CuI is the elemental symbol abbreviation for the ionic chemical compound copper (I) iodide, also known as cuprous iodide. CuI is a solid formed from a mixture of the metallic element copper and the halogen iodine. It has various applications in chemistry and industry.

An ionic compound forms when an atom of one element donates one or more electrons to an atom of a different element. The first atom becomes positively charged and the second becomes negatively charged. The two atoms now stick together due to the electrostatic attraction between their opposite charges. This is known as an ionic bond. Sodium chloride, or table salt, is a well-known ionic compound.

CuI is an ionic compound that has each molecule made from one atom of copper (Cu) and one atom of iodine (I). The copper atom is positively charged and the iodine is negatively charged, so there is an ionic bond between them. It is written in full as copper (I) iodide to show that the copper has an oxidation state of 1, which means it has given up one electron.

CuI is a white crystalline powder with a density of 5.7 grams per cubic centimeter. It melts at 606 degrees C. It is essentially insoluble in water, which is unusual for an ionic compound. It is found naturally as the mineral marshite but can also be synthesized chemically.

CuI is an ingredient in various synthetic chemical reactions. It is also added to nylon to increase resistance to heat and light and has been used to produce a test paper to show the presence of mercury vapor. CuI has been used to "seed" clouds to produce rain.