Full Idea
Pauli explained the maximum number of electrons successive shells can accommodate, ...but it does not explain the lengths of the periods, which is the really crucial property of the periodic table.
Gist of Idea
Pauli explained the electron shells, but not the lengths of the periods in the table
Source
Eric R. Scerri (The Periodic Table [2007], 07 'Pauli')
Book Reference
Scerri,Eric R.: 'The Periodic Table' [OUP 2007], p.203
A Reaction
Paulis' Exclusion Principle says no two electrons in an atom can have the same set of four quantum numbers. He added 'spin' as a fourth number. It means 'electrons cannot be distinguished' (243). Scerri says the big problem is still not fully explained.