This convention is entirely a matter of aesthetics and formatting practicality; a rarely used wide-formatted periodic table ( 32 columns ) shows the lanthanide and actinide series in their proper columns, as parts of the table's sixth and seventh rows ( periods ).
22.
Thus it was not until 1945, after Glenn T . Seaborg and his team had discovered the transuranic elements americium and curium, that Seaborg realised that thorium was the second member of the actinide series and was filling an f-block row, instead of being the heavier congener of hafnium and filling a fourth d-block row.
