What is the building up principle in electron configuration?

The building-up principle, often referred to as the Aufbau principle, describes how electrons are arranged in an atom's electron configuration. According to this principle, electrons fill atomic orbitals starting from the lowest available energy levels and move to higher energy levels as the lower ones are filled.

In this context, the correct answer aligns with Hund's rule, which states that electrons will occupy degenerate orbitals (orbitals of the same energy) singly before they pair up. This helps to minimize electron-electron repulsions and results in a more stable electronic configuration. The statement accurately reflects how electron configurations are established in multi-electron atoms, showcasing the importance of maximizing mutual spin within similar energy levels for stability.

The other choices do not accurately represent the building-up principle. Filling the highest energy orbitals first contradicts the very mechanism of electron configuration, as electrons adhere to a lower energy-first filling order. Similarly, the idea that electrons cannot occupy the same orbital is more related to the Pauli Exclusion Principle rather than the Aufbau process. Thus, the correct choice underscores the order of filling that promotes the stability of atomic structure.