Group number of lanthanides and actinides

IUPAC has not recommended a specific format for the periodic table, so different conventions are permitted and are often used for the group number of lanthanides and actinides. There is no pre-existing physical entity for a "group" of elements. The notion is simply a convenient classification, since some elements have similar periodicities in their properties. This allows them to be arranged in columns that also happen to correspond with electron configurations.

• Some tables [1] include lanthanum (La) and actinium (Ac), (the beginnings of the lanthanide and actinide series of elements, respectively) in group 3 with scandium (Sc) and yttrium (Y). In their most commonly encountered tripositive ion forms, these elements do not possess any partially filled f orbitals, thus resulting in more d-block-like behaviour.

• Some tables [2] include lutetium, (Lu) and lawrencium, (Lr) in group 3. These elements terminate the lanthanide and actinide series, respectively. Since the f-shell is nominally full in the ground state electron configuration for both these metals, they behave most like d-block metals out of all the lanthanides and actinides, and thus exhibit the most similarities in properties with Sc and Y. For Lr, this behavior is expected, but it has not been observed because sufficient quantities are not available. (Also see Periodic table (wide) and Periodic table (extended).)

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Some tables [3] refer to all lanthanides and actinides by a marker in group 3. This is the periodic table used by Wikipedia. A third and fourth alternative are suggested by this arrangement:

• The third alternative is to regard all 30 lanthanide and actinide elements as included in Group 3. Lanthanides, as electropositive trivalent metals, all have a closely related chemistry, and all show many similarities to Sc and Y.

• The fourth alternative is to include none of the lanthanides and actinides in group 3. The lanthanides possess additional properties characteristic of their partially-filled f orbitals which are not common to Sc and Y. Furthermore, the actinides show a much wider variety of chemistry (for instance, in range of oxidation states) within their series than the lanthanides, and comparisons to Sc and Y are even less useful.