Aamc #4 q 39

[oxelhs]

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Questions asks what is emitted in the decay of 40 K?

a. Gamma ray
b. Alpha particle
c. Electron
d. Positron

Took a random guess with gamma ray (stupid, I know). Answer is given as positron as mass number does not change but protons go from 19 to 18.

My question is, how are we to infer that the mass did not change? If it did change, it should be B, but the answer says it decays into 40 Ar. Is this just how the process works or am I missing smth here?

EDIT: Seems I didn't read the passage properly, the 40 K decays into 40 Ar as mentioned, so A and B are out, and then it has to be positron since Ar 18 has one less positive charge. Am I correct?

 

[rickrun]

Yes, you are corrent. A positron is when a proton loses its charge, which turns it into a neutron. So, the atom now has one more neutron than it did but one less proton, so it becomes Ar.