Why are iodine precipitates yellow? [closed]

Question

Why do iodine precipitates appear yellow? Learning about precipitates I noticed that compounds like Lead Iodide and Silver Iodide are yellow.

Answer 1

There is certainly some variance between different cases, but a major factor is that bonds to iodine are weaker than bonds to lighter halogens. The large size of the iodine atom decreases electrostatic forces that are ultimately responsible for both ionic and covalent bonding.

The lowered bond energy then leads to lowered electronic transition energies, so that these transitions which are ordinarily in the ultraviolet region end up overlapping into the violet/blue portion of the visible region; absorbing that out of white light then leaves a complementary yellow or greenish yellow color. Silver iodide nanoparticles generated by pulsed electric discharge show this overlap, given that visible light lies between 400 and 800 nm wavelength. From Ref. [1]:

The effect is actually most dramatic with carbon. Steric effects weaken the bonding of the relatively small carbon atom to multiple iodine atoms, leading to decreasing electronic transition energies; we see a deepening in color (absorption moves more into the visible region) as we add more iodine atoms to an iodine-substituted methane:

$\ce{CH3I}$: Colorless liquid

$\ce{CH2I2}$: Colorless liquid

$\ce{CHI3}$: Pale, light yellow, opaque crystals

$\ce{CI4}$: Dark violet crystals, forming a dark red solution in diethyl ether

Reference

1. Tseng, KH., Yeh, CT., Chung, MY. et al. (2021). "A study of preparing silver iodide nanocolloid by electrical spark discharge method and its properties." Sci Rep 11, 20457. https://doi.org/10.1038/s41598-021-99976-5