Is it possible to deduce the number of protons from the mass

[rwooduk]

...number?

If you are give say Lead, ²⁰⁸Pb. What is the 208? and is it possible to deduce the number of protons Z from it with no other information?

Haven't done chemistry yearts so any help would be appreciated.

 

[Borek]

Nope. Number of protons is defined by the Pb symbol. 208 is sum of number of protons and neutrons (number of nucleons), and doesn't tell anything about how many of each there is.

It is also not uncommon to have several isotopes of different elements all having the same total number of nucleons, these are called isobars.

 

[rwooduk]

Nope. Number of protons is defined by the Pb symbol. 208 is sum of number of protons and neutrons (number of nucleons), and doesn't tell anything about how many of each there is.

It is also not uncommon to have several isotopes of different elements all having the same total number of nucleons, these are called isobars.

Many thanks for that, the lecturer seemed to assume we knew, no other info was given in class. So youre reply is reassuring thanks!

One more thing if you have a free minute, for Pb 206, he said that there are no energetically favourable decay modes, can this be deduced from the 206 value? or would more information be required? or it is common knowledge and something to memorise?

 

[Borek]

It is not a chemistry question - I will move the thread to nuclear physics. 206 is definitely not enough, but perhaps there are nucleus models that allow predicting decay modes from known Z and A.

 

[rwooduk]

It is not a chemistry question - I will move the thread to nuclear physics. 206 is definitely not enough, but perhaps there are nucleus models that allow predicting decay modes from known Z and A.

Ok. Many thanks for your replies!

 

[DaveC426913]

Nope. Number of protons is defined by the Pb symbol. 208 is sum of number of protons and neutrons (number of nucleons), and doesn't tell anything about how many of each there is.

Am I misreading something here?

If you have a mass composed of Pb²⁰⁸, as the OP specified, then all the atoms contain 82 protons and 126 neutrons.

If they didn't, they would either not be the 208 isotope, or they would not be lead at all..

 

[rwooduk]

Am I misreading something here?

If you have a mass composed of Pb²⁰⁸, as the OP specified, then all the atoms contain 82 protons and 126 neutrons.

If they didn't, they would either not be the 208 isotope, or they would not be lead at all..

Did you deduce the bolded statement from the Pb²⁰⁸? Is there any chance that you could explain how?

 

[DaveC426913]

Yes.

(My 'shut up and keep your head down' alarms are going off in my head, so it is against my better judgement that I'm challenging Borek, but) it seems to me that
- you specified the mass contains entirely Pb²⁰⁸
- lead contains 82 protons, so the rest (126) are neutrons.

Here it is, listed in the isotopes of lead chart:
http://en.wikipedia.org/wiki/Isotopes_of_lead

208Pb 82 126 207.9766521

 

[Borek]

Am I misreading something here?

Definitely there is some misunderstanding, could be my English failed.

As far as I understand the original question asked "is it possible to deduce number of protons from a single 208 number" - and then the answer is "no". At the same time when you are given entire "²⁰⁸Pb" information number of protons is given by the identity of the element, given by its symbol (and I believe that's what I wrote).

 

[DaveC426913]

Definitely there is some misunderstanding, could be my English failed.

As far as I understand the original question asked "is it possible to deduce number of protons from a single 208 number" - and then the answer is "no". At the same time when you are given entire "²⁰⁸Pb" information number of protons is given by the identity of the element, given by its symbol (and I believe that's what I wrote).

That's what I thought you were saying, yes. Given only 208, you do not know which element it is, and thus the ratio of protons/neutrons.

But the OP did specify Pb²⁰⁸. Which is enough to deduce the ratio.

 

[mfb]

One more thing if you have a free minute, for Pb 206, he said that there are no energetically favourable decay modes, can this be deduced from the 206 value? or would more information be required? or it is common knowledge and something to memorise?

You have to look up the masses of possible decay products to see if the decays are possible. It is possible to estimate them theoretically but to do that precisely is way beyond the scope of homework problems. Alternatively, look up directly if the nucleus is stable.