Question

Why are Chromate and Dichromate Anions colored?

Answer 1

Hint: The chromate anion, $Cr{O_4}^{2-}$, is found in chromate salts. The dichromate anion, $C{r}_2{O_7}^{2-}$, is found in dichromate salts. They are moderately strong oxidising agents and are chromium oxyanions in the 6+ oxidation state. The chromate and dichromate ions can interconvert in an aqueous solution.

Complete answer:
The presence of transition ions (chromium (vi) ions) is one of the main reasons why chromate and dichromate anions are coloured. Transition elements, in general, produce coloured ions. Alternatively, we could say that the colours we see are the complementary colours of a bond's vibration wavelength. $C{r}^{6+}$ is found in the chromate ion. Due to its high charge, this ion is highly oxidising, causing charge transitions to occur in the visible region of the spectrum, giving rise to the colour. They have full orbital permission, so the bands are thick and the colour is deep. Dichromate is similar to chromium oxide, but it has two chromium atoms and seven oxide atoms (i.e., $C{r}_2{O}_7$). In acidic conditions, chromate and dichromate oxidise rapidly, but only slowly in basic conditions.
For a chromate ion, we see red slash orange and yellow when we look at the dichromate ion. The colours we see are primarily light wavelengths that are absorbed by the molecules.

Note:
Chrome plating uses chromates and dichromates to protect metals from corrosion while also improving paint adhesion. Heavy metal chromate and dichromate salts, lanthanides, and alkaline earth metals are used as pigments because they are only very slightly soluble in water. Chrome yellow, a lead-containing pigment, was used for a long time before environmental regulations made it illegal.