Radioactive elements have a half-life. Half-life occurs naturally in some of the radioactive elements while it could be artificially stimulated in some other elements.

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 half-life there will be 5 grams of the radioactive element left.

After another half-life, there will be 2.5 g of the original element left, after another half-life, 1.25 g will be left.

Each and every radioactive element has its own half-life. For instance, 238U has a half-life of 4.5billion years.

Example

A radioactive substance has a half-life 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 half-lives. 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 half-life of 14C is 5730 years and this is very helpful in geological dating of any archaeological material. (CARBON DATING)

CARBON DATING

The ratio of normal carbon (carbon-12) to carbon-14 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 carbon-14 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 carbon-14 decays with its half-life of 5,700 years, while the amount of carbon-12 remains constant in the sample. By looking at the ratio of carbon-12 to carbon-14 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.