The formula of dichromate ion is

The dichromate ion is a polyatomic ion composed of chromium and oxygen. Its chemical formula is represented as Cr₂O₇^2-. This formula is derived from the combination of two chromium atoms and seven oxygen atoms, with a total charge of -2.

Dichromates are often found in acidic solutions and are known for their bright orange color. They are chemically related to chromates (CrO₄^2-), and both can interconvert depending on the pH of the solution. In acidic environments, the chromate ion transforms into the dichromate ion, as shown in this equilibrium:

2 CrO₄^2- + 2 H⁺ ↔ Cr₂O₇^2- + H₂O

The correct formula of the dichromate ion is:

b. Cr₂O₇^2-

• Option a (Cr₂O₄^-) is incorrect because the dichromate formula does not match this structure.
• Option c (Cr₂O₄⁺²) and option d (Cr₂O₄⁺³) are also incorrect since the dichromate formula does not have positive charges; it is always negatively charged.