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# Half-life question watch

1. Given that I have a sample of a radioactive isotope X and it contains 1.8 x 10^15 atoms, and the half-life of the isotope is 8 hours.
The question basically asks :

1. The number of atoms remaining after a period of time (8 hours).
2. And the number of atoms that would have decayed (8 hours)

I can't tell the difference between the remaining atoms and the atoms that have decayed. I do not know what formula to use.

Any help would be appreciated!
2. (Original post by Clem H Fandango)
Given that I have a sample of a radioactive isotope X and it contains 1.8 x 10^15 atoms, and the half-life of the isotope is 8 hours.
The question basically asks :

1. The number of atoms remaining after a period of time (8 hours).
2. And the number of atoms that would have decayed (8 hours)

I can't tell the difference between the remaining atoms and the atoms that have decayed. I do not know what formula to use.

Any help would be appreciated!
After 8 hours, the number of atoms has halved as 8 hours is time period.

So u list 50% of the original total, and u gave 50% of original total left too.
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3. (Original post by Shaanv)
After 8 hours, the number of atoms has halved as 8 hours is time period.

So u list 50% of the original total, and u gave 50% of original total left too.
Hi

Thank you for your help.

So for question 1. All I need to do is half the original total?

I don’t understand question 2. and the decay 😩
4. (Original post by Clem H Fandango)
Hi

Thank you for your help.

So for question 1. All I need to do is half the original total?

I don’t understand question 2. and the decay 😩
In this case as one half life (8 hours) has passed then the number of atoms in the sample will have halved.

So u now have half as many atoms remaining. If you do the initial number of atoms minus the number of atoms after 8 hours, then u will work out how many atoms have decayed.

In a more general sense if the time was not as nice a number then u would use n=n0e^(-lambda*t) where n is the number of atoms remaining. Then n0-n will give u the number pf atoms that have decayed.

If the formatting is an issue or my explanation was unclear please let me know and ill try and rectify that.
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