I am given the following question:
228-88 Ra is an isotope of Radium.
A-Z Ra is a neutral atom of a different isotope of radium. State the possible value for A and Z.
The value for Z is 88 because the proton number is the same between isotopes.
According to the mark scheme, the value for A is 228 +- 10, but I don't know why.
My uneducated intuition told me this means there are 20 other isotopes of Radium: 10 with nucleon numbers incrementing from 229-238, and 10 from 227-218.
However, according to the table of nuclides, there are only two other isotopes of Radium with a nucleon number above 228 (229 and 230).
This means that anything above 228+2 would be wrong, but they have given +10.
What am I missing?