Iodine (I). Diagram of the nuclear composition and electron configuration of an atom of iodine-127 (atomic number: 53), the most common isotope of this element. The nucleus consists of 53 protons (red) and 74 neutrons (yellow). 53 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Iodine, present in thyroid hormones and some seaweeds, is a halogen in group 17, period 5, and the p-block of the periodic table. In elemental form it is a blue-black solid (I2) that melts at 113 degrees Celsius.