Actinide
The actinide or actinoid (IUPAC nomenclature) series encompasses the
15 metallic chemical elements with atomic numbers from 89 to 103, actinium
through lawrencium.
Strictly
speaking, both actinium and lawrencium have been labeled as group 3 elements,
but both elements are often included in any general discussion of the chemistry
of the actinide elements.
Actinium is the more often omitted of the two,
because its placement as a group 3 element is somewhat more common in texts and
for semantic reasons: since "actinide" means "like
actinium", it has been argued that actinium cannot logically be an
actinide, even though IUPAC acknowledges its inclusion based on common usage.
The actinide
series derives its name from the first element in the series, actinium. The
informal chemical symbol An is used in general discussions of actinide
chemistry to refer to any actinide. All but one of the actinides are f-block
elements, with the exception being either actinium or lawrencium. The series
mostly corresponds to the filling of the 5f electron shell, although actinium
and thorium lack any f-electrons, and curium and lawrencium have the same
number as the preceding element. In comparison with the lanthanides, also
mostly f-block elements, the actinides show much more variable valence. They
all have very large atomic and ionic radii and exhibit an unusually large range
of physical properties. While actinium and the late actinides (from americium
onwards) behave similarly to the lanthanides, the elements thorium,
protactinium, and uranium are much more similar to transition metals in their
chemistry, with neptunium and plutonium occupying an intermediate position.
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 reactors and
nuclear weapons. Uranium and thorium also have diverse current or historical
uses, and americium is used in the ionization chambers of most modern smoke
detectors.
Of the
actinides, primordial thorium and uranium occur naturally in substantial
quantities. The radioactive decay of uranium produces transient amounts of
actinium and protactinium, and atoms of neptunium and plutonium are
occasionally produced from transmutation reactions in uranium ores. The other
actinides are purely synthetic elements.
Nuclear weapons tests have
released at least six actinides heavier than plutonium into the environment;
analysis of debris from a 1952 hydrogen bomb explosion showed the presence of
americium, curium, berkelium, californium, einsteinium and fermium.[8]
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,[2]
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 wide-formatted periodic table
inserts the lanthanide and actinide series in their proper places, as parts of
the table's sixth and seventh rows (periods).
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