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Halflife (element)
 This article is about the property of radioactive elements. For the video game, see HalfLife (video game).
Number of halflives that have happened 
Parts remaining 
As power of 2 

0  1/1  [math]1/2^0[/math] 
1  1/2  [math]1/2^1[/math] 
2  1/4  [math]1/2^2[/math] 
3  1/8  [math]1/2^3[/math] 
4  1/16  [math]1/2^4[/math] 
5  1/32  [math]1/2^5[/math] 
6  1/64  [math]1/2^6[/math] 
7  1/128  [math]1/2^7[/math] 
...  ...  
[math]N[/math]  [math]1/2^N[/math]  [math]1/2^N[/math] 
The halflife of a substance is the time it takes for half of the substance to decay. The word "halflife" was first used when talking about radioactive elements where the number of atoms get smaller over time. It is now used in many other situations. A GeigerMuller detector can be used to measure the halflife; it is the time when the activity is half of the original.
Note that halflife is defined as a probability. Halflife is the expected value when half the number of atoms have decayed. Carbon14 has a halflife of 5730 years. Taking one atom of C14, this will either have decayed after 5730 years, or it will not. But if this experiment is repeated again and again, it will be seen that the atom decays within the half life 50% of the time.
Radioactive atoms have unstable nuclei. Their nuclei are unstable because the arrangement of protons and neutrons in them are unsteady, making them shake until that they begin to decay and change into completely different types of atoms by releasing certain types of radioactive particles, such as alpha particles, beta particles or gamma rays, and possibly by fissioning into smaller nuclei. This is known as radioactive decay. For example, a radioactive carbon14 atom releases a beta particle to become nitrogen14. As an example of fission decay, a Fermium256 atom can split into Xenon140 and Palladium112 atoms, releasing 4 neutrons in the process.
As an example, U^{232} has a halflife of about 69 years. Pu^{238} has a halflife of 88 years. C^{14}, which is used to find the age of fossils, has a halflife of 5730 years.

