Explain the following observations:
a) The elements of the d-series exhibit a larger number of oxidation states than the elements of the f-series.
b) Cu⁺ salts are colourless while Cu²⁺ salts are coloured (Z of Cu = 29).
a) The elements of the d-series exhibit a larger number of oxidation states than the elements of the f-series.
b) Cu⁺ salts are colourless while Cu²⁺ salts are coloured (Z of Cu = 29).
Solution
(a) In the d-series the (n−1)d and ns orbitals are close in energy, so a variable number of electrons can take part in bonding — a wide range of oxidation states. In the lanthanoids the 4f electrons lie deep inside and are shielded from the surroundings; essentially only the 5d/6s electrons bond, so +3 dominates with few exceptions.
(b) Cu: [Ar]3d — the d-shell is full, no d–d transition is possible, so Cu(+1) salts are colourless.
Cu: [Ar]3d — an incomplete d-shell allows d–d transitions that absorb visible light, giving Cu(+2) salts their blue colour.
(b) Cu: [Ar]3d — the d-shell is full, no d–d transition is possible, so Cu(+1) salts are colourless.
Cu: [Ar]3d — an incomplete d-shell allows d–d transitions that absorb visible light, giving Cu(+2) salts their blue colour.
CBSE Class 12 Board · 2003 · Delhi · Subjective