Beta Decay for the MCATs

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10+ Year Member
Jun 14, 2012
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Hey Everyone,

So, today I was taking Exam Krackers Section 4 and ran across an interesting problem. Questino 90 states

^218PO undergoes two alpha decays and four beta decays to become:
A. ^210Bi
B. ^226Ra
C. ^210Pb
D. ^210Po

Now, the credited answer is D. And the reasoning is that two alpha decays means you'll lose 4 Protons and 4 neutrons (becoming 210), and that with four beta decays you'll gain 4 more protons (at the expense of neutrons) so the Atomic Symbol stays the same.

This is great and all, except for the fact that the term Beta Decay does not only incompass Electron Emission (A neutron turns into a proton and an electron) but also Positron Emission (A proton turns into a positron and a neutron.) Knowing this, how can one reliably figure out the answer?

For example, with three Electron Emissions and One Positron Emision, you have Answer C!

So, for the MCAT is their a simple, hard and fast rule to approach this problem?

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I can't give you a "hard and fast" rule to apply, but as a general rule of thumb, if they just say "beta decay" you should probably assume it's a beta-minus particle. If they want positron decay, they'll specify beta-plus.
Alright. Fair enough. It looks like that's pretty much the assumption that EK is going off of as well.
-Beta decay means you lose an electron (same effect as positron capture)
-Beta capture means you gain an electron ( which is the same effect as positron emission)
If they mean Beta + decay (aka positron emission) they will always specify that and usually as using the terms positron emission occurs rather than Beta+ at least that's what i've seen so far!