Radioactive elements have a halflife. Halflife occurs naturally in some of the radioactive elements while it could be artificially stimulated in some other elements.
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The half life of any given element is the time that is required for one half of the sample to decay.
For example: If you have 10 grams of a radioactive element to start with….. after one halflife there will be 5 grams of the radioactive element left.
After another halflife, there will be 2.5 g of the original element left, after another halflife, 1.25 g will be left.
Number of halflives elapsed 
Fraction remaining 
Percentage remaining 

^{1}/_{1}  100  
1  ^{1}/_{2}  50  
2  ^{1}/_{4}  25  
3  ^{1}/_{8}  12  .5 
4  ^{1}/_{16}  6  .25 
5  ^{1}/_{32}  3  .125 
6  ^{1}/_{64}  1  .563 
7  ^{1}/_{128}  .781  
…  …  …  
n  1/(2^{n})  100/(2^{n}) 
Each and every radioactive element has its own halflife. For instance, ^{238}U has a halflife of 4.5billion years.
Example
A radioactive substance has a halflife of 20 minutes. If we begin with a 500 g sample, how much of the original sample remains after two hours?
Two hours is 120 minutes > six halflives. At the end of the stated time period, 7.8 g remains.
500 g > 250 g > 125 g > 62.5 g > 31.25 g > 15.625 g > 7.8125 g
7.8 g remains
Another interesting fact is halflife of ^{14}C is 5730 years and this is very helpful in geological dating of any archaeological material. (CARBON DATING)
CARBON DATING
The ratio of normal carbon (carbon12) to carbon14 in the air and in all living things at any given time is nearly constant. At this moment, your body has a certain percentage of carbon14 atoms in it, and all living plants and animals have the same percentage.
As soon as a living organism dies, it stops taking in new carbon. The carbon14 decays with its halflife of 5,700 years, while the amount of carbon12 remains constant in the sample. By looking at the ratio of carbon12 to carbon14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely.
Three types of natural radioactive decay include alpha radiation, beta radiation and gamma radiation.