(a) Which metal in the first transition series (3d series) exhibits +1 oxidation state most frequently and why?
(b) Which of the following cations are coloured in aqueous solutions and why?
`Sc^(3+),V^(3+),Ti^(4+),Mn^(2+)(At. "no".Sc=21,V=23,Ti=22,Mn=25)`

Explanation

(a) Cu is the only metal in the first transition series (3d series) which shows+ 1 oxidation state most frequently. This is because the electronic configuration of Cu is `3d^10 4s^1` and after losing one electron it acquires cl stable `3d^1`0 configuration.
(b) The colour of cations is dependent on the number of unpaired electrons present in d-orbital. The electronic configuration of the following cations is as follows :
`"Sc(Atomic number 21)"=3d^14S^2and Sc^(3+)`
`=3d^@4S^@`
As d - orbital is empty, it is colourless. V(Atomic number 23)
`3d^34S^2andV^(3+)=3d^24S^@`
As d- orbital is having 2 unpaired electrons, it undergoes d-d- transition and show green colour.
Ti(Atomic number 22)=`3d^24s^2andTi^(4+)=3D^@4S^@`
As d- orbital is empty, it is colourless.Mn (Atomic number 25)
`=3d^54S^2`
`and Mn^(2+)=3d^54S^@`
As d- orbital is having 5 unpaired electrons, it shows pink colour.