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A lanthanide (sometimes abbreviated as Ln) is the element lanthanum (atomic number 57) or one of the 14 following elements, through atomic number 71. Their isotopes have mass numbers in the 130 to 180 range. Lanthanides are trans-iron elements, and their creation (lanthanide nucleosynthesis) is of research interest because it is not obvious where (enough of) the necessary extreme environment exists. The presumed creation process of neutron capture (r-process or s-process) requires many free neutrons. Lanthanides make up the bulk of the rare earth elements.
An actinide (An) is an analogous category of elements consisting of actinium (atomic number 89) and the 14 following elements, through 103. They present the same puzzle regarding their synthesis (actinide nucleosynthesis).