Final answer:
The rule that states electrons fill energy levels to maximize the number of unpaired electrons before pairing up is Hund's rule. It ensures that each degenerate orbital is singly occupied before electrons are paired up.
Step-by-step explanation:
The rule that states electrons will fill an energy level in order to maximize the number of unpaired electrons before pairing up is known as Hund's rule. According to Hund's rule, all orbitals will be singly occupied before any is doubly occupied. Therefore, two p orbital get one electron and one will have two electrons.
Hund's rule also stipulates that all of the unpaired electrons must have the same spin.This rule plays a crucial role in determining the most stable electron configuration for an atom and is often used in conjunction with the Aufbau principle and the Pauli Exclusion Principle when deducing electron arrangements.
Hund's rule dictates that when electrons occupy degenerate orbitals, they must occupy empty orbitals singly, with parallel spins, to minimize electron repulsion and achieve greater stability. Only after all the degenerate orbitals are half-filled do electrons start pairing up in these orbitals.