Final answer:
An iodine atom becomes an ion by gaining one electron, resulting in an approximate 1.89% increase in the number of electrons, with no significant change in mass.
Step-by-step explanation:
When an iodine atom (I) becomes an ion (I-), there is a change in the number of electrons it has. The atomic number of iodine is 53, which means a neutral iodine atom has 53 protons and 53 electrons. If an iodine atom gains an extra electron, it becomes an iodine anion with a mass number of 127, having 53 protons, 74 neutrons (127 - 53 = 74), and 54 electrons. There isn't a percent change in terms of mass or charge because the addition of one electron does not significantly change the mass, and the concept of percent charge change isn't applicable in this context. However, we can talk about the increase in the number of electrons, which would be approximately 1.89% [(54 - 53) / 53 * 100%].