Se
Reactive nonmetal
Selenium
Z 34A 79mass 78.971
Decay curve
Selenium (illustrative)
half-life
not measured here
half-lives elapsed1 half-life
0123456
left
50.0%
in half-lives
1 t½
Selenium has no radioactive isotope in this lab's data, so the curve shows the mathematics every nucleus follows. Pick uranium, radon or cobalt to read real numbers off the same curve.
Sample of 240 nuclei
129 leftEach dot is one nucleus. Decay is random for each one — but the pattern that emerges is the exponential curve on the left.
The numbers
| Half-lives | Remaining | Of 1000 |
|---|---|---|
| 0 | 100.0% | 1000 |
| 1 | 50.0% | 500 |
| 2 | 25.0% | 250 |
| 3 | 12.5% | 125 |
| 4 | 6.25% | 63 |
| 5 | 3.13% | 31 |
| 6 | 1.56% | 16 |
N = N₀ × (½)ⁿ — never exactly zero
Real half-lives, for scale
Thorium-23214.05 billion years
Uranium-2384.47 billion years
Potassium-401.25 billion years
Neptunium-2372.14 million years
Plutonium-23924,110 years
Carbon-145,730 years