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Full Idea
The order of shell filling has not yet been deduced from first principles, and this issue cannot be avoided if one is to really ask whether quantum mechanics explains the periodic system in a fundamental manner.
Gist of Idea
To explain the table, quantum mechanics still needs to explain order of shell filling
Source
Eric R. Scerri (The Periodic Table [2007], 09 'From')
Book Ref
Scerri,Eric R.: 'The Periodic Table' [OUP 2007], p.229