Final answer:
Iodine-127 is the more stable isotope, being non-radioactive, while iodine-129 is radioactive with a long half-life. Generally, isotopes with shorter half-lives are more radioactive, so while iodine-129 is more radioactive than stable iodine-127, it is less so compared to isotopes like iodine-123.
Step-by-step explanation:
When comparing the two isotopes of iodine, iodine-127 and iodine-129, it is noted that iodine-127 is the more stable isotope. Iodine-127 is actually considered non-radioactive, while iodine-129 is indeed radioactive but with a relatively long half-life. The stability of an isotope is determined by the neutron to proton ratio within the nucleus, and iodine-127 has a balance that results in a stable, non-radioactive nucleus.
In regards to which isotope would be more radioactive, in general, isotopes with shorter half-lives are more radioactive as they decay at a faster rate. For instance, using isotopes iodine-131 with a half-life of 8.04 days and iodine-123 with a half-life of 13.2 hours as examples, iodine-123 is more radioactive than iodine-131 due to its shorter half-life.
Therefore, iodine-129, having a lower activity and a longer half-life than these examples, would be less radioactive in comparison to an isotope like iodine-123, but is still more radioactive than the stable iodine-127.