A silver halide is one of the compounds formed between silver and one of the halogens — silver bromide (AgBr), chloride (AgCl) and iodide (AgI). As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX. Although most silver halides involve silver atoms with oxidation states of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of which silver(II) fluoride is the only known stable one.
Silver halides are used in photographic film and photographic paper where an emulsion of silver halide crystals in gelatin is coated on to a film base, glass or paper substrate. The gelatin is a vital part of the emulsion as it contains trace elements (such as sulfur) which increase the light sensitivity of the emulsion. Photons cause electrons to be promoted to a conduction band (de-localized electron orbital with higher energy than a valence band) which can be attracted by the trace element (dopant) and then combined with a silver ion to form silver metal. Silver bromide and silver chloride may be used separately or combined, depending on the sensitivity and tonal qualities desired in the product. Silver iodide is always combined with silver bromide or silver chloride, except in the case of daguerreotype production where a daguerreotype (one of the oldest photographic processes) is developed with pure red light instead of mercury vapors (a method known as the Bequerelle method, named for the inventor who discovered the phenomenon).
When a silver halide crystal is exposed to light, a sensitivity speck on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image). If the speck of silver contains approximately four or more atoms, corresponding to an absorption of four or more photons, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver. Areas of the emulsion receiving larger amounts of light (reflected from a subject being photographed, for example) undergo the greatest development.
Silver halides are extremely insoluble in water. Silver nitrate can be used to precipitate halides; this application is useful in quantitative analysis of halides. The precipitation of silver halides via silver nitrate is also useful for abstracting halide leaving groups.