Actinide series

The 15 metallic elements from actinium through lawrencium, characterized by progressive filling of 5f orbitals. All are radioactive, and most are synthetic.

f-block: The periodic-table block in which the differentiating electrons enter f orbitals. The actinides occupy its 5f portion.
Actinide oxidation states: The formal charges adopted by actinide atoms in compounds, often spanning several values. Accessible 5f, 6d, and 7s electrons allow more varied states than in most lanthanides.
Uranium: A radioactive actinide element with atomic number 92, used chiefly as nuclear fuel. Its fissile isotopes sustain most commercial nuclear power reactors.
André-Louis Debierne: A French chemist who discovered actinium in 1899 while studying uranium residues. His discovery supplied the element that begins the series.
Actinium: A radioactive metallic element with atomic number 89 and the first member of the actinide series. It anchors the series at its lightest end.
5f orbital: An orbital in the fifth electron shell that can hold up to 14 electrons across its subshell. Progressive occupation of these orbitals defines the series.
Actinide bonding: Chemical bonding involving actinide atoms, with contributions from 5f, 6d, and 7s orbitals. The radial reach of 5f orbitals changes bonding across the series.
Plutonium: A radioactive actinide element with atomic number 94, produced in reactors and used as fuel or in weapons. Its isotopes support reactor fuel cycles and nuclear weapons.
Glenn T. Seaborg: An American chemist who helped discover several transuranium elements and proposed the actinide concept. His placement of heavy elements in a separate series reshaped the periodic table.
Thorium: A naturally occurring radioactive metallic element with atomic number 90. Its abundance and potential as reactor fuel distinguish it among early actinides.

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