The lanthanide or lanthanoid series comprises the fifteen elements with atomic numbers 57 through 71, fromlanthanum to lutetium. All lanthanides are f-block elements, corresponding to the filling of the 4f electron shell.
The actinoid series encompasses the 15 chemical elements that lie between actinium and lawrencium included on the periodic table, with atomic numbers 89 - 103. The actinoid series derives its name from the first element in the series, actinium, and ultimately from the Greek aktis, "ray," reflecting the elements' radioactivity.
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