actinoid

1 of 2

adjective

ac·​ti·​noid ˈak-tə-ˌnȯid 

1

: resembling a ray especially of a radially symmetrical animal

2

: exhibiting radial symmetry

actinoid

2 of 2

noun

ac·​ti·​noid ˈak-tə-ˌnȯid 

plural actinoids

chemistry

: actinide

The actinoid elements (or actinides; An) constitute a series of 14 elements which are formed by the progressive filling of the 5f electron shell and follow actinium in the periodic table (atomic numbers 90-103).

—Mirtat Bouroushian, Electrochemistry of Metal Chalcogenides, 2010

Word History

Etymology

Adjective

actin(ia) + -oid entry 2 (probably after actinoida or actinoidea )

Noun

actin(ide) + -oid entry 1

First Known Use

Adjective

1846, in the meaning defined at sense 1

Noun

1958, in the meaning defined above

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