Using Hund’s Rule and Spin Multiplicity to Assess Competing Versions of Group 3 and f-Block Constituency

The article explores Group 3 constituency and f-element representation. In considering those intertwined topics, we frame the discussion in terms of two varieties of periodic table: the one seen in most chemistry texts, where Group 3 is comprised of Sc-Y-La-Ac, and a lesser-known variety, where Group 3 is Sc-Y-Lu-Lr (which makes it akin to Janet’s Left-Step Periodic Table). We also discuss the type seen most often in physics texts, where Group 3 takes the form Sc-Y-*-**, thus footnoting La and Ac to join the other lanthanides/actinides. This variety we look at less closely since it may be regarded as a variant of the first type mentioned. From Hund's rule, spin multiplicity and ground-level microstate data we erect a framework for judging which of the types seems most attuned to atomic structure. Our conclusion is that the type with Sc-Y-Lu-Lr accords best; it possesses an f-block that ends, unequivocally, on the first 4f14 element (Yb). By contrast, the f-elements of the other two types pass through Yb to halt at the second 4f14 element (Lu), as if in a delayed reaction or stutter.