Half-life: definition in chemistry
Half-life: The time it takes for half of the radioactive nuclei in a sample to decay, given the symbol t½.
Each radioactive isotope has its own half-life, which is practically unaffected by temperature, pressure or chemical form. After one half-life half of the original nuclei remain, after two a quarter, after three an eighth, and so on. Half-lives range from fractions of a second to billions of years, and knowing them is essential for radioactive dating, nuclear medicine and managing nuclear waste. The term is also used for reactions and medicines, meaning the time for a concentration to fall by half.
Examples
- Carbon-14 — about 5,730 years, used to date ancient wood and bone
- Iodine-131 — about 8 days
- Technetium-99m — about 6 hours, widely used in medical imaging
- Uranium-238 — about 4.5 billion years, close to the age of Earth
On the periodic table
Half-lives tend to become very short among the heaviest elements at the end of the table: the known isotope of oganesson lasts less than a thousandth of a second.