Actinoids — Revision Notes

⚡ 30-Second Revision

Actinoids: — $Z=90-103$ (Th to Lr), 5f-block elements.
General Config: — $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$ (irregular filling).
Key Exception: — Thorium ($Th, Z=90$) is $[Rn] 5f^0 6d^2 7s^2$.
Oxidation States: — Wide range (+3 to +7), +3 most common/stable. Higher states for lighter actinoids (U, Np, Pu).
Actinoid Contraction: — Decrease in radii across series. Caused by poor 5f shielding. *More pronounced* than lanthanoid contraction.
Radioactivity: — ALL actinoids are radioactive.
Transuranic Elements: — $Z > 92$ (synthetic, radioactive).
Color: — Ions are generally colored (f-f transitions).
Magnetic Prop: — Paramagnetic (unpaired 5f e-), complex magnetic moments.

2-Minute Revision

Actinoids are the second f-block series, from Thorium ($Z=90$) to Lawrencium ($Z=103$), characterized by the filling of 5f orbitals. Their electronic configurations are irregular due to the close energy levels of 5f, 6d, and 7s subshells, with Thorium ($5f^0$) being a notable exception.

A defining feature is their universal radioactivity; all actinoids are unstable and decay. Elements with $Z > 92$ are called transuranic and are synthetic. Actinoids exhibit a wide range of oxidation states (+3 to +7), with +3 being the most common and stable, but higher states are significant for lighter members like Uranium (+6 in $UO_2^{2+}$).

This variability is due to the accessible 5f, 6d, and 7s electrons. They also show 'actinoid contraction,' a gradual decrease in atomic/ionic radii across the series, which is more pronounced than lanthanoid contraction due to the even poorer shielding of 5f electrons.

Actinoids are highly reactive metals and form colored, generally paramagnetic ions.

5-Minute Revision

Actinoids, the 5f-block elements from Thorium ($Z=90$) to Lawrencium ($Z=103$), are crucial for NEET. Their general electronic configuration is $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$, but remember the irregularities, especially Thorium's $5f^0 6d^2 7s^2$ configuration.

This irregularity stems from the very close energy levels of the 5f, 6d, and 7s orbitals, which also explains their most distinctive feature: a wide range of oxidation states. While +3 is the most common and stable state (e.

g., $Am^{3+}$), lighter actinoids like Uranium, Neptunium, and Plutonium readily exhibit higher states such as +4, +5, +6, and even +7 (for Np and Pu). For example, Uranium forms the stable uranyl ion, $UO_2^{2+}$, where U is in the +6 oxidation state.

Another key property is 'actinoid contraction,' the gradual decrease in atomic and ionic radii across the series. This is caused by the poor shielding effect of the diffuse 5f electrons. As the nuclear charge increases, the outer electrons are pulled more strongly, shrinking the atom.

Critically, actinoid contraction is *more pronounced* than lanthanoid contraction because 5f electrons are even poorer shielders than 4f electrons. All actinoids are radioactive, a fundamental characteristic.

Elements with atomic numbers greater than Uranium ($Z=92$) are termed transuranic elements and are synthetic. Actinoids are highly reactive metals, form colored ions (due to f-f transitions), and are generally paramagnetic due to unpaired 5f electrons, though their magnetic moments are complex.

When comparing with lanthanoids, remember actinoids have more accessible 5f electrons, leading to greater reactivity and complex-forming ability.

Prelims Revision Notes

1
Definition & Position: — Actinoids are the 14 elements from Thorium ($Z=90$) to Lawrencium ($Z=103$), where 5f orbitals are progressively filled. They are part of the f-block.
2
Electronic Configuration: — General: $[Rn] 5f^{1-14} 6d^{0-1} 7s^2$. Irregularities are common due to comparable energies of 5f, 6d, 7s orbitals. Key Exception: Thorium ($Z=90$) is $[Rn] 5f^0 6d^2 7s^2$.
3
Oxidation States:

* Show a wider range (+3, +4, +5, +6, +7) than lanthanoids. * +3 is the most common and stable oxidation state, especially in aqueous solutions. * Higher oxidation states are more stable for lighter actinoids (e.g., U, Np, Pu). Example: $UO_2^{2+}$ (uranyl ion) has U in +6 state. * Reason: Close energy levels of 5f, 6d, and 7s electrons allow more electrons to participate in bonding.

1
Actinoid Contraction:

* Gradual decrease in atomic and ionic radii across the series. * Cause: Poor shielding effect of 5f electrons, leading to increased effective nuclear charge. * Comparison: More pronounced than lanthanoid contraction (5f electrons are poorer shielders than 4f).

1
Radioactivity: — All actinoids are radioactive.
2
Transuranic Elements: — Elements with $Z > 92$ (beyond Uranium). All are synthetic (man-made) and radioactive.
3
Color: — Actinoid ions are generally colored due to f-f transitions.
4
Magnetic Properties: — Generally paramagnetic due to unpaired 5f electrons. Magnetic moments are complex and often deviate from spin-only formula.
5
Reactivity: — Highly reactive metals, more reactive than lanthanoids.
6
Complex Formation: — Greater tendency to form complexes than lanthanoids due to larger size, higher charge, and more accessible 5f orbitals.

Vyyuha Quick Recall

To remember key Actinoid properties, think: All Radioactive, Contract More, Variable Oxidation States.

All Radioactive: Every actinoid is radioactive.
Contract More: Actinoid contraction is more pronounced than lanthanoid contraction.
Variable Oxidation States: They show a wide range of oxidation states, not just +3.