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JEE Main levelSingle correct optionDifficulty 2 of 3cc026

Which of the following octahedral aqua ions is expected to be colourless?

A $[Cu (H X_{2} O) X_{6}] X^{2 +}$
B $[Ti (H X_{2} O) X_{6}] X^{3 +}$
C $[Cr (H X_{2} O) X_{6}] X^{3 +}$
D $[Sc (H X_{2} O) X_{6}] X^{3 +}$

Answer: D

Colour in these ions arises from d-d transitions, which need at least one d electron and a vacant d orbital to move into.

Sc X^{3 +}

is

d^{0}

so no d-d transition is possible. Ti(III) is

d^{1}

, Cr(III)

d^{3}

, Cu(II)

d^{9}

, all coloured.

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