Which ions in radiographic film change when exposed to X-rays?

• A. Silver bromide ions
• B. Silver chloride ions
• C. Iodide ions
• D. Sodium bromide ions

Answer: (C)

Radiographic film relies on silver halide crystals to form an image. When X-rays strike the film, energy is deposited in these crystals and electrons are liberated, allowing some silver ions (Ag+) to be reduced to metallic silver (Ag0) at exposed sites. In crystals that contain iodide, the iodide component is the part most directly involved in the initial chemical changes during exposure. The iodide ions (I−) respond to the X-ray energy, setting up the conditions for silver ions to be reduced and metallic silver to deposit where the exposure occurred. This change creates the latent image that is then amplified during development. The other ions listed are part of different silver halide chemistries, but the iodide-containing crystals are the ones whose halide ions undergo the primary changes that drive image formation.