Why is silver iodide yellow

Silver chloride is white silver bromide is pale yellow and silver iodide has a rich yellow colour. We might first think that the change in colour was due to Agl incorporating the iodide anion, yet Nal or HI are both colourless, so the colour does not come from the iodide ions on their own. We need to find a different explanation. 

Silver iodide also has other anomalous properties it is physically soft - it can even be beaten into a sheet, unlike the overwhelming majority of ionic compounds. More unusual still, it is slightly soluble in ethanol. Clearly, silver iodide is not a straightforward ionic compound. In fact, its properties appear to overlap between covalent (see Table 2.6) and ionic (see Table 2.7). 

In other words, the valence bonds approach is suitable for compounds showing purely ionic or purely covalent behaviour we require molecular orbitals for a more mature description of the bonding in such materials. So the yellow colour of silver iodide reflects the way the bonding is neither ionic nor covalent. We find, in fact, that the charge clouds of the silver and iodide ions overlap to some extent, allowing change to transfer between them. We will look at charge transfer in more detail on p. 459. 

Valency is the number of electrons lost, borrowed or shared in a chemical bond. Formal charges are indicated with Arabic numerals, so the formal charge on a copper cation is expressed as Cu2+, meaning each copper cation has a deficiency of two electrons. In this system of thought, the charge on the central carbon of methane is zero. 

A trimer is a species comprising three components (the Latin tri means three ). The W3O9 trimer has a triangle structure, with a W03 unit at each vertex. 

---