Actinide

An actinide is a metallic chemical element in the series from actinium through lawrencium. These elements are associated with filling the 5f electron subshell.

5f orbital: An atomic orbital subshell whose electrons have principal quantum number 5 and angular momentum quantum number 3. Actinide chemistry reflects the filling of this subshell, though electron configurations can be irregular.
Actinide contraction: The gradual decrease in atomic and ionic size across the actinide series as nuclear charge increases. Poor shielding by 5f electrons makes later actinides smaller than a simple shell model predicts.
Uranium: A naturally occurring actinide element with atomic number 92 and several long-lived radioactive isotopes. Uranium-235 sustains fission chain reactions, while uranium-238 is a major fertile isotope.
Actinium: A radioactive metallic element with atomic number 89 and the first member of the actinide series. Its name gives the series its name, though its placement among the actinides was established later.
Lawrencium: A synthetic radioactive element with atomic number 103, generally placed at the end of the actinide series. Its electron configuration and placement expose the limits of a simple 5f-filling definition.
Lanthanide: A series of metallic elements from lanthanum through lutetium, associated with filling the 4f subshell. Lanthanides are the neighboring inner-transition series, with 4f electrons generally less chemically available.
Oxidation state: A formal charge assigned to an atom in a compound under a set of electron-allocation rules. Early actinides can access several oxidation states because 5f, 6d, and 7s electrons are close in energy.
Plutonium: A radioactive actinide element with atomic number 94, produced naturally in trace amounts and in nuclear reactors. Plutonium-239 is fissile and is used in nuclear fuel and weapons.
Glenn T. Seaborg: An American chemist who helped discover several transuranium elements and proposed the actinide concept. His actinide hypothesis reorganized the periodic table to place the heavy elements in a separate series.
Superheavy element: A chemical element with an atomic number high enough that relativistic and nuclear effects strongly shape its properties. The actinide series leads into elements whose chemistry challenges ordinary periodic trends.

Linked from 24 pages