by Former Oceano » 15/02/09, 19:58
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The half-life time is the time for half of a compound to transform.
It may be a chemical molecule. If its half-life is 10 years, this means that half of the molecules will have disappeared (transformed into degradation compounds) after 10 years.
The same is true for atoms. At the end of the half-life, half of the atoms have disintegrated to give other atoms which may also be radioactive.
Thus, after 4,5 billion years, half of the uranium atoms will have disintegrated. The radioactivity of Uranium will not have decreased, but the number of atoms will have decreased, which will result - for a given sample - by a possible fall in radioactivity (remember the existence of atoms from decay and can be them radioactive).
This explains why there is still Uranium left today. Basically, when the Earth was created, there were twice as many as today.
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