Each element has a unique atomic number. The atomic number is defined as the number of protons that an element has. Using a periodic table (unless you have all the atomic numbers memorized, which would be AMAZING!), I can determine that Iodine has an atomic number (AKA- number of protons) of 53.
The atomic mass on the other hand is the number of protons plus the number of neutrons (so, essentially, it is the mass of the nucleus). Iodine has an atomic mass of 126.90 amus (atomic mass units), which we will round to 127. Therefore,
number of protons + number of neutrons = 127
We already know the number of protons is 53 from the atomic number. So,
53 + number of neutrons = 127
number of neutrons = 127 - 53
number of neutrons = 74
Hence, Iodine has an atomic number of 53 and an atomic weight of 127 (53 protons and 74 neutrons)
The most common, stable form of iodine has an atomic number of 53 (protons) and an atomic weight of 127 (53 protons plus 74 neutrons). Because its nucleus has the "correct" number of neutrons, it is stable and is not radioactive.