Alpha Decay
Alpha decay is ^A_ZX -> ^(A-4)_(Z-2)Y + ^4_2He. It occurs when the final rest mass is lower; quantum tunnelling through the Coulomb barrier permits emission even when classical escape is forbidden.
Why this shows up in the exam
Finding alpha-decay daughters · Computing alpha energy and recoil · Tracking natural radioactive series
Learn the idea
Alpha decay lowers mass number by four and atomic number by two, with Q shared by alpha and daughter recoil. The emitted alpha particle is a helium-4 nucleus. Conservation immediately fixes the daughter, while momentum conservation explains why the much lighter alpha usually carries most of the released energy.
🧠 Memory hook: Alpha takes 4 from A and 2 from Z.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- ^A_ZX -> ^(A-4)_(Z-2)Y + ⁴₂He — change in A and Z during alpha decay
- Q = (M_X - M_Y - M_alpha)c² — alpha-decay energy using a consistent mass convention
- K_alpha approximately Q (A-4)/A — non-relativistic heavy-daughter approximation for a stationary parent
How to approach it
- 1Subtract 4 and 2 from parent A and Z
- 2Compute Q from consistent masses
- 3Share Q with recoil when requested
Common slip-ups that cost marks
- •Changing Z by four
- •Giving the alpha all of Q exactly
- •Treating an alpha particle as a neutral helium atom
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
In hydrogen, an electron transitions from n = 2 to n = 1. Using E_n = -13.6/n^2 eV, find the emitted photon energy.
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