Iodine (I) is a chemical element with atomic number 53. It's part of the group of elements called halogens.
Iodine is fairly reactive, but much less reactive than the lighter halogens. Iodine plays an important role in several bodily functions, for example in the synthesis of thyroid hormones, and is recommended in small amounts in our diets. For that reason, it is often added to table salt.
The only stable iodine isotope is 127, with 53 protons and 74 neutrons.