Note: This article is currently in the Proposed Content section.

The International Union of Pure and Applied Chemistry (IUPAC) publishes the one of the recognised global standards for the table of chemical elements.

But the layout of the Periodic Table, which groups chemical elements according to their electronic properties (i.e. free electrons), has not been universally agreed upon.

There are continuing disagreements about 'Group 3' (which, according to IUPAC, currently contains Scandium, Yttrium, Lanthanoids and Actinoids)

The question of precisely which elements should be placed in group 3 has been debated from time to time. An IUPAC project has been recently initiated to resolve the question. Will group 3 consist of Sc, Y, Lu, and Lr or, will it consist of Sc, Y, La and Ac?

In March 2021, IUPAC published a discussion document on the subject. For the time being, the official version is from 2018 - and some chemists refer to it as a 'compromise'.

Editor's note: This article seems more focused on lack of agreement about classification than 'unknowns' as such.