Q.Write any three differences between Lanthanoids and Actinoids.

Lanthanoids (filling 4f) and actinoids (filling 5f) differ mainly in radioactivity, the range of oxidation states shown, and how strongly the f-orbitals participate in bonding/complex formation.

Three key differences:

1. Radioactivity: Actinoids are all radioactive (uranium onward are entirely man-made, 'transuranic'), while lanthanoids (except promethium, Pm) are non-radioactive/stable.
2. Oxidation states: Lanthanoids show mainly one dominant oxidation state, +3, with only occasional +2/+4 states; actinoids show a much wider range of oxidation states (+3 up to +6 or even +7 for some, e.g. U, Np, Pu), because 5f, 6d, and 7s orbitals in actinoids lie closer in energy than the corresponding 4f, 5d, 6s orbitals in lanthanoids, allowing more electrons to take part in bonding.
3. Orbital involvement/complex formation: In lanthanoids, the 4f orbitals are deeply buried (well shielded by the 5s and 5p orbitals) and take almost no part in bonding, so lanthanoid complexes are relatively few and less stable; in actinoids, the 5f orbitals extend further and participate more in bonding, so actinoids form more, and more stable, complexes with ligands. …

Lanthanides and actinides differ chiefly in which f-subshell they fill, the range of oxidation states they show, and their radioactivity.

Both series are inner-transition (f-block) elements, but they show significant differences arising from the different sizes and shielding behaviour of the 4f and 5f orbitals:

1. Orbital filled / general electronic configuration: In lanthanides (atomic numbers 58-71, Ce to Lu) the differentiating electron progressively fills the 4f subshell (general configuration $[Xe]4f^{1-14}5d^{0-1}6s^2$). In actinides (atomic numbers 90-103, Th to Lr) the differentiating electron fills the 5f subshell (general configuration $[Rn]5f^{0-14}6d^{0-2}7s^2$).

2. Oxidation states: Lanthanides show predominantly the +3 oxidation state in almost all their compounds (occasionally +2 or +4 for a few members, e.g. $Ce^{4+}$, $Eu^{2+}$), because the 4f electrons are deeply buried and shielded, and are not easily involved in bonding beyond the outer 3 electrons. Actinides show a much wider range of oxidation states, from +3 up to +7 (e.g. $UO_2^{2+}$ has U in +6, and Np/Pu/Am can reach +7), because the 5f, 6d and 7s energy levels lie close together in energy, so more electrons of the 5f/6d subshells can take part in bonding.

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