What is the correct order of increasing strength of the following spe…

The correct answer is Option (2) → Cr₂O₇²⁻ < Cr³⁺ < Cr²⁺

A reducing agent gives away electrons and is itself oxidised. The three species hold chromium in the +6, +3 and +2 states; the lower the oxidation state of chromium, the more readily the species gives away electrons and the stronger it is as a reducing agent.

• \( \ce{Cr2O7^2-} \) (weakest): 2x + 7(−2) = −2, so x = +6. Cr (Z = 24, [Ar]3d⁵4s¹) has lost all six valence electrons, so it has none left to give. Dichromate takes up electrons instead: it is a strong oxidising agent in acid and is reduced to \( \ce{Cr^3+} \):

      \( \displaystyle \ce{Cr2O7^2- + 14H+ + 6e- -> 2Cr^3+ + 7H2O} \)

• \( \ce{Cr^3+} \) (middle): 3d³, the stable state into which both \( \ce{Cr^2+} \) and dichromate change. It has little tendency to give up more electrons.

• \( \ce{Cr^2+} \) (strongest): 3d⁴. It readily loses one electron to reach the stable d³ state, so it is a strong reducing agent and even liberates hydrogen from a dilute acid:

      \( \displaystyle \ce{2Cr^2+(aq) + 2H+(aq) -> 2Cr^3+(aq) + H2(g)} \)

NCERT (2026-27 edition):
4.3.4 Oxidation States – “… Thus Cr(VI) in the form of dichromate in acidic medium is a strong oxidising agent, whereas MoO₃ and WO₃ are not.”

Example 4.4 – “Cr²⁺ is reducing as its configuration changes from d⁴ to d³, the latter having a half-filled t_2g level (see Unit 5).”

4.3.8 Chemical Reactivity and E^o Values – “… The ions Ti²⁺, V²⁺ and Cr²⁺ are strong reducing agents and will liberate hydrogen from a dilute acid, e.g.,

    \( \displaystyle \ce{2Cr^2+(aq) + 2H+(aq) -> 2Cr^3+(aq) + H2(g)} \)”