d and f Block Elements — CBSE Class 12 · 2022 Board Question with Solution

Give reasons for the following statements:
a) Scandium (Z = 21) is a transition element but Zn (Z = 30) is not.
b) [Ti(H₂O)₆]³⁺ is coloured while [Sc(H₂O)₆]³⁺ is colourless.
c) The physical and chemical properties of the 4d and 5d series of transition elements are more similar than expected.

Solution

(a) A transition element must have incompletely filled d-orbitals in its atom or common ions. Sc ([Ar]3d

^{1}

^{2}

) qualifies; Zn ([Ar]3d

^{10}

^{2}

) and Zn

^{2 +}

(3d

^{10}

) have a completely filled d-subshell in every state, so zinc is not a transition element.
(b) [Ti(H

_{2}

_{6}

]

^{3 +}

: Ti

^{3 +}

= 3d

^{1}

; the electron undergoes d–d transitions in the ligand-split orbitals, absorbing visible light — purple colour.
[Sc(H

_{2}

_{6}

]

^{3 +}

: Sc

^{3 +}

= 3d

^{0}

; with no d electron there is no d–d transition — colourless.
(c) Because of the lanthanoid contraction the 5d elements have practically the same radii as their 4d congeners; with equal size and charge, each 4d/5d pair (Zr–Hf, Nb–Ta, Mo–W) shows unexpectedly similar physical and chemical properties.

CBSE Class 12 Board · 2022 · Subjective

CBSE Class 12 · 2022 · d and f Block ElementsShort Answer

Solution

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