Question

Do electrons fill the lower energy levels first?

Answer 1

Yes. The Aufbau diagram can be used to determine the order of filling of atomic sublevels, although there are some exceptions among the transition metals.

Explanation:

The Aufbau diagram shows the order of filling of atomic sublevels in order of increasing energy. Typically, each sublevel must be filled with the maximum number of electrons before filling the next highest energy sublevel, though there are exceptions to this rule among the transition metals.

The Aufbau diagram can help you determine the electron configuration for most elements.

You should notice that the order of filling is not simply from a lower sublevel to the next sublevel. For example, the 3d sublevel fills after the `"4s"` sublevel and before the `"4p"` sublevel.