Lanthanoids vs. Actinoids

Feature

Lanthanoids (4f-series)

Actinoids (5f-series)

Span

Cerium ($$Z = 58$$) to Lutetium ($$Z = 71$$)

Thorium ($$Z = 90$$) to Lawrencium ($$Z = 103$$)

General Config.

$$[Xe]\ 4f^{1-14}\ d^{0-1}\ 6s^2$$

$$[Rn]\ 5f^{1-14}\ 6d^{0-2}\ 7s^2$$

Config. Predictability

Highly predictable; anchor $$6s^2$$ is highly stable.

Highly volatile; narrow energy gap between $$5f$$ and $$6d$$ causes electron slipping.

Oxidation States

Dominated by stable $$+3$$ ; rare exceptions ($$+2,+4$$).

Highly variable; reaches up to $$+7$$ for early elements ($\text{Np}, \text{Pu}$).

Electron Shielding

$$4f$$ electrons are deeply buried; poor shielding causes steady contraction.

$$5f$$ electrons extend further out, participate more in bonding, and shield even more poorly.