Why do radium isotopes have nucleon numbers within ±10 of 228?

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CAH
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A neutral atom of radium isotope may be represented by:
228
Ra (88 proton number, 228 nucleon number).
88

A
Ra (A:nucleon number, Z: proton number).
Z
...Is a neutral atom of different isotope of radium. State a possible value for A and for Z.

I know that neutral means the number of protons will be the same as electrons so net charge=0 but I am confused as to why (in the mark scheme) it shows: 228 ± 10.
I know the proton number will stay the same, but why is the nucleon number only plus or minus 10??

Thanks
 
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CAH said:
Im confused as to why (in the mark scheme) it shows: 228 ± 10.

Where does it show 228 ± 10?

(Also, if this is a homework question, it should be posted in the appropriate homework forum and the homework template should be used. If it isn't, a reference for the source material you are getting this from would be helpful.)
 
Question 2 on this paper: http://filestore.aqa.org.uk/subjects/AQA-PHYA1-W-QP-JAN11.PDF
Mark scheme: http://filestore.aqa.org.uk/subjects/AQA-PHYA1-W-MS-JAN11.PDF
This is just revision for my exams, not homework.

Thanks
 
CAH said:
This is just revision for my exams, not homework.

Exam questions are treated the same as homework here on PF. I have closed this thread; please re-post a new thread in the homework forum with the template filled out.