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Old Dec 29th 2015, 02:38 PM   #1
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Nuclear physics questions

What is N?, where N = Avogadro's number / mass number

How can I intuitevely imagine this N? What do we get?



What is fission rate? In my notes it is written as Power / Fission work.

Another thing which I can't imagine. Some example might help.
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Old Dec 29th 2015, 07:33 PM   #2
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Originally Posted by Nforce View Post
What is N?, where N = Avogadro's number / mass number

How can I intuitevely imagine this N? What do we get?

What is fission rate? In my notes it is written as Power / Fission work.

Another thing which I can't imagine. Some example might help.
This is way too vague to answer. A variable is not uniquely defined by a particular letter. For example; if you're looking in a text on nuclear physics then N might be the number of nucleons in the nucleus of particular isotope of an atom. An isotope is the term used to distinguish between atoms of the same element but having a different number of neutrons. E.g. there are fifteen "isotopes" of carbon, two of which are stable. A list of then can be found at: https://en.wikipedia.org/wiki/Isotopes_of_carbon

N is then the sum of the number of protons and the number of neutrons.

The rate of fission of an isotope is the number of fissions per unit time.
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Old Dec 30th 2015, 06:33 AM   #3
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Avogadro's number is (essentially) the number of atoms in 1 gram of hydrogen.
There are some tweaks to the detail of this definition see
Avogadro_constant
, but I think that for the purpose of your question this definition is satisfactory.

The mass number is the number of protons plus the number of neutrons in an atom.
Hydrogen will have a mass number of 1 (deuterium 2, tritium 3, helium 4, etc.).

Note that for every proton there will be an electron and the mass of a neutron is (essentially) the same as the mass of a proton plus the mass of an electron.
Again there are small tweaks due to (for example) the binding energy, but I think these will be small enough to be ignored for the purpose of this question.

From this we can see that N in your equation will be the number of atoms in 1 gram of the element being considered.
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Old Dec 31st 2015, 01:39 AM   #4
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Originally Posted by MBW
Note that for every proton there will be an electron and the mass of a neutron is (essentially) the same as the mass of a proton plus the mass of an electron.
Note: This only holds for electrically neutral atoms, not for ions.

Originally Posted by MBW
Again there are small tweaks due to (for example) the binding energy, but I think these will be small enough to be ignored for the purpose of this question.
If you don't mind me asking, tweaks to what?

From this we can see that N in your equation will be the number of atoms in 1 gram of the element being considered.[/QUOTE]

Thanks for correcting me pal. I mixed them up.

Z = number of protons
N = number of neutrons (this is different than the OPs N)
A = Z + N

https://en.wikipedia.org/wiki/Mass_number
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Old Dec 31st 2015, 02:28 AM   #5
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tweaks

The mass number (in its simplest form) assumes that the mass of an atom can be defined completely by counting the number of protons and neutrons.
This includes the assumption that the mass of a proton plus the mass of its associated electron is the same as the mass of a neutron.
It also includes the assumption that all the mass of the atom is totally accounted for within the neutrons and proton/electron pairs.
(which as you point out includes the assumption it is not ionized).
However the binding energy between neutrons and protons introduces an additional source of mass.
OK it's a very very small effect, which has no particular bearing on the original question, I just included it for "completeness".
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Old Dec 31st 2015, 04:09 AM   #6
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Originally Posted by MBW View Post
The mass number (in its simplest form) assumes that the mass of an atom can be defined completely by counting the number of protons and neutrons.
I disagree. I believe that you're confusing atomic mass with mass number. The atomic is not directly related to the mass of the nucleus. It's actually defined as the number of nucleons in the nucleus of an atom. which is sum of protons and neutrons. Mass number is unit-less, unlike mass which has units.

See: http://chemistry.about.com/od/chemis...ass-Number.htm
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Old Dec 31st 2015, 10:22 AM   #7
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No,
I was trying to describe the (small) difference between mass number and isotopic mass.

Atomic mass includes the effect of the relative abundances of different isotopes of the element.
Isotopic mass is for a single isotope of the element.

However, simply adding the number of nucleons (mass number) does not give exactly the isotopic mass,
due the the different binding energies required to hold nuclei of different compositions together.

https://en.wikipedia.org/wiki/Atomic_mass
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Old Dec 31st 2015, 09:44 PM   #8
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Originally Posted by MBW View Post
The mass number (in its simplest form) assumes that the mass of an atom can be defined completely by counting the number of protons and neutrons.
May I make a comment which I hope will make your responses better in the future? It's not meant as a criticism but merely a suggestion. First off, what is the definition of isotopic mass? I tried to look it up but ran into trouble.

Second, it's not clear at all that this was what you were trying to do. And third, you can't compare the two. They're like apples and oranges just like I mentioned above. You're confusing atomic mass with mass number.
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Old Jan 1st 2016, 06:25 AM   #9
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Atomic mass is based on the mix of isotopes that will be naturally found in a sample (which has not been artificially enriched to favor one isotope).
https://en.wikipedia.org/.../Relative_atomic_mass
Thus (for example) chlorine, which has an abundance of naturally occurring isotopes, has a standard atomic mass of 35.453.
https://en.wikipedia.org/.../Isotopes_of_chlorine

Isotropic mass is based on a single isotope of the element
https://en.wikipedia.org/.../Relative_isotopic_mass:

The mass number of the original post can be related to the mass via the atomic mass unit:
https://en.wikipedia.org/.../Atomic_mass_unit

However a simple multiplication of the atomic mass unit by the mass number of an isotope will not give exactly the correct mass (except of course for carbon 12),
because of the different binding energies:
https://en.wikipedia.org/.../Binding_energy_curve - common_isotopes

Note also the isotropic masses for chlorine in this table:
https://en.wikipedia.org/.../Isotopes_of_chlorine#Table
None of them are integer numbers (although all of them are close).
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