On the position of La, Lu, Ac and Lr in the periodic table: a perspective

The periodic table of elements, organised as blocks of elements that contain similar properties, occupies a central role in chemistry. However, the position of some of the elements in the periodic table is a debate that has been ensuing over the past one and a half long centuries. Particularly, the positions of lanthanum (La), lutetium (Lu), actinium (Ac) and lawrencium (Lr) in the periodic table have been quite controversial. Different kinds of studies carried out by various research groups have yet left the fate of these elements undecided as the results of these investigations suggested that these elements could potentially be placed in the d-block, p-block or all four in the f-block. Our recent work looked into this question from a new perspective, involving encapsulation of La, Lu, Ac and Lr into Zintl ion clusters, Pb122− and Sn122−. These clusters were chosen as they provide a fitting environment for the determination of structural, thermodynamic and electronic properties of the encapsulated species. Various results that have been evaluated and subsequently analysed (Joshi et al. in Phys. Chem. Chem. Phys. 20:15253–15272, 2018) in order to seek out similarities and differences for making justified conclusions about the placement of all these four elements in the periodic table are the subject matter of this review article. This review highlights a unique methodology wherein Ln and An encapsulated Pb122− and Sn122− clusters provided in-depth insights on the long-drawn controversy: positions of lutetium, lawrencium, lanthanum and actinium in the modern periodic table. Electronic, thermodynamic and structural parameters have been comprehensively investigated by DFT to yield a justified conclusion.