Q.Why nobelium is the only actinoid with +2 oxidation state?

Matching each actinoid to its maximum oxidation state gives Th–$+4$, Pa–$+5$, Lr–$+3$, U–$+6$, Pu–$+7$, i.e. option (b).

Thorium (Th, Z = 90) belongs to the actinoid series; the other three options are not actinoids.

Actinoids are the 14 elements from Th (Z=90) to Lr (Z=103) that follow actinium, in which the 5f orbitals are progressively filled.

Checking each option:

• Thorium (Z=90): an actinoid.
• Cerium (Z=58): a lanthanoid (4f block), not an actinoid. …

Because of their larger size and lower ionisation enthalpies (and comparable energies of 5f, 6d, 7s orbitals), actinoids are more reactive than lanthanoids, especially in the finely divided state.

Lanthanoids (4f series) are comparatively less reactive, tarnishing slowly in air, while actinoids (5f series) — especially the early actinoids like thorium, uranium, plutonium — are much more reactive: they combine readily with most non-metals, tarnish rapidly, and some (like plutonium) are pyrophoric in powdered form. This higher reactivity of actinoids is attributed to their larger atomic/ionic radii and t …

Actinides can show oxidation states from +3 up to a maximum of +7, because the 5f, 6d and 7s electrons are close in energy and all can participate in bonding.

+3 is the most common and stable oxidation state throughout the actinide series (like +3 is typical for lanthanides), but because the 5f orbitals extend further from the nucleus than the 4f orbitals of lanthanides and are less effectively shielded, more of the outer electrons of an actinide atom can be involved in bonding. This lets some early actinides reach higher oxidation states than lanthanides ever d …

The near-equal energies of 5f/6d/7s in actinoids let more electrons participate in bonding than the more energetically separated 4f/5d/6s orbitals of lanthanoids allow.

In lanthanoids, the 4f orbitals are deeply buried beneath the filled 5s and 5p subshells and are well shielded from the nucleus; they lie considerably lower in energy than the 5d and 6s orbitals. As a result, 4f electrons rarely take part in bonding, and lanthanoids show an overwhelmingly dominant +3 oxidation state, with only occasional +2/+4 states (when they give a specially stable f⁰, f⁷ or f¹⁴ configuration).

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Terbium belongs to the lanthanide (4f) series; curium, californium and uranium are actinides (5f).

Actinides are the 5f-series elements (Z = 90-103), which include uranium (92), curium (96) and californium (98).

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Like the lanthanoids, actinoids show a wide range of oxidation states, but $+3$ is the most common/characteristic one.

Actinoids exhibit a greater range of oxidation states than lanthanoids (owing to the closer energies of $5f$, $6d$ and $7s$ orbitals), but the most common and characteristic oxidation state throughout the series, as with the lanthanoids …

A transuranic element is any element with atomic number greater than 92 (uranium); Neptunium (Np, Z = 93) is one example.

Elements with atomic number beyond that of uranium (Z = 92) are called transuranic (or transuranium) elements. They are all synthetic/man-made, produced in nuclear reactors or particle accelerators by bombarding heavy nuclei with neutrons or light nuclei, and all are radioactive. Examples include Neptunium (Np, Z = 93), Plutonium (Pu, Z = 94), …