It's the law! Matter cannot be created or destroyed in chemical reactions. This is the law of conservation of mass. In every chemical reaction, the same mass of matter must end up in the products as started in the reactants.
The simple trick which one can consider in such problem where it is asked for positron emission is :
<span>When the atomic number goes DOWN by one and mass number remains unchanged, then a positron is emitted.
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<span>a. </span>

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Here the atomic number decreases by one.
Similarly, options b and d are eliminated.
Option c is also not the answer.
For c, Count the atomic number on left side and compare it with right side. You will see it is 9 on left and 8 on right. Atomic no. did go down by 1. But the atomic mass is changed as well.
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Answer:
Pb^+7
Explanation:
I placed the carrot there to signify the +7 is supposed to be small. It is plus seven because seven nuetrons are added
I attached a photo of my work below. Please tell me if I got it wrong; hope you got a good score on your assignment! ^v^
No. bacteria only needs food to live along with other special needs depending on the type of bacteria