Answer the question on the basis of passage given below:
In the periodic table, the d-block contains the elements of group 3 to 12. The d-orbitals are progressively filled in each of the four long periods. The elements of d-block referred as transition metals have partly filled d-orbitals and exhibit certain characteristic properties such as variety of oxidation states, formation of coloured ions, act as catalyst and show paramagnetic behaviour.
The two inner transition metals series 4f and 5f are known as Lanthanoids and Actinoids respectively. The lanthanoids resemble one another more closely as compared to ordinary transition elements in any series.
The bonds present in the structure of dichromate ion are
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Six equivalent \(Cr–O\) bonds and one \(Cr–O–Cr\) bond
In the dichromate ion, \( \ce{Cr2O7^{2-}} \), each chromium (+6: 2x + 7(−2) = −2, x = +6) is surrounded tetrahedrally by four oxygen atoms. The structure consists of two tetrahedra sharing one corner: one oxygen atom bridges the two Cr atoms, with a Cr–O–Cr bond angle of 126°.
Each Cr has three terminal oxygens: 2 × 3 = 6 terminal Cr–O bonds, all equivalent because of resonance.
The bridging oxygen forms the Cr–O–Cr link (two longer Cr–O bonds).
So the ion has six equivalent Cr–O bonds and one Cr–O–Cr bond.
Why the other options are incorrect
Option (1) Six equivalent Cr-O bonds and one O-O bond — Incorrect. There is no peroxide (O–O) linkage in \( \ce{Cr2O7^{2-}} \); the two halves are joined through one oxygen atom.
Option (2) Six equivalent Cr-O bonds and one Cr-Cr bond — Incorrect. The two chromium atoms are not bonded to each other; they are joined through the bridging oxygen.
Option (3) Eight equivalent Cr-O bonds — Incorrect. There are eight Cr–O bonds in all, but the two bonds to the bridging oxygen are longer and are not equivalent to the six terminal ones.
NCERT (2026-27 edition):
Potassium Dichromate – …
