# Which of the following transition metal ions are colourless?

$(a)\;Ti^{2+}\qquad(b)\;Cu^{+2}\qquad(c)\;Ni^{+2}\qquad(d)\;Zn^{+2}$

Answer: Zn$^{+2}$
Since Zn$^{+2}$ is a 3d$^{10}$ ion, it does not absorb visible light even though the d-orbitals are correct for absorbing visible wavelengths. Hence it's colourless.
edited Jul 31, 2014