Actinide concept

The proposal that elements heavier than actinium form a series analogous to the lanthanides, with electrons filling the 5f subshell. It organizes the chemistry and periodic placement of elements 90 through 103.

Glenn T. Seaborg: An American chemist who helped discover several transuranium elements and proposed the actinide series. His 1940s interpretation placed the transuranium elements in a new series beneath the lanthanides.
Lanthanide: A series of chemical elements in which the 4f subshell is progressively filled. The proposed actinide series mirrors this established pattern with the 5f subshell.
5f orbital: An atomic orbital in the fifth f subshell, occupied in many actinide atoms and ions. Progressive occupation of these orbitals is the series' defining electronic pattern.
Dmitri Mendeleev: A Russian chemist who formulated an early periodic table and predicted properties of undiscovered elements. His periodic framework predates the actinide concept and could not anticipate the transuranium series.
Periodic table: A tabular arrangement of chemical elements by atomic number and recurring properties. The actinide concept changed how the heaviest elements fit into its structure.
Electron configuration: The distribution of an atom's electrons among its orbitals and subshells. The 5f-filling proposal is fundamentally a claim about heavy-element electron configurations.
Actinide contraction: The gradual decrease in actinide atomic and ionic sizes as atomic number increases. Increasing nuclear charge across the series draws electrons inward despite imperfect 5f shielding.
Niels Bohr: A Danish physicist whose atomic model and electronic-configuration ideas influenced early periodic-table arrangements. His proposed placements for heavy elements differed from Seaborg's actinide-series interpretation.
Transuranium element: A chemical element with an atomic number greater than uranium's 92. The concept arose as newly discovered transuranium elements challenged existing periodic placements.
Aufbau principle: A rule of thumb for predicting electron configurations by filling lower-energy orbitals first. It provides a starting model for assigning electrons across the actinide subshells.

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