Mercury iodide is a chemical compound

Chemical compound

A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 of mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 and iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

. The chemical formula is Hg₂I₂. It is photosensitive and decomposes easily to mercury and HgI₂.

Synthesis

Mercury(I) iodide can be prepared by direct combination of mercury

Mercury (element)

Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...

 and iodine

Iodine

Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

:

2 Hg + I₂ → Hg₂I₂

Structure

In common with other Hg(I) (mercurous) compounds which contain linear X-Hg-Hg-X units, Hg₂I₂ contains linear IHg₂I units with an Hg-Hg bond length of 272 pm (Hg-Hg in the metal is 300 pm) and an Hg-I bond length of 268 pm. The overall coordination of each Hg atom is octahedral as it has in addition to the two nearest neighbours there are four other I atoms at 351 pm. The compound is often formulated as Hg₂²⁺ 2I⁻.

Uses

Mercury(I) iodide, called Protiodide

Protiodide

Protiodide is an iodide of mercury and was a very commonly used drug in the 19th century, prescribed for everything from acne to kidney disease. It was also the treatment of choice for syphilis...

, was used as a medicine in early years. However, due to the high toxicity of the substance, it is no longer used.