Two electrons in the 1s orbital must have different spin quantum numbers to satisfy Hund's Rule

The Magnetic Rule

The Pauli Exclusion Principle

The Aufbau Principle

The Pauli Exclusion Principle

Explanation:

The electrons are filled in orbitals of equal energy with one electron entering each orbital until all the orbitals contain one electron with the same spin. The pairing of electrons take place thereafter.

The first electron in an orbital could have +1/2 spin or -1/2 spin. All of the electrons in singly occupied orbitals have the same spin so as to maximize total spin. which is given by Hunds rule.

Two electrons must have opposite spin and thus different spin quantum numbers is given by Pauli's exclusion principle.

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