actinide

The actinide (or actinoid) series encompasses the 15 chemical elements that lie between actinium and lawrencium on the periodic table, with atomic numbers 89 - 103. The actinide series derives its name from the first element in the series, actinium.The actinide series is included in some definitions of the rare earth elements. IUPAC is currently recommending the name actinoid rather than actinide, as the suffix "-ide" generally indicates ions. There are alternative arrangements of the periodic table that exclude actinium or lawrencium from appearing together with the other actinides.
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n. actinide, any of a series of radioactive elements with atomic numbers 89 through 103

Ac
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No

Elements 89-102 are called actinides. Electrons added during the Aufbau construction of actinide atoms go into the 5f subshell. Actinides are unstable and undergo radioactive decay. The most common actinides on Earth are uranium and thorium.

Noun
1. any of a series of radioactive elements with atomic numbers 89 through 103
(synonym) actinoid, actinon
(hypernym) group, grouping