Iodine (I), atomic number 53, is a chemical element found in the fifth period of the periodic table. It belongs to Group 17, a column of elements collectively known as the Halogens.
Defining the Halogen Group
Group 17 contains the highly reactive nonmetals known as the halogens, a name derived from Greek words meaning “salt-former.” The group includes fluorine, chlorine, bromine, iodine, astatine, and the synthetic element tennessine. All halogens share a fundamental characteristic: they possess seven valence electrons in their outermost shell.
This means each atom is only one electron shy of achieving a stable, full octet, which drives their strong chemical affinity. Halogens readily gain an electron to form a negatively charged ion, known as a halide anion with a charge of -1. Reactivity generally decreases down the group, making fluorine the most reactive and iodine one of the least reactive non-radioactive halogens.
Distinct Properties of Iodine
Iodine exhibits physical characteristics that distinguish it from its lighter halogen counterparts. At standard room temperature, iodine exists as a nearly black or purplish-gray crystalline solid, contrasting sharply with fluorine and chlorine (gases) and bromine (a liquid). This difference is due to iodine’s larger atomic size, which results in stronger intermolecular forces between its diatomic molecules.
Elemental iodine is notable for its ability to sublime, transitioning directly from a solid into a distinctive, deep violet vapor without passing through the liquid phase. When gently heated, solid iodine produces this vapor. The presence of this purple gas is a hallmark of the element, a property less pronounced in other halogens under normal conditions.
Iodine’s Biological Role
Beyond its chemical properties, iodine is recognized as a trace mineral necessary for human physiology and health. Since the body does not produce this element internally, it must be obtained through diet or supplementation. Iodine’s primary function involves the synthesis of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3).
These hormones, produced by the thyroid gland, regulate numerous metabolic processes. They control the rate of protein synthesis and enzymatic activity, determining a person’s overall metabolic rate. Adequate iodine intake is also important for the proper development of the central nervous system and skeletal structure in fetuses and infants.
A deficiency in this element can trigger health issues as the thyroid gland attempts to compensate. Insufficient iodine causes the pituitary gland to secrete increased amounts of thyroid-stimulating hormone. This leads to the thyroid gland enlarging, a condition termed goiter. Prolonged deficiency also causes hypothyroidism, resulting in symptoms like fatigue, weight gain, and cold sensitivity.
For pregnant women, severe iodine deficiency can lead to irreversible developmental issues in the child, such as cognitive impairment. To prevent these deficiency disorders, many countries have implemented salt iodization programs, making iodized table salt a primary source of the mineral. Other reliable sources of iodine include seafood, such as fish and seaweed, dairy products, and eggs.