the neutrino in 1930. Beta particles are stopped by a few millimetres of aluminium or about 1 cm of body tissue. Beta-plus (β⁺) decay is the mirror process: too many protons, a proton becomes a neutron, emitting a positron and a neutrino. PET scanners detect the two gamma photons produced when the positron annihilates with an electron.

Gamma (γ) Decay

Gamma decay does not change the nucleus's composition (Z and A are unchanged). Instead, a nucleus in an excited energy state (often produced right after alpha or beta decay) drops to a lower energy state by emitting a high-energy photon called a gamma ray, with energies typically 0.1–10 MeV. General form: ᴬ_Z X → ᴬ_Z X + γ. The star () denotes an excited nuclear state. Because gamma rays are pure electromagnetic radiation with no charge and no mass, they interact weakly with matter and are the most penetrating — requiring several centimetres of lead or ~1 metre of concrete for significant attenuation. Gamma rays are used in cancer treatment (gamma knife surgery) and in sterilising medical equipment. Electron capture is a related process: the nucleus absorbs an inner-shell electron, converting a proton to a neutron, and emits an X-ray rather than a gamma ray.

Conservation Laws in Nuclear Equations

Every nuclear decay equation must satisfy two bookkeeping rules: (1) Conservation of mass number A — the sum of nucleon numbers on both sides must be equal. (2) Conservation of atomic number Z — the sum of proton numbers (charges) on both sides must be equal. Lepton number is also conserved, which is why beta decay always produces a neutrino or antineutrino alongside the electron or positron. These laws allow you to identify unknown daughter nuclei: if you know the parent and the decay type, you can deduce the product. For example, if polonium-210 (Z=84) undergoes alpha decay: 210 − 4 = 206 (new A), 84 − 2 = 82 (new Z), so the daughter is lead-206.

The Key Equation

ZAX→Z−2A−4Y+24He{}^{A}_{Z}X \rightarrow {}^{A-4}_{Z-2}Y + {}^{4}_{2}He
  • A — Mass number (dimensionless): Total number of protons plus neutrons in the nucleus
  • Z — Atomic number (dimensionless): Number of protons, which determines the chemical element
  • X — Parent nucleus (—): The original unstable nucleus before decay
  • Y — Daughter nucleus (—): The resulting nucleus after alpha emission
  • {}^{4}_{2}He — Alpha particle (—): Helium-4 nucleus: 2 protons and 2 neutrons, ejected at high speed

Worked Example

Problem. Radium-226 (Ra, Z = 88) undergoes alpha decay. (a) Write the balanced nuclear equation and identify the daughter nucleus. (b) Then the daughter nucleus is in an excited state and undergoes gamma decay — write that equation too.

88226Ra→86222Rn∗+24Hethen86222Rn∗→86222Rn+γ{}^{226}_{88}Ra \rightarrow {}^{222}_{86}Rn^* + {}^{4}_{2}He \quad \text{then} \quad {}^{222}_{86}Rn^* \rightarrow {}^{222}_{86}Rn + \gamma

Answer. Daughter from alpha decay is Radon-222. Gamma decay then produces stable Radon-222 plus a high-energy photon.

Figure
Diagram
Types of Radioactive Decay & Penetrating PowerSourceαβγPaper~0.1 mmStops αAluminium~3 mmStops βLeadseveral cmAttenuates γDecay Type SummaryAlpha (α)Emits: ⁴₂He nucleusZ−2, A−4High ionisation, low penetrationBeta (β)Emits: e⁻ + antineutrinoZ+1, A unchangedMedium ionisation & penetrationGamma (γ)Emits: high-energy photonZ, A unchangedLow ionisation, high penetration
Origins · 1 minMarie Curie and how this was discovered
MC
Marie Curie
1867–1934 · Polish-French

Marie Curie pioneered the study of radioactivity — a term she coined. Working with her husband Pierre, she isolated the radioactive elements polonium and radium and showed that radioactivity was an atomic property of certain elements rather than a chemical reaction. Her meticulous measurements of radiation intensity laid the groundwork for understanding all three decay modes and earned her two Nobel Prizes: Physics (1903) and Chemistry (1911), making her the only person to win in two different sciences. Curie's personal notebooks from the 1890s are still so radioactively contaminated that they are stored in lead-lined boxes at the Bibliothèque nationale de France. Researchers who wish to read them must sign a waiver and wear protective clothing.

  • Radioactive Half-Life — Half-life quantifies how quickly a radioactive sample decays — it depends directly on which decay process the nucleus undergoes.
  • Nuclear Stability and Binding Energy — Whether a nucleus decays by alpha, beta, or gamma emission is determined by its binding energy and position on the chart of nuclides.
  • Ionising Radiation and Safety — The biological hazard of radiation depends on the type of decay: alpha is most dangerous internally, gamma most penetrating externally.
  • Nuclear Equations and Conservation Laws — Balancing nuclear equations requires the conservation of mass number and atomic number introduced in decay processes.