"Electrons always occupy the lowest empty energy level" is a statement of

Step 1: Analyze the question The statement says electrons fill the lowest available energy level first. We need the principle or rule that describes this.

Step 2: Review the concepts

• Aufbau Principle: "Aufbau" is German for "building up". Electrons fill orbitals starting from the lowest energy level and move to higher ones. The order is 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p, and so on.
• Hund's rule: Electrons occupy degenerate orbitals (orbitals of the same energy, such as the three 2p orbitals) singly first, with parallel spins, before they pair up.
• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers. An orbital holds a maximum of 2 electrons, and they must have opposite spins.
• Periodic law: The properties of elements are a periodic function of their atomic numbers. It has nothing to do with how electrons fill orbitals.

Step 3: Match the statement "Electrons always occupy the lowest empty energy level" describes filling orbitals in order of increasing energy. That is the Aufbau Principle.

Step 4: Answer The correct option is A. Aufbau Principle.

Exam tip: Link each rule to a keyword:

• Aufbau = "build up", lowest energy first
• Hund = "hundred buses": passengers sit singly in each seat before sharing
• Pauli = "pairs have opposite spins"