The actinide series encompasses the 14 chemical elements that lie between actinium and nobelium on the periodic table with atomic numbers 89 - 102 inclusive. They are similar in characteristics to the elements of the lanthanide series. The actinides with the higher atomic numbers are not found in nature and have short half-lives.
The actinides are typically placed below the main body of the periodic table, in the manner of a footnote. The full-width version of the periodic table shows the position of the actinides more clearly.
It is known, however, that all members of the series resemble actinium and each other in their chemical properties and that they have a strong chemical resemblance to their homologs in the lanthanide series.
The actinides are reactive and assume a number of different valences in their compounds.
Elements with atomic numbers greater than 103 are not members of the actinide series; element 104 (rutherfordium) is the first of the transactinide elements.
Other isotopes of the actinideelements that have found application are Pu-238 which is used to power heart pacemakers and space instrumentation, americium-241 which is used in smoke detectors, and californium-252 which is used in analytical techniques that involve production of isotopes of other elements by neutron-capture.
The actinides display less similarity in their chemical properties than the lanthanide series, for instance exhibiting a wider range of oxidation states, which initially led to confusion as to whether actinium, thorium and uranium should be considered d-blockelements.
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