Multiple choice question HELP! Watch

Yatayyat
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https://imgur.com/SmUaQie

I'm now sure how to find how many alpha decays happen in this decay series

All I know is that when an alpha particle is ejected from the nucleus of an element, that element's mass number goes down by 4 while the atomic number goes down by 2, since an alpha particle is known to be the nucleus of a helium atom.

What I have tried to do is work out the total number of protons that have been lost during the decay series so this is simply 92 - 82 = 10, so 10 protons have been lost.

In doing so I thought that to find the number of alpha decays, then you do 10/2 = 5, so five alpha decays happen because that is the only number of decays that can lose 10 protons.

But I also know that the total number of protons and neutrons lost is 236 -204 = 32, hence 32 protons and neutrons are lost all together. If I divide 32 by 4 that gives 8. So is there meant to be 8 alpha decays then? When I thought before that only 5 alpha decays happen.

I'm not sure what the answer could be, if anyone can help me that would be really appreciated. Thanks!
Last edited by Yatayyat; 2 months ago
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Pangol
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(Original post by Yatayyat)
https://imgur.com/SmUaQie

I'm now sure how to find how many alpha decays happen in this decay series

All I know is that when an alpha particle is ejected from the nucleus of an element, that element's mass number goes down by 4 while the atomic number goes down by 2, since an alpha particle is known to be the nucleus of a helium atom.

What I have tried to do is work out the total number of protons that have been lost during the decay series so this is simply 92 - 82 = 10, so 10 protons have been lost.

In doing so I thought that to find the number of alpha decays, then you do 10/2 = 5, so five alpha decays happen because that is the only number of decays that can lose 10 protons.

But I also know that the total number of protons and neutrons lost is 236 -204 = 32, hence 32 protons and neutrons are lost all together. If I divide 32 by 4 that gives 8. So is there meant to be 8 alpha decays then? When I thought before that only 5 alpha decays happen.

I'm not sure what the answer could be, if anyone can help me that would be really appreciated. Thanks!
Note that the question says that there is a series of decays, not that they are all alpha decays.

Everything you say about alpha decay is correct. But what are the consequences for the mass number and atomic number of other kinds of decay?
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Yatayyat
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(Original post by Pangol)
Note that the question says that there is a series of decays, not that they are all alpha decays.

Everything you say about alpha decay is correct. But what are the consequences for the mass number and atomic number of other kinds of decay?
So are you implying that there could also be beta minus decay involved too? That being the other decay since it cannot all be alpha decays, right?

So if I was going to say there were going to be 8 alpha decays on top of 6 beta minus decays to compensate for the loss of protons, it would still end up getting an element of Pb. Is that okay to say?
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Pangol
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(Original post by Yatayyat)
So are you implying that there could also be beta minus decay involved too? That being the other decay since it cannot all be alpha decays, right?

So if I was going to say there were going to be 8 alpha decays on top of 6 beta minus decays to compensate for the loss of protons, it would still end up getting an element of Pb. Is that okay to say?
That is indeed it. Only alpha decays can change the mass number, so basing your answer on the change from 236 to 204 is fine. But beta decays can change the atomic number, so if you try to base your answer on the change from 92 to 82, you can't guarantee that all of the changes are due to alpha decays.
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Yatayyat
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(Original post by Pangol)
That is indeed it. Only alpha decays can change the mass number, so basing your answer on the change from 236 to 204 is fine. But beta decays can change the atomic number, so if you try to base your answer on the change from 92 to 82, you can't guarantee that all of the changes are due to alpha decays.
Thanks a lot! That was the perfect answer that I needed. All makes sense now.
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