Difference Between Lanthanoids and Actinoids

• Position: These 14 elements follow Lanthanum (Atomic Numbers 58 to 71).
• Electron Filling: The extra electrons are filled progressively into the deep 4f orbital.
• Oxidation States: They primarily show a very stable +3 oxidation state. Sometimes +2 and +4 are shown, but higher states are not possible.
• Radioactivity: Except for Promethium (Pm), all lanthanoids are non-radioactive and relatively stable.
• Complex Formation: They have a low tendency to form complex compounds.

• Position: These 14 elements follow Actinium (Atomic Numbers 90 to 103).
• Electron Filling: The extra electrons are filled progressively into the 5f orbital.
• Oxidation States: While they also show a +3 state, they frequently exhibit much higher oxidation states like +4, +5, +6, and +7 (e.g., Uranium). This is because the 5f, 6d, and 7s energy levels are very close to each other.
• Radioactivity: All actinoids are radioactive. The heavier ones are highly unstable and artificial (made in labs).
• Complex Formation: They have a very high tendency to form complex compounds.

Both series experience a steady decrease in atomic size across the row.

• Lanthanoid Contraction: Caused by the poor shielding effect of the 4f electrons.
• Actinoid Contraction: The contraction here is much greater from element to element because the shielding effect of the 5f electrons is even poorer than that of the 4f electrons.