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Looking for someone with solid chemistry knowledge to answer this (1 Viewer)

Azzamaral

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2018
After completing my first assessment on nuclear chemistry I am in the process of disputing a mark for one of the questions and would like clarification if possible from someone who has a solid understanding of chemistry.

The question is "Describe the conditions under which a nucleus is unstable and the decay processes that such unstable nuclei undergo."

The question was valued at 3 marks which surprised me as I am sure this is only a two mark question.

My response was "There are two conditions to which a nucleus can be unstable. The first is if the nucleus has an amount of protons greater than 82. If this is the case, the element undergoes alpha decay. The second condition is if the ratio of neutrons to protons is too large ie. there are excess neutrons. Atomic #20 1:1 Atomic #50 1.3:1 Atomic #80 1.5:1 (These numbers represent the ideal n:p ratio, any higher causes beta decay)"

I was awarded 2/3 for the question because the third mark on the marking criteria was awarded for writing that the third condition for a nucleus to be unstable is if it is metastable and releases excess energy as gamma rays.

From my understanding however, being metastable does not necessarily mean that the nucleus is unstable, it only means that it is in an excited state where it contains excess energy which is shed away as gamma radiation. Furthermore there is no decay pathway for it on the zone of stability graph because Gamma radiation is an accompanying form of radiation to both alpha and beta decay. Ultimately what I am arguing is that "metastable" is not a condition for instability and should therefore not be one of the awarded marks.
I would love to have some light shed on whether being metastable does count as a reason for instability of a nucleus.
Thankyou
 
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Chematics

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Feb 13, 2018
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After completing my first assessment on nuclear chemistry I am in the process of disputing a mark for one of the questions and would like clarification if possible from someone who has a solid understanding of chemistry.

The question is "Describe the conditions under which a nucleus is unstable and the decay processes that such unstable nuclei undergo."

The question was valued at 3 marks which surprised me as I am sure this is only a two mark question.

My response was "There are two conditions to which a nucleus can be unstable. The first is if the nucleus has an amount of protons greater than 82. If this is the case, the element undergoes alpha decay. The second condition is if the ratio of neutrons to protons is too large ie. there are excess neutrons. Atomic #20 1:1 Atomic #50 1.3:1 Atomic #80 1.5:1 (These numbers represent the ideal n:p ratio, any higher causes beta decay)"

I was awarded 2/3 for the question because the third mark on the marking criteria was awarded for writing that the third condition for a nucleus to be unstable is if it is metastable and releases excess energy as gamma rays.

From my understanding however, being metastable does not necessarily mean that the nucleus is unstable, it only means that it is in an excited state where it contains excess energy which is shed away as gamma radiation. Furthermore there is no decay pathway for it on the zone of stability graph because Gamma radiation is an accompanying form of radiation to both alpha and beta decay. Ultimately what I am arguing is that "metastable" is not a condition for instability and should therefore not be one of the awarded marks.
I would love to have some light shed on whether being metastable does count as a reason for instability of a nucleus.
Thankyou

The question given in your exam, as you might already know, comes straight from the syllabus.

The two conditions you wrote about are perfectly fine and is the reason you got 2 out of the 3 marks. Personally, my textbook does not state the 'metastable' condition, however, there are at least 3 conditions under which nucleus is unstable and this is regarded as one of those conditions.

As you said metastable, is a higher and excited state within a nucleus, compared to a stable ground state of other nuclei. If a nucleus is metastable it will undergo gamma decay. This emission of gamma radiation translates the fact that if a nucleus is metastable, it is theoretically unstable. Thus, if a nucleus is metastable then it is technically unstable.

The three conditions I always remember are:
- Excess protons (beta-positive decay)
- Excess neutrons (beta-negative decay)
- Heavy nucleus (Too many nucleons, leading to alpha particle decay)

I stick by these to avoid controversial responses and marking criterions. Feel free to invest your time on those as well.

Hope this helps!
 

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