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Radioactive decay is a random process. Each nucleus in a sample of an isotope decays at
It happens regardless of:
- outside conditions such as temperature and pressure
- the chemical state of the isotope
- whether the radioactive substance is a solid, liquid, gas or solution.
E.g. uranium would decay in the same way whether it was in a compound or in the free
free state as an element.
As more of the nuclei in a sample decay, the sample becomes less and less radioactive.
Half-life: the time for half of radioactive nuclei to decay
In the course of one half-life, half of the radioactivity of the sample always disappears.
The sample never completely disappears because we are still able to detect it many years
Some isotopes and their half-lives
Uranium-238 4.5 x 10 years
Carbon-14 5.7 x 10³ years
Strontium-90 28 years
Iodine-131 8.1 days
Bismuth-214 19.7 minutes
Polonium-214 1.5 x 10
If you started with 8g of strontium-90, in 28 years' time half the nuclei would have decayed
and only 4g of strontium-90 would be left. In 56 years' time only 2g would be left.
By that time the isotope would have gone through two half-lives: the original quantity
would have been halved and halved again.
Half-life calculations can be used to date archaeological artefacts made from living things
e.g. wood, cotton and bones. Ages of igneous rocks contains radioactive isotopes can also
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Calculating and Using Half-life
If you know the half-life of a radioactive isotope, you can use it to calculate:
- The time needed for the activity, or the mass of the isotope, to fall to a certain value
- The mass of the isotope, or its activity, after it has decayed for a certain time
The half-life of the radioisotope carbon-14 is 5730 years.…read more