Electrons will not pair in a set of equal energy orbitals if an empty orbital is available"" is a statement of

Hund’s (First) Rule

Explanation:

According to the first rule, electrons always enter an empty orbital before they pair up. Electrons are negatively charged and, as a result, they repel each other. Electrons tend to minimize repulsion by occupying their own orbitals, rather than sharing an orbital with another electron. Furthermore, quantum-mechanical calculations have shown that the electrons in singly occupied orbitals are less effectively screened or shielded from the nucleus. Electron shielding is further discussed in the next section.

explanation:

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15.3 cos 65 degrees trig