Question

Explain with reasons:
(i) Transition elements show variable oxidation states.
(ii) Transition elements form coloured compounds.
(iii) The size of Lanthanide elements decreases from La to Lu.

Answer 1

(i)Transition elements show variable oxidation state because the energy difference between $ns$ and $(n-1)d$ orbitals is very small. Therefore, electrons of (n-1)d orbital can also participate in bond formation after utilizing $ns$ electrons.

(ii)Transition metal atoms in free state (non-bonded state) have five degenerate d-orbitals ($d_{xy},d_{yz},d_{zx},d_{x^2-y^2},d_{z^2}$). On formation of complex with ligands, the degeneracy breaks and forms $d_{xy},d_{yz},d_{zx}$ and $d_{x^2-y^2},d_{z^2}$ as two set of degenerate d-orbital. Upon irradiation of light, the electron from lower set of d-orbitals is excited to higher set of d-orbitals. In case of transition metal complexes, the energy difference between these two sets of d-orbitals lies in the visible range of the spectrum. One colour of visible light (white light) is absorbed and other colour is reflected due to this, the complex appears coloured. The colour of the complex is complementary to the colour that is absorbed.

(iii) The size of an atom or ion is determined by the effective nuclear charge ($Z_{eff}=Z-\sigma$). The shielding effect ($\sigma$) of inner electrons and the number of electrons in the shell is considered. Since the shielding effect of 4 f-electrons is poor, with increase in the atomic number, the increase in nuclear charge is not properly shielded. Hence, the effective nuclear charge increases along lanthanoid series. It results in decrease in size of elements with increasing atomic number. This phenomenon is called Lanthanoid contraction.