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.

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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 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.

Number of
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/(2n) 100/(2n)

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


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)


What is Half-Life? (Chemistry)

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.

Three types of natural radioactive decay include alpha radiation, beta radiation and gamma radiation.

Cite this article as: William Anderson (Schoolworkhelper Editorial Team), "Half-Life: Radioactive Elements Decay Over Time," in SchoolWorkHelper, 2019,
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