The rule of decay, specifically regarding radioactive decay, states that a fixed fraction of a substance will decay within a specific unit of time. This principle is often described by exponential decay.
Understanding Exponential Decay
Rutherford and Soddy's work established the foundation for understanding radioactive decay. Here's a breakdown of the core concept:
- Fixed Fraction Decay: In each unit of time, a constant proportion of the existing radioactive material will decay. This proportion is quantified by the decay constant.
- Example: Consider a sample of a thorium product. If half of it decays in four days, then half of the remaining sample will decay in the next four days, and so on. It's not a linear decrease; the rate of decay slows as less of the radioactive material remains.
Exponential Decay Explained Simply
Imagine you start with 100 radioactive atoms. Let's say the "half-life" (the time it takes for half to decay) is 1 hour.
| Time (hours) | Remaining Atoms | Atoms Decayed in the Last Hour |
|---|---|---|
| 0 | 100 | - |
| 1 | 50 | 50 |
| 2 | 25 | 25 |
| 3 | 12.5 | 12.5 |
| 4 | 6.25 | 6.25 |
Notice that while the fraction decaying per hour (50%) remains constant, the number of atoms decaying each hour decreases. This is the essence of exponential decay.
Key Concepts
- Decay Constant: A value that describes how quickly a radioactive substance decays. A larger decay constant means faster decay.
- Half-Life: The time it takes for half of the radioactive material to decay. It is inversely proportional to the decay constant. Substances with shorter half-lives decay more rapidly.
