Which of the following statements is true regarding electron-electron repulsion in filled orbitals?

The correct statement regarding electron-electron repulsion in filled orbitals is that it is a significant factor contributing to pairing energy. In quantum chemistry, when two electrons occupy the same atomic orbital, they must pair up with opposite spins due to the Pauli exclusion principle. This pairing results in an increase in energy because of the repulsive forces between the two electrons, which are negatively charged.

This repulsion occurs because electrons repel each other due to their like charges, and when they are forced to occupy the same orbital, it leads to an increase in the overall energy of the system. This concept is crucial in understanding how atoms form their electron configurations and why certain arrangements are more stable than others.

In essence, the pairing energy directly arises from the need to overcome the repulsion between electrons, making it a key factor in determining the stability and configuration of electrons in an atom. This is particularly significant when considering transition metals, where the filling of d-orbitals and the associated pairing can greatly influence the stability and reactivity of the elements.