Iodine is a chemical element represented by the symbol I and has the atomic number 53. The standard atomic mass of Iodine is officially reported as 126.90447 atomic mass units (u). This value is used for nearly all chemical calculations involving the element.
Defining the Atomic Mass of Iodine
The number 126.90447 u represents the standard atomic weight of Iodine, which is a weighted average of the masses of all its naturally occurring isotopes. Iodine is considered a monoisotopic element because virtually all naturally found Iodine consists of a single stable isotope, Iodine-127 (¹²⁷I).
Atomic mass units (u) are a standard measure used to express mass on an atomic or molecular scale. One atomic mass unit is defined as one-twelfth of the mass of a carbon-12 atom. The mass number, in contrast, is simply the total count of protons and neutrons in an atom’s nucleus, which is a whole number. For Iodine-127, the mass number is 127, while its actual atomic mass is slightly less due to the mass lost as binding energy holding the nucleus together.
Stable and Radioactive Forms
Iodine-127 is the only stable form of the element. It is known to have 37 different isotopes, ranging from Iodine-108 to Iodine-147, with these variations arising from differing numbers of neutrons. The difference in neutron count results in the different atomic masses of these isotopes, even though they all have the same chemical properties.
Radioactive isotopes are unstable forms of Iodine that undergo decay over time. For example, Iodine-131 (¹³¹I) is a well-known radioisotope used in medicine, which has an atomic mass close to 131 and a half-life of approximately 8.02 days. Another trace radioisotope, Iodine-129 (¹²⁹I), has an exceptionally long half-life of about 16 million years and is used in geochronological studies. The relatively short half-life of ¹³¹I makes it suitable for medical applications where the radiation must clear the body quickly.
Applications in Biology and Medicine
The stable isotope, Iodine-127, is a required trace element for human health. The thyroid gland uses it to synthesize the hormones thyroxine and triiodothyronine, which regulate metabolism, growth, and neurological development in the body. A lack of this stable form can lead to deficiency disorders, such as goiter.
The radioactive isotope Iodine-131 is used because the thyroid gland cannot distinguish it from the stable ¹²⁷I. This allows the targeted use of ¹³¹I for diagnosis and treatment, as the gland absorbs the radioactive material that destroys thyroid cells. This targeted destruction is used to treat hyperthyroidism and specific types of thyroid cancer. Other radioisotopes, such as Iodine-123 (¹²³I), which has a half-life of 13.2 hours, are used in medical imaging because their shorter decay time minimizes patient exposure.