Question 2

[michaelgar7090]

I got this right, but it was a guess. I understand that for this Mg isotope, n=15 and p=12. I don't understand how this relates to the graph. Can someone help me out?

[nedaa.asba6809]

ok then by taking the ratio of the n/p [number of neutrons /number of protons] It would give us something more than 1. In the figure it shows that the beta decay region is after the stable (n/p) ratio, and this indicates that the radioactive decay is the beta- decay region.

[StoneColdPS14]

I think, for MCAT you should know the criteria of stable nuclei, which is n:p ratio close to 1. the rest can be reasoned through the graph which implies B-Decay for n/p > 1.
if you read closely, the passage says that n/p line has 'unit slope' its a complicated way of saying 'slope=1'.

[nedaa.asba6809]

When we say unit slope, we really mean that the slope is equal to 1. Same as many other examples such as unit vector that basically represent a vector having a magnitude of 1.

[tjk447963]

This questions confused me because the passage says that the graph exhibits straight line behavior with unit slope up to p = 25. In this question with Mg, p = 12, so since its below 25 shouldn't it still exhibit straight line behavior?

[mcat_premed3832]

If you look at the periodic table (which of course is available to you during the exam), you will see that Mg is atomic number 12 and its atomic weight is 24. Thus its n/p ratio is 1 which falls precisely on the graph's unit slope for p < 25.

However, the question is about the "radioisotope" of Mg which is not balanced thus radioactive and so it falls outside of the standard curve with an n/p ratio of 15/12.

[dnpgr16513]

Above p = 25, those with an n/p ratio lying below the stable region usually undergo electron capture while those with n/p ratios lying above the stable region usually undergo beta decay. Very heavy isotopes (p > 83) are unstable because of their relatively large nuclei and they undergo alpha decay. Gamma ray emission does not involve the release of a particle. It represents a change in an atom from a higher energy level to a lower energy level.


Hi, I'm confused as to why the answer would be beta decay. In the paragraph listed above, they mention the P values as indicators as to what kind of decay it is. For example: p>83= alpha. Mg has p=12 Is it wrong looking at problem this way? Also, I wasn't which beta decay to choose because there are 2 types of beta decay ( positive and negative).

Any clarification would be greatly appreciated.

thanks.

[jellywing_2058]