Rules for placing electrons

1. Aufbau Principle: electrons enter orbitals of lowest energy first

https://en.wikipedia.org/wiki/Aufbau_principle

1. Pauli Exclusion Principle: an orbital can only contain two electrons with opposite spin

https://www.premedhq.com/pauli-exclusion-principle-2

1. Hund's rule: within a sub level, electrons enter singly before pairing up

http://ch301.cm.utexas.edu/section2.php?target=atomic/e-config/hunds-rule.html