(i) Compounds of transition elements are coloured due to the presence of incomplete d-subshell, i.e., unpaired electrons. These unpaired electrons gain energy and get excited to higher energy level and when they one come back, they radiate energy in vidible rang due to that from coloured compounds.
(ii) MnO is basic while `Mn_(2)O_(7)` is acidic bacause the oxides in lower oxidation states of metals are basic and in their higher oxidation states, they are acidic.
`underset("Basic")underset(("lower oxidation"))overset(+2)(MnO)and underset("Acidic")underset(("higher oxidation"))overset(+7)(Mn_(2)O_(7))`
(b) Magnetic moment `(mu)=sqrt(n(n+2))B.M`
`underset(("ion"))underset(M^(2+)=3d^(6))(m(26)=1s^(2))2s^(2)2p^(6)3s^(2)3p^(6)4s^(2)3d^(6).`
Unpaied `e^(-)s=4,i.e., n=4`
`mu-sqrt(4(4+2))`
`mu=sqrt(24)=4.90B.M`
