<human>:  I have a question about quantum chemistry. What is the Aufbau principle?
<bot>: The Aufbau principle is a rule that governs the way that atoms are built up in molecules. It states that the electrons in a molecule are arranged in order of increasing energy. This means that the electrons in the lowest energy level are arranged in the outermost shell, and the electrons in the highest energy level are arranged in the innermost shell.