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# binding energy/mass defect watch

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1. Hi there,

In my OCR Physics textbook it says: 'When the mass of a given nucleus is compared with the mass of its constituent nucleons, the total mass of the seperated nucleons is always greater than the mass of the nucleus.'

So by this I would assume: 'Mass of parent nucleus < daughter nuclei'

But in a OCR exam question about nuclear fission it says that mass decreases in fission:

This seems to contradict what my textbook says, but I must be missing something.

Thanks for any help,
Malc.
2. In fission, you are using a nucleus, but what your text book is saying, which is correct, is that the nucleons, when separated and not held together by strong forces, actually have more mass, than when they are joined together.

Your statement of "Parent nuclei < Daughter nuclei" is wrong, but if you think about what fission is, its where a nucleus becomes unstable and ejects a helium nucleus or an electron, or just energy, and if it is ejecting something out of itself, it is losing mass and energy, so the daughter nuclei will be lighter than the parent nuclei, so the statement of Parent nuclei > Daughter Nuclei is right
3. (Original post by Sir_Malc)
Hi there,

In my OCR Physics textbook it says: 'When the mass of a given nucleus is compared with the mass of its constituent nucleons, the total mass of the seperated nucleons is always greater than the mass of the nucleus.'

So by this I would assume: 'Mass of parent nucleus < daughter nuclei'

But in a OCR exam question about nuclear fission it says that mass decreases in fission:

This seems to contradict what my textbook says, but I must be missing something.

Thanks for any help,
Malc.
What is your reason to say that 'When the mass of a given nucleus is compared with the mass of its constituent nucleons, the total mass of the separated nucleons is always greater than the mass of the nucleus.' imply that (for fission) 'mass of parent nucleus < daughter nuclei'?

“the total mass of the separated nucleons is always greater than the mass of the nucleus” means that (for example) the mass of uranium-235 nucleus is less than sum of the mass of 92 individual protons and 143 individual neutrons.

mass of uranium-235 nucleus = 235.0439299u

mass of 92 individual protons = 92 × 1.00727647u = 92.66943524u
mass of 143 individual neutrons = 143 × 1.00866492u = 144.23908356u

235.0439299u < 92.66943524u + 144.23908356u
235.0439299u < 236.9085188u

mass of uranium-235 nucleus < (mass of 92 individual protons + mass of 143 individual neutrons)
4. (Original post by m005eman)
....Your statement of "Parent nuclei < Daughter nuclei" is wrong, but if you think about what fission is, its where a nucleus becomes unstable and ejects a helium nucleus or an electron, or just energy, and if it is ejecting something out of itself, it is losing mass and energy, so the daughter nuclei will be lighter than the parent nuclei, so the statement of Parent nuclei > Daughter Nuclei is right
if you think about what fission is, its where a nucleus becomes unstable and ejects a helium nucleus or an electron, or just energy,

Are you sure about it? Nuclear fission is about "splitting of heavier nucleus". There is no ejection of helium nucleus.
5. (Original post by Eimmanuel)
if you think about what fission is, its where a nucleus becomes unstable and ejects a helium nucleus or an element ctron, or just energy,

Are you sure about it? Nuclear fission is about "splitting of heavier nucleus". There is no ejection of helium nucleus.
I think i’ve gotten mixed up with radioactive decay and fission

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