a) Give one example each of an amphoteric and an acidic oxide of transition metals.
b) Describe the trends in the following cases:
i) melting points of elements in the 3d transition series
ii) atomic sizes of elements in the 4f inner transition series
b) Describe the trends in the following cases:
i) melting points of elements in the 3d transition series
ii) atomic sizes of elements in the 4f inner transition series
Solution
(a) Amphoteric oxide: CrO (also VO); acidic oxide: CrO or MnO (higher oxidation states are acidic).
(b)(i) Melting points first rise from Sc, peaking around V–Cr where the number of unpaired d electrons (hence metallic-bond strength) is maximum, show an anomalous dip at Mn (stable 3d4s electrons participate poorly), and then fall steadily to Zn (no unpaired electrons).
(ii) Across the 4f series the atomic (and Ln) radii decrease slowly and steadily from La to Lu — the lanthanoid contraction, due to poor shielding by 4f electrons.
(b)(i) Melting points first rise from Sc, peaking around V–Cr where the number of unpaired d electrons (hence metallic-bond strength) is maximum, show an anomalous dip at Mn (stable 3d4s electrons participate poorly), and then fall steadily to Zn (no unpaired electrons).
(ii) Across the 4f series the atomic (and Ln) radii decrease slowly and steadily from La to Lu — the lanthanoid contraction, due to poor shielding by 4f electrons.
CBSE Class 12 Board · 2001 · Delhi Compartment · Subjective