guy - 13-7-2006 at 11:28
Why is PbI2 yellow? Its not because of Pb2+ or I-, but somehow when they form a bond they have a color. Does anyone have any ideas?
woelen - 13-7-2006 at 12:08
PbI2 is not a purely ionic compound, but it is covalent. So, the colorless ions I(-) and Pb(2+) combine into a completely new entity, PbI2. That
entity is yellow.
There are many examples of this, just to name a few:
Hg(2+) + 2I(-) --> HgI2 (colorless --> very bright red/orange)
Pb(2+) + S(2-) --> PbS (colorless --> black)
Cd(2+) + S(2-) --> CdS (colorless --> bright yellow)
Cd(2+) + Se(2-) --> CdSe (colorless --> red)
Many oxides of colorless ions also are colored, e.g. Ag2O is black, HgO is red.
The reason that these compounds have colors has to do with the formation of larger orbitals. The ions have orbitals spread out over an atom, the
covalent compounds have larger orbitals, spread out over the molecules. These larger orbitals allow interaction with longer wave length light, and
this interaction can go into the visible spectrum (the smaller ions absorb in the UV range) and that gives them color.