Give reasons:
(i) Scandium (At. No. 21) is a transition element but calcium (At. No. 20) is not.
(ii) The transition metal ions such as Cu⁺ Ag⁺ and Sc³⁺ are colourless.
(iii) The melting points of transition metals of 3d series are much higher than the melting point of zinc of the same period.
(iv) The radius of Fe²⁺ is less than that of Mn²⁺.
(v) Actinides cations are coloured.

(i) An element is called a transition element if its atom or ion has incomplete i.e., partly filled d-orbital. In scandium (1s²2s²2p⁶3s²3p⁶3d¹4s²) the penultimate 3d orbital is partly filled and it is a transition element. In calcium (1s²2s²2p⁶3s² 3p⁶3d⁰4s²) the 3d orbital does not contain any electron, therefore, it is not a transition element.

(ii) An ion appears colourless if it does not contain any unpaired electron in its d-orbitals.
(a) Cu⁺(3d¹⁰4s⁰) is colourless because it does not contain any unpaired electron and 3d¹⁰orbital is completely filled.
(b) Ag⁺(14d¹⁰5s¹⁰) is colourless because it does not contain any unpaired electron in its 4d-orbital and 4d¹⁰ orbital is completely filled.
(c) Sc³⁺ (3d⁰4s⁰) is colourless because it does not contain any unpaired electron in its 3d orbital and 3d-orbital is empty.

(iii) The high values of melting points indicate that the atoms in transition metals are held together by strong metallic bonds. This is attributed to the availability of vacant d-orbitals in transition metals. This facilitates the formation of strong metallic bonds by delocalization of electron clouds in the metallic crystal lattice. In the 3d-series, Zinc has exceptionally low melting point and low boiling point, because it has completely filled d-orbitals. Therefore, the atoms in the crystal lattice of zinc are held less strongly as compared to the other members in their respective crystal lattices.

(iv) The radius of Fe²⁺ (1s²2s²2p⁶3s²3p⁶3d⁶4s⁰) is less than that of Mn²⁺(1s²2s²2p⁶3s²3p⁶3d⁵4s⁰) because in Fe²⁺ the electrostatic attraction between its nucleus and the outer electron clouds is stronger than that in case of Mn²⁺. Strong attraction causes a decrease in the distance between the nucleus and outer electron clouds.

(v) The cations of actinides are coloured because of the presence of unpaired electrons. The colour observed depends on the number of unpaired electrons. The colour is due to electronic transition within the 5f-levels.