Q.Write the structure of dichromate ion.

Cr2O7²⁻ = two tetrahedral CrO4 units linked through one shared (bridging) oxygen atom, i.e. an O3Cr–O–CrO3 arrangement with a Cr–O–Cr bridge.

Structure of the dichromate ion (Cr2O7²⁻):

• It is made of two chromate-type tetrahedral units.
• Each chromium atom is at the centre of a tetrahedron of four oxygen atoms (Cr is sp3, tetrahedral).
• The two tetrahedra are joined together by sharing one corner oxygen atom, which acts as a bridge between the two chromium atoms — this gives the Cr–O–Cr linkage.
• So the ion can be written as O3Cr–O–CrO3, with three terminal oxygens on each Cr plus one central bridging oxygen (6 terminal + 1 bridging = 7 O atoms in total, charge 2−).
• The bridging Cr–O–Cr bond (about 126 pm) is longer than the terminal Cr–O bonds (about 116 pm); the Cr–O–Cr bond angle is about 126°.

A simple sketch:

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