A borax bead containing a trace of a metal salt is heated in the oxidising flame and then cooled. It is violet, but becomes colourless in the reducing flame. The ion responsible for the violet colour and its spin-only magnetic moment (in BM, taking the ion as high spin) are

Answer: D

Manganese gives an amethyst (violet) bead as

Mn (BO X_{2}) X_{3}

in the oxidising flame; the reducing flame converts

Mn X^{3 +}

to colourless

Mn X^{2 +}

Mn X^{3 +}

d^{4}

with four unpaired electrons (high spin), so

μ = 4 \times 6 = 24 = 4.90

BM.

Cr X^{3 +}

would give a green bead in both flames.