Q.Which of the following does not belong to actinoid series?

Th, Am, and Np are actinoids (5f-block, Z = 89-103); Sm (samarium, Z = 62) is a lanthanoid (4f-block, Z = 58-71).

The actinoid series consists of the 14 elements following actinium (Z = 89 to 103) in which the 5f orbitals are progressively filled: it includes Th (thorium, Z=90), Np (neptunium, Z=93), and Am (americium, Z=95). Samarium (Sm, Z=62), on the other hand, lies in the lanthanoid series (Z=58 to 71, 4f-filling), which comes one f …

Although actinoids display a wider range of oxidation states than lanthanoids (due to the close energies of 5f, 6d, and 7s orbitals), the +3 state remains the single most common oxidation state throughout the actinoid series.

Actinoids ($_{90}Th$ to $_{103}Lr$) show more variable oxidation states than lanthanoids (e.g. Th commonly +4, U up to +6, Np/Pu/Am up to +6/+7) because the 5f, 6d and 7s orbitals lie close in energy, so more electrons can participate in bonding. Even so, as one moves across the series (particularly for the later, heavier actinoids from curium onward), the +3 oxidation state becomes progressively the most stable and most commonly observed state, ma …

The actinide series (Z=90-103, following actinium) fills the 5f subshell, analogous to how lanthanides fill 4f, with the general valence configuration $[Rn]\,5f^{0-14}\,6d^{0-2}\,7s^2$ — the $7s^2$ shell stays filled throughout, $6d$ occupancy varies (0, 1, or 2 depending on the element due to close 5f/6d energy proximity), and 5f fills progressively from 0 to 14 electrons across the series.

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Actinide elements characteristically form linear dioxo-cations (e.g. uranyl $UO_2^{2+}$), a feature not shown by lanthanides, noble gases or alkali metals.

Actinides can access higher oxidation states (+4, +5, +6) more readily than lanthanides because their 5f, 6d and 7s orbitals are close in energy and can participate in bonding, allowing stable M=O multiple bonding as seen in oxocations like $UO_2^{2+}$ (uranyl) and $PuO_2^{2+}$ (plutonyl). Lanthanides are overwhelmingly confined to the +3 state and do not form comparable s …