The physical basis for this claim is that all orbitals need to have like one electron at least to hinder repulsion by the action of pairing of element.
What the ways that electrons are filled in Subshells?
Shells and subshells are known to be filled in the level of energy order, and as such, electrons need to fill the 4s subshell before they can be able to fill 3d shell.
Note that Orbitals are often and singly filled by electrons and can double up only when all orbitals has like one electron to hinder repulsion by the action of pairing of element.
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