Which rule states that electrons will fill lower-energy orbitals before filling higher-energy orbitals?

The principle that describes how electrons occupy atomic orbitals is foundational in understanding electron configurations. The Aufbau Principle specifically states that electrons will first fill the lowest-energy orbitals available before moving on to higher-energy orbitals. This establishes a more stable electron configuration since lower-energy states are more favorable energetically.

In the atomic structure, orbitals are arranged in a hierarchy of energy levels. For example, the 1s orbital has lower energy than the 2s and 2p orbitals. According to the Aufbau Principle, electrons will fill the 1s orbital fully before any electrons are placed in the 2s or 2p orbitals. This systematic filling order helps to predict and explain the arrangement of electrons in an atom, which in turn influences the atom's chemical properties and reactivity.

Understanding the Aufbau Principle is essential in chemistry and helps guide the overall structure of the periodic table and the behavior of elements during chemical bonding.