Final answer:
Iodine-131 transforms into xenon-131 when it emits a beta particle during radioactive decay. This transformation is used in the medical field for the diagnosis and treatment of thyroid issues.
Step-by-step explanation:
When iodine-131 gives off a beta particle, the new isotope formed is xenon-131. The nuclear equation representing this beta decay process is I-131 → Xe-131 + electron. In this process, an iodine-131 nucleus emits a beta particle (an electron) and transforms into a xenon-131 nucleus, which is a stable isotope.
Iodine-131 has important medical applications, specifically in the diagnosis and treatment of thyroid cancer. It is also used for evaluating thyroid gland activity, as the thyroid naturally absorbs iodine. When used for therapy, the iodine-131 accumulates in the thyroid, and its beta emissions can then destroy excess thyroid tissue or thyroid cancer cells.