Apologies if this seems like a silly question.

Rare earth elements (the 4d series) are called by the names Lanthanides or Lanthanoids interchangeably. The same applies for the 5d series, which are called Actinides or Actinoids.

Which one of these two terms is more preferred? Is there an IUPAC directive preferring one over the other in scientific documents? If one is to write a paper or book regarding these elements, which term would one use? Or can both of them be used interchangeably?

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    $\begingroup$ Briefly, since "-ide" implies an anion, using suffix "-oid" is more preferable. I bet I saw this question before either on Chemistry.SE or on English.SE. $\endgroup$
    – andselisk
    Commented Dec 19, 2019 at 9:26
  • 2
    $\begingroup$ @andselisk I did search for a similar question on Chemistry SE but couldn't find it. Hence I posted the question. $\endgroup$ Commented Dec 19, 2019 at 9:38

1 Answer 1


According to p 52, Nomenclature of Inorganic Chemistry: IUPAC Recommendations 2005 (‘Red Book’):

The ending ‘ide’ normally indicates a negative ion, and therefore lanthanoid and actinoid are preferred to lanthanide and actinide.

In practice this is almost never followed, and consequently both terms are (for all intents and purposes) interchangeable, unless the context demands a differentiation.

  • 1
    $\begingroup$ Let me just add that lanthanide/lanthanoid anions do not exist (at least under any reasonable conditions) so there is no concern of confusion. $\endgroup$
    – Gimelist
    Commented Mar 18, 2020 at 5:50

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