Editing Period 7 element (section)

=== Actinides ===
{{main|Actinide}}
[[File:Nagasakibomb.jpg|thumb|The [[Fat Man|atomic bomb]] [[Atomic bombings of Hiroshima and Nagasaki|dropped on Nagasaki]] had a [[plutonium]] charge.<ref>[https://web.archive.org/web/20101122185847/http://www.cfo.doe.gov/me70/manhattan/nagasaki.htm The Manhattan Project. An Interactive History]. US Department of Energy</ref>]]
The '''actinide''' or '''actinoid''' ([[Chemical nomenclature|IUPAC nomenclature]]) series encompasses the 15 [[metal]]lic [[chemical element]]s with [[atomic number]]s from 89 to 103, [[actinium]] through [[lawrencium]].<ref name="Gray" >{{cite book|last=Gray|first=Theodore|title=The Elements: A Visual Exploration of Every Known Atom in the Universe|year=2009|publisher=Black Dog & Leventhal Publishers|location=New York|isbn=978-1-57912-814-2|page=[https://archive.org/details/elementsvisualex0000gray/page/240 240]|url=https://archive.org/details/elementsvisualex0000gray/page/240}}</ref><ref>[https://www.britannica.com/EBchecked/topic/4354/actinoid-element Actinide element], Encyclopædia Britannica on-line</ref><ref>Though "actinoid" (rather than "actinide") means "actinium-like" and therefore should exclude actinium, that element is usually included in the series.</ref><ref>{{cite book|last=Connelly|first=Neil G.|title=Nomenclature of Inorganic Chemistry|publisher=[[Royal Society of Chemistry]]|location=London|year=2005|chapter-url=https://books.google.com/books?id=w1Kf1CakyZIC&pg=PA52|page=52|chapter=Elements|isbn=978-0-85404-438-2|display-authors=etal}}</ref>

The actinide series is named after its first element actinium. All but one of the actinides are [[f-block]] elements, corresponding to the filling of the 5f [[electron shell]]; lawrencium, a [[d-block]] element, is also generally considered an actinide. In comparison with the [[lanthanides]], also mostly [[f-block]] elements, the actinides show much more variable [[valence (chemistry)|valence]].

Of the actinides, [[thorium]] and [[uranium]] occur naturally in substantial, [[Primordial nuclide|primordial]], quantities. Radioactive decay of uranium produces transient amounts of [[actinium]], [[protactinium]] and [[plutonium]], and atoms of [[neptunium]] and [[plutonium]] are occasionally produced from [[Nuclear transmutation|transmutation]] in [[uranium ore]]s. The other actinides are purely [[synthetic elements]], though the first six actinides after plutonium would have been produced at [[Oklo]] (and long since decayed away), and [[curium]] almost certainly previously existed in nature as an [[extinct radionuclide]].<ref name="Gray" /><ref name=g1250>Greenwood, p. 1250</ref>  Nuclear tests have released at least six actinides heavier than plutonium into the [[natural environment|environment]]; analysis of debris from a 1952 [[hydrogen bomb]] explosion showed the presence of [[americium]], [[curium]], [[berkelium]], [[californium]], [[einsteinium]] and [[fermium]].<ref>{{cite journal|last1=Fields|first1=P.|last2=Studier|first2=M.|last3=Diamond|first3=H.|last4=Mech|first4=J.|last5=Inghram|first5=M.|last6=Pyle|first6=G.|last7=Stevens|first7=C.|last8=Fried|first8=S.|last9=Manning|first9=W. |title=Transplutonium Elements in Thermonuclear Test Debris|journal=Physical Review|volume=102|issue=1|page=180|year=1956|doi=10.1103/PhysRev.102.180|bibcode = 1956PhRv..102..180F  }}</ref>

All actinides are [[radioactive]] and release energy upon radioactive decay; naturally occurring uranium and thorium, and synthetically produced plutonium are the most abundant actinides on Earth. These are used in [[nuclear reactor]]s and [[nuclear weapons]]. Uranium and thorium also have diverse current or historical uses, and americium is used in the [[ionization chamber]]s of most modern [[smoke detector]]s.

In presentations of the [[periodic table]], the lanthanides and the actinides are customarily shown as two additional rows below the main body of the table,<ref name="Gray" /> with placeholders or else a selected single element of each series (either [[lanthanum]] or [[lutetium]], and either [[actinium]] or [[lawrencium]], respectively) shown in a single cell of the main table, between [[barium]] and [[hafnium]], and [[radium]] and [[rutherfordium]], respectively.  This convention is entirely a matter of [[aesthetics]] and formatting practicality; a rarely used [[Periodic_table_(detailed_cells)#32-column_layout|wide-formatted periodic table]] (32 columns) shows the lanthanide and actinide series in their proper columns, as parts of the table's sixth and seventh rows (periods).