Alpha Decay
Alpha decay is a type of radioactive decay in which an unstable nucleus emits an alpha particle — a helium-4 nucleus consisting of two protons and two neutrons — thereby reducing its atomic number by 2 and its mass number by 4. This process is common in heavy nuclei (Z > 82) such as uranium and radium, where the nuclear repulsion between protons becomes too great to maintain stability. Alpha particles have low penetrating power and can be stopped by a sheet of paper, but are highly ionising and dangerous if ingested or inhaled.
Key Formula
A_Z X → (A-4)_(Z-2) Y + 4_2 He
LaTeX: {}^{A}_{Z}X \rightarrow {}^{A-4}_{Z-2}Y + {}^{4}_{2}He
| Symbol | Meaning | Unit |
|---|---|---|
| A | Mass number of parent nucleus | dimensionless |
| Z | Atomic number of parent nucleus | dimensionless |
| X | Parent nuclide symbol | — |
| Y | Daughter nuclide symbol | — |
| ⁴₂He | Alpha particle (helium-4 nucleus) | — |
Worked Example
Problem
Uranium-238 (³⁸₉₂U) undergoes alpha decay. Write the decay equation and identify the daughter nuclide.
Solution
Step 1: Apply conservation of mass number: A_daughter = 238 − 4 = 234. Step 2: Apply conservation of atomic number: Z_daughter = 92 − 2 = 90. Step 3: Element with Z = 90 is Thorium (Th). Step 4: Write the equation: ²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He.
Answer
The daughter nuclide is Thorium-234 (²³⁴₉₀Th).
Properties of Alpha Particles vs Other Radiation Types
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| Composition | He-4 nucleus (2p + 2n) | Electron or positron | High-energy photon |
| Charge | +2e | ±e | 0 |
| Mass (u) | 4.0015 | 0.000549 | 0 |
| Penetrating power | Very low (cm air) | Low (mm Al) | High (cm Pb) |
| Ionising ability | Very high | Moderate | Low |
| Stopped by | Paper / skin | Aluminium sheet | Lead / thick concrete |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Radioactive Decay
Radioactive decay is the spontaneous transformation of an unstable atomic nucleus into a more stable configuration by emitting radiation in the form of particles or electromagnetic waves. This process occurs because the nucleus has too many protons, too many neutrons, or excess energy, making it thermodynamically unstable. It is the foundation of nuclear medicine, radiometric dating, and nuclear power generation.
Beta Decay
Beta decay is a type of radioactive decay mediated by the weak nuclear force, in which a neutron converts to a proton (beta-minus decay, emitting an electron and an antineutrino) or a proton converts to a neutron (beta-plus decay, emitting a positron and a neutrino). Unlike alpha decay, the mass number of the nucleus remains unchanged, but the atomic number increases or decreases by one. Beta decay is responsible for the natural transmutation of elements and is exploited in positron emission tomography (PET scanning) and food irradiation.
Nuclear Binding Energy
Nuclear binding energy is the energy required to completely separate a nucleus into its individual protons and neutrons, or equivalently, the energy released when these nucleons combine to form the nucleus. It arises from the strong nuclear force overcoming electromagnetic repulsion between protons, and is directly related to the mass defect — the difference between the mass of the nucleus and the sum of masses of its constituent nucleons via Einstein's E = mc². The binding energy per nucleon peaks around iron-56, explaining why both fusion of light nuclei and fission of heavy nuclei can release energy.
Named "alpha" by Ernest Rutherford in 1899 using the first letter of the Greek alphabet (α), as it was the first type of radiation he identified. Rutherford later established that alpha particles are helium-4 nuclei through spectroscopic analysis in 1909.