Concept:Track the oxidation state of the metal as it transitions from a free element into an ionic compound.
Solution:- On the reactant side, Copper (Cu) is in its elemental, uncombined state. By rule, its oxidation number is \( 0 \).
- On the product side, Copper is part of the ionic compound Copper(II) nitrate, \( Cu(NO_3)_2 \).
- The nitrate polyatomic ion (\( NO_3^- \)) carries a fixed charge of \( -1 \).
- To balance two nitrate ions (a total of \( -2 \) charge), the single Copper atom must carry an oxidation state of \( +2 \).
- Therefore, the oxidation number changes from \( 0 \) to \( +2 \).
Why other options are incorrect:Copper very rarely exhibits a \( -1 \) or \( -2 \) state (it is a metal and prefers to lose electrons). While \( +1 \) is possible (Cu2O), the formula here demands \( +2 \).
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