Which color is associated with the nickel ion in solution, as stated?

• A. Red
• B. Blue
• C. Green
• D. Yellow

Answer: (C)

The color seen for nickel(II) in solution comes from transitions of its d-electrons when nickel is surrounded by water ligands. Nickel(II) has a d^8 configuration, and in the octahedral field created by six water molecules, the d-orbitals split. The electrons can absorb light for those d-d transitions, and for Ni2+ in water the absorbed light is in the red part of the spectrum. What we actually perceive is the complementary color, which is green. So the aqueous nickel ion typically appears green (often described as pale green to green). The other color options would reflect different ions or ligand environments, but for Ni2+ in aqueous solution green is the characteristic color.