Final answer:
The oxidation number of nickel in NiCO₃ is +2. This is determined by balancing the -2 charge of the carbonate ion (CO₃²⁻) with the positive oxidation number of nickel, which must be +2 to achieve a neutral compound.
Step-by-step explanation:
To determine the oxidation number of nickel in NiCO₃, we must consider the oxidation states of each component in the compound. The compound is neutral overall, so the sum of oxidation numbers must equal zero. The carbonate ion (CO₃²⁻) has a charge of -2. Therefore, the nickel ion must balance this with a positive oxidation number that results in the compound having no overall charge.
Since the carbonate ion is -2, and we know that nickel is balancing this charge, the oxidation number for nickel must be +2 to ensure that the total charge of the compound is zero. Thus, the correct answer is c) +2, giving us Ni²+ and CO₃²⁻ in NiCO₃.