Why is pairing electrons in the same orbital less favorable?

Pairing electrons in the same orbital is less favorable primarily due to the repulsive forces that occur because both electrons have the same negative charge. According to Pauli's exclusion principle, each orbital can hold a maximum of two electrons, but these electrons must have opposite spins. When electrons occupy the same orbital, they must pair up with opposite spins, which provides a type of accommodation for their presence in the same space.

However, while opposite spins satisfy the Pauli principle, the two electrons still experience mutual repulsion because they are both negatively charged. This repulsion leads to an increase in energy within the system, making it energetically less favorable for electrons to pair in the same orbital rather than occupy separate orbitals when there are available ones.

In contrast, when electrons occupy different orbitals, they can remain unpaired, which reduces the overall repulsion since they are spread out in different spatial locations. This distribution is favored as it minimizes the total energy of the atom. Thus, while there is a mechanism that allows pairing (due to opposite spins), the inherent electrostatic repulsion makes it a less favorable option when alternatives exist.