The only information that was important was the size of the orbit, which was described by the n quantum number. The basis of this prediction is a rule known as the aufbau principle , which assumes that electrons are added to an atom, one at a time, starting with the lowest energy orbital, until all of the electrons have been placed in an appropriate orbital. Problems Suppose that all you know about a certain electron is that its principal quantum number is 3. Why or why not? In another example, the possible values of l are 0, 1, 2, 3, and 4 when the value of n is 5. Aufbau Principle.