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Why do actinoid elements have similar chemical properties?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — transuranium element Modern version of the periodic table of the elements (printable).

The actinoid elements exhibit similar chemical properties primarily because of their atomic structure. As the atomic number increases across the series, additional electrons are placed into internal f orbitals (specifically the 5f subshell).1

Because these f orbitals are partially buried within the atom, the added electrons are not chemically active. Consequently, the outer valence electrons—which determine how an element forms chemical bonds—remain largely the same for each element in the series. This configuration causes the actinoids to share a close resemblance in their chemical behavior, much like the lanthanoids, which similarly fill internal 4f orbitals.1

While the actinoids are chemically similar, they do exhibit more variable valences than the lanthanoids because their 5f electrons are farther from the nucleus and less tightly bound, occasionally participating in chemical reactions. Additionally, while many actinoids share crystal structures similar to lanthanoids, some lighter actinoids display unique metallic structures due to the mixing of 5f and 6d orbitals.12