Concept:To determine electron transfer per atom, calculate the change in the oxidation state of that specific element from reactant to product.
Solution:- First, find the oxidation state of Chromium (Cr) in the dichromate ion (\( Cr_2O_7^{-2} \)):
\( 2x + 7(-2) = -2 \implies 2x - 14 = -2 \implies 2x = +12 \implies x = +6 \).
- On the product side, Chromium is \( Cr^{+3} \), so its oxidation state is \( +3 \).
- The oxidation state of a single Chromium atom goes from \( +6 \) down to \( +3 \).
- This mathematical reduction of 3 indicates a gain of 3 electrons (\( +3e^- \)) per Chromium atom.
Why other options are incorrect:The entire dichromate molecule (which contains two Cr atoms) gains a total of 6 electrons. However, the question specifically asks for the transfer "by chromium atom" (singular), which is 3.
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