Radioactive decay is a spontaneous, random change in an unstable atomic nucleus. Think of an unstable nucleus as a wobbling top that eventually tips over and releases energy. The top can fall in different ways, just like a nucleus can emit different kinds of radiation.
A decay can be written like a chemical reaction. For example, radium‑226 decays to radon‑222 by emitting an α‑particle:
\$^{226}{88}\text{Ra} \;\longrightarrow\; ^{222}{86}\text{Rn} \;+\; \alpha\$
The numbers (mass and atomic) balance on both sides, just like in a balanced chemical equation.
Unlike a clock that ticks at regular intervals, radioactive decay follows a probability distribution. Each nucleus has a 50 % chance of decaying in a fixed time called the half‑life (\$t_{1/2}\$). The exact moment a particular atom decays is unpredictable, but the overall rate for a large number of atoms is predictable.
1. Identify the type of radiation: Look for clues like “α” or “β” in the question. Remember: α = 2 protons + 2 neutrons, β = electron or positron, γ = photon.
2. Balance the equation: Check that the mass and atomic numbers match on both sides. Use the notation \$^{A}_{Z}\text{X}\$.
3. Half‑life concept: If the question asks about time, use the relation \$N = N_0 \, e^{-\lambda t}\$ or the simple rule that after one half‑life, half the nuclei remain.
4. Randomness reminder: Emphasise that decay is spontaneous and random; you cannot predict when a single atom will decay, only the statistical behaviour of many atoms.
1️⃣ What is emitted in α‑decay?
2️⃣ Which decay mode involves a photon?
3️⃣ If a sample has a half‑life of 5 years, how many years will it take for 75 % of the nuclei to decay?
Remember the wobbling top analogy: just as the top can fall in many directions, an unstable nucleus can emit α, β, or γ radiation. The direction (type) is random, but the overall tendency (half‑life) is predictable.