The dichromate ion (Cr₂O₇^2–) has a structure consisting of two CrO₄ tetrahedra sharing one oxygen atom. This shared oxygen atom acts as a bridge between the two chromium atoms, forming a Cr–O–Cr bond. Each chromium atom is in the +6 oxidation state.

The structure can be visualized as follows:

Two chromium (Cr) atoms are each surrounded by four oxygen (O) atoms in a tetrahedral arrangement.
One oxygen atom is shared between the two chromium atoms (bridging oxygen).
The remaining six oxygen atoms are terminal (not shared).
The overall ion carries a charge of -2, which is delocalized over the oxygen atoms.

The bond lengths in the Cr₂O₇^2– ion are not all equal. The Cr–O bond lengths for the terminal oxygen atoms are shorter (approximately 1.65 Å) due to double bond character (Cr=O), while the Cr–O bonds involving the bridging oxygen are longer (approximately 1.78 Å) and are single bonds.

The dichromate ion is isoelectronic and isostructural with the dichromate ion (S₂O₇^2–).

Related Topics

Oxidation States: Chromium in dichromate is in the +6 oxidation state. Dichromate ions are strong oxidizing agents, especially in acidic medium, where they are reduced to Cr³⁺ ions.

Chromate-Dichromate Equilibrium: In aqueous solution, chromate (CrO₄^2–) and dichromate (Cr₂O₇^2–) ions exist in equilibrium, which is pH-dependent. The equilibrium shifts towards dichromate in acidic conditions and towards chromate in basic conditions.

Formulae

The conversion between chromate and dichromate involves a condensation reaction:

2 CrO 4_{}^{2 -} + 2 H^{+} \leftrightarrow Cr 2_{} O 7_{}^{2 -} + H 2_{} O