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?

• Wikipedia gives even more: 202 to 234. en.m.wikipedia.org/wiki/Isotopes_of_radium – badjohn Mar 27 at 18:17
• I've added the homework-and-exercises tag. In the future, please use this tag on this type of question. – Ben Crowell Mar 27 at 18:24
• @badjohn The biggest one there is still only +7 – Gerard Way Mar 27 at 18:43
• I suspect that you have been given some heuristics on, given Z, how broad a range of A one might suspect. – Jon Custer Mar 27 at 18:51
• Yes but more than you were expecting. It seems that the example is towards the heavier end of the range. – badjohn Mar 27 at 19:18

• The OP is talking about radium, not radon, but I suppose you actually meant radium. But anyway, both elements are radioactive, with no stable isotopes. The most stable radium isotope, $^{226}$Ra, has a halflife of 1600 years. – PM 2Ring Mar 28 at 5:10