Question

The incorrect statement among the following is :

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• A. Actinoid contraction is greater for element than Lanthanoid contraction.
• B. Lanthanoids are good conductors of heat and electricity.
• C. Most of the trivalent Lanthanoid ions are colourless in the solid state.
• D. Actinoid are highly reactive metals, especially when finely divided.

Answer 1

The correct option is C Most of the trivalent Lanthanoid ions are colourless in the solid state.

most of the trivalent lanthanoid compounds except that of $L{a}^{+3}$ and $L{u}^{+3}$ are coloured, both in the solid state and in the aqueous solution. These ions are coloured due to the presence of unpaired f-electrons.
Lanthanides are good conductors of heat and electricity.
Actnoids:
Pyrophoricity: Many actinide metals, hydrides, carbides, alloys and other compounds may ignite at room temperature in a finely divided state.
Actinoid contraction is greater for element than Lanthanoid contraction because in actinoids, 5f orbitals are filled. These 5f orbitals have a poorer shielding effect than 4f orbitals (in lanthanoids). Thus, the effective nuclear charge experienced by electrons in valence shells in case of actinoids is much more than that experienced by lanthanoids. Hence, the size contraction in actinoids is greater as compared to that in lanthanoids.
Hence option (a) will be the answer