Last min Question

[Leonine]

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I was going over AAMC 4R and question # 58 is confusing me.

Background :-
In radioactive dating, the age of a rock can be determined by measuring the amount of radioactive potassiam (K^40 , half life = 2.4 * 10^8 years) in the rock and the amount of its decay product, argon gas (Ar^40), in the sample.

Question
What kind of object is emitted in the decay of 40
K?

A) A gamma ray
B) An alpha particle
C) An electron
D) A positron

I would pick C since

40 40 -
K --- --> Ar + e
19 18

The correct answer however is D. I know this is last minute but its really frusterating me - can someone belay my anxiety? 🙁

Thanks

 

[AxlxA]

D positron is my choice.

You are going from K to Ar, which is one less proton. This means you have to convert that proton into a neutron some how.

By grabbing an electron from nowhere(with creation of positron) your new proton+electron becomes a neutron and u emit a positron.