Which light-sensitive chemical is used to coat the celluloid film in an intra-oral x-ray film packet?

The appropriate light-sensitive chemical used to coat the celluloid film in an intra-oral x-ray film packet is silver bromide salts. This compound is integral to the photographic process in dental radiography. When exposed to x-rays, silver bromide undergoes a chemical reaction that creates a latent image on the film. Upon subsequent processing, this latent image is developed into a visible image, allowing for the examination of dental structures.

Silver bromide salts are particularly effective due to their sensitivity to the wavelengths emitted by x-rays, enabling the capture of high-quality images needed for diagnostic purposes. The process relies on their ability to undergo changes in the presence of radiation, thus making them essential in dental and other forms of radiography.

In contrast, lead is primarily used as a radiation shielding material to protect both patients and dental healthcare providers from unnecessary exposure, without having any role in film development. Mercury, while it has historical uses in some dental applications, is not relevant in the context of film exposure or development. Rare earth chemicals are used in some modern imaging systems, but not in the traditional context of celluloid x-ray film packets. Therefore, silver bromide salts' unique properties highlight their significance as the correct answer for this question.