Iodide

Quick Facts

Formula: I⁻

Pronunciation: EYE-oh-dide

Etymology: Iodine + -ide

Meaning: The negative ion formed when iodine gains one electron.

What It Really Means

Iodide is the largest of the common halide ions, and that size gives it a unique personality in chemistry. Because it is big and polarizable, iodide is an excellent nucleophile in polar aprotic solvents. It can attack carbon quickly. But because the charge is spread out over a large volume, it is a weak base. That is a classic example of the difference between nucleophilicity and basicity, and examiners love testing it. Now here is a common confusion. Iodide, iodine, and iodate are three different things. Iodide is I minus, the ion. Iodine is I2, the grey-black solid that sublimes to a purple vapour. Iodate is IO3 minus, an oxyanion. Same root, very different chemistry. Also, iodide is important in your body. Your thyroid gland uses iodide to make thyroid hormones. Without enough iodide, you can develop goitre, an enlarged thyroid. That is why table salt is often iodised. A small amount of iodide prevents a serious health problem. In the lab, iodide is used in titration with thiosulfate, a technique called iodometric titration. And potassium iodide is added to some table salts to prevent caking. So iodide is not just a chemistry concept. It is part of public health.

Common Examples

• Potassium iodide (KI)
• Sodium iodide (NaI)
• Hydroiodic acid (HI)
• Silver iodide (AgI)

Related Terms