Alpha decay
The radioactive decay of an unstable nucleus by emitting an alpha particle (helium-4 nucleus: ⁴He or ⁴₂He), reducing the atomic number by 2 and mass number by 4. The equation: ᴬ_Z X → (A−4)_(Z−2) Y + ⁴_2 He. Alpha particles have kinetic energy up to several MeV and are easily stopped by paper or a few centimetres of air.
Formula
ᴬ_Z X → ᴬ⁻⁴_(Z-2) Y + ⁴_2 He
Real World
Americium-241 in household smoke detectors undergoes alpha decay, emitting alpha particles that ionise air inside the detector — when smoke disrupts this ionisation current, the alarm triggers.
Exam Focus
Always balance both mass number and atomic number in decay equations; losing marks for unbalanced equations is the most common error.
How well did you know this?