Physics problem I'm confused on

[DeFused]

A parent nucleus at rest beta decays into a daughter nucleus and emits an electron. What is the mass condition for this beta decay to occur?

A. The mass of the daughter nucleus must be less than the mass of the parent nucleus
B. The combined masses of the parent nucleus and an electron must be greater than the mass of the daughter nucleus
C. The mass of the daughter nucleus must be less than the sum of the masses of its constituent nucleons
D. The mass of the parent nucleus must be greater than the combined masses of an electron and daughter nucleus.

The answer is D here. I'm a little confused by the answer here because I thought at least the mass of the parent nucleus should be equal to the combined masses of the electron and daughter nucleus? I thought A made the most sense but I know that electron masses are so small they dont even really matter - but still it would have a little less mass nonetheless right?

Can someone please explain why D is the best answer?

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[sdm33]

The rules for radioactive decay are based on conservation laws. Examination of the example reveals that the number of nucleons and the electric charge are conserved; that is, the total on one side of the equation equals the total on the other side of the equation. Other conservation laws that must be observed are those of energy, momentum, and angular momentum.

 

[DeFused]

Maybe i'm misunderstanding here but wouldn't that mean that the sum of the mass numbers of the products is equal to the mass number of the parent nuclide?

Maybe if we looked at something specifically you could explain it better?

So wouldn't the mass on the right equal the mass on the left?

 

[NuttyEngDude]

I think that problem is testing your knowledge of the mass defect or mass excess:
look at the example in this:
http://en.wikipedia.org/wiki/Mass_excess

 

[DeFused]

Thanks - that really helped.