Which of the following is the INCORRECT statement for dichromate ion?

The correct answer is Option (2) → Cr-O (Terminal) bond length is 189 pm.

The dichromate ion, Cr₂O₇²⁻, is made of two CrO₄ tetrahedra sharing one corner oxygen. Each chromium (+6) is bonded to three terminal oxygens and to the one bridging oxygen:

• The terminal Cr–O bonds are short, 163 pm, because they have partial double bond character.

• The bridging Cr–O bonds are longer, 179 pm, as they are single bonds.

• The Cr–O–Cr angle at the bridging oxygen is 126°.

• Option (1) Cr-O-Cr bond angle is 126°. — A correct statement for dichromate ion. This is the angle at the shared bridging oxygen.

• Option (2) Cr-O (Terminal) bond length is 189 pm. — An incorrect statement (terminal Cr–O is 163 pm). A terminal bond of 189 pm would be longer even than the bridging bond (179 pm).

• Option (3) The dichromate ion consists of two tetrahedra. — A correct statement for dichromate ion. The two \( \ce{CrO4} \) tetrahedra are joined through one common oxygen atom.

• Option (4) The dichromate ion is orange in colour. — A correct statement for dichromate ion. Yellow chromate turns into orange dichromate when the solution is acidified: \( \ce{2CrO4^2- + 2H^+ -> Cr2O7^2- + H2O} \)

NCERT (2026-27 edition):
Potassium Dichromate – …