Question

all the compounds of scandium are colourless give reason

Answer 1

Complexes of transition metal ions (e.g. Ti3+ and V3+) are usually coloured due to d-d transitons within the partly filled d orbitals. Although there is a partially filled d level in Scandium ( [Ar] 3d14s2 ), when it forms Sc3+, it loses all three outer electrons so the 3d level is empty. So although scandium is a member of the d block like titanium and vanadium, its ion (Sc3+) hasn't got any d electrons left to move around. Sc3+ complexes are therefore colourless because no visible light is absorbed.

Answer 2

Compounds of transition metals are coloured because they absorb light for d-d transition, however in case of Sc as the d orbital is empty after formation of compound hence there is no d-d transition thus the compounds appear colourless.