Read the following passage and answer the next five questions based on it.
The transition metals are very hard and have low volatility. Their melting and boiling points are high. In any row, the melting points of these metals rise to a maximum at d5 and fall regularly as atomic number increases. The high melting points of these metals are attributed to the involvement of greater number of electrons from (n – 1)d in addition to ns electrons in the interatomic metallic bonding.
How many electrons are needed in reduction of Cr2O72− to Cr3+ ?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Six
In \( \ce{Cr2O7^2-} \) each chromium is in the +6 state (2x + 7(−2) = −2, so x = +6). Reduction to \( \ce{Cr^3+} \) lowers each chromium by 3 units, and there are two chromium atoms, so 2 × 3 = 6 electrons are needed. The balanced half-reaction in acid shows the same:
\( \displaystyle \ce{Cr2O7^2- + 14H+ + 6e^- -> 2Cr^3+ + 7H2O} \)