Kaplan Chemistry Question

[goodie]

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I'm going through some Kaplan chemistry questions and the following question is giving me some trouble:

What is the value of Z in the reaction below

60 top (mass #), 27 bottom (atomic #) CO --> 60 top Z bottom (atomic #) X + electron

I thought the answer would be 26 since Z is the # or protons and electrons but the answers say 28???

 

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the answer is 28. this is Beta decay

z is atomic number = number of protons

in this question, electron is beta particle which means that a neutron disappears and a proton takes its place. (a neutron --> electron + proton)
--> the mass number remains the same, the atomic number increases by 1