In lanthanoids, the 4f orbitals are being filled, while in actinoids, the 5f orbitals are occupied. The 5f electrons penetrate less into the inner core and are more effectively shielded from the nuclear charge than the 4f electrons of corresponding lanthanoids.

Both series exhibit contraction effects. The actinoid contraction is greater from element to element than the lanthanoid contraction. Actinoids show greater structural variability due to irregularities in metallic radii which are far greater than in lanthanoids. These contractions have extended effects on sizes and properties of elements succeeding them.

Actinoids display a wide range of oxidation states from +3 to +7, making their chemistry more complicated. Lanthanoids predominantly show +3 oxidation state, though some like Cerium exhibit +4 state. Early actinoids resemble lanthanoids in showing gradual variation in properties without change in oxidation state.

The ionisation enthalpies of early actinoids are lower than for early lanthanoids. Since outer electrons in actinoids are less firmly held, they are more available for bonding. Actinoid chemistry is more complicated due to wide range of oxidation states and radioactivity. Behaviour similar to lanthanoids is not evident until the second half of the actinoid series.