The 半衰期 or radioactive period of a radioactive isotope is the time required for half of the nuclei in a sample to decay. It is a characteristic quantity of an unstable nucleus and a probabilistic measure of its stability. The half-life is independent of the amount of material or external conditions (temperature, pressure, etc.) because radioactive decay is a purely quantum process, governed by the laws of quantum mechanics.
短半衰期意味着快速衰变（极不稳定的元素）。长半衰期则表示元素更为稳定。
No, not all chemical elements have a measurable half-life, because 一些同位素是稳定的。. For example, carbon-12 (12C), oxygen-16 (16O), or iron-56 (56Fe) do not decay spontaneously, even over timescales comparable to the age of the Universe. On the other hand, 放射性元素 such as uranium, thorium, or polonium have one or more unstable forms (isotopes) with half-lives ranging from microseconds to billions of years.
放射性半衰期，通常记作 \( T_{1/2} \)，是指一个化学元素（更确切地说，是一种放射性核素）的样本中，一半原子核自发衰变所需的时间。它是衡量原子核稳定性的指标。
对于在 \( t = 0 \) 时刻含有 \( N_0 \) 个原子核的样本，放射性衰变定律为：\( N(t) = N_0 \cdot e^{-\lambda t} \)
半衰期 \( T_{1/2} \) 与 \( \lambda \) 的关系为：\( T_{1/2} = \frac{\ln(2)}{\lambda} \)
| 元素 | 同位素 | 半衰期（秒） | 半衰期（可读形式） |
|---|---|---|---|
| 钋 | Po-212 | 0.0000003 | 0.3 微秒 |
| 锂 | Lv-293 | 0.061 | 61毫秒 |
| 锎 | Cf-251 | 0.9 | 0.9秒 |
| 钫 | Fr-223 | 1320 | 22分钟 |
| 锝 | 锝-99m | 21600 | 6 h |
| 碘 | I-131 | 693,728 | 8.02 天 |
| 钴 | 钴-60 | 166,824,000 | 5.27年 |
| 氢 | 氚 | 388,435,680 | 12.32年 |
| 碳 | C-14 | 180,622,080,000 | 5730年 |
| 钚 | 钚-239 | 760,585,760,000 | 24,100年 |
| 铀 | U-234 | 7,740,138,000,000 | 245,500年 |
| 铀 | 铀-235 | 22,174,070,000,000 | 7.03亿年 |
| 铀 | U-238 | 141,379,620,000,000 | 44.68亿年 |
| 钍 | 钍-232 | 443,538,150,000,000 | 140.5亿年 |
| 碲 | Te-128 | 6.945×10²⁴ | 2.2×10²⁴秒（约7×10¹⁶年） |
该图展示了选定同位素的半衰期，按递增顺序排列。对数坐标轴使人们能够直观地看到从微秒到超过10²⁴秒的巨大差异。
原子的各种形态：从古代直觉到量子力学 
