Aufbau principle

The Aufbau principle is a rule used in chemistry to determine the electron configuration of an atom or ion. It states that electrons fill atomic orbitals in order of increasing energy levels. Lower energy orbitals are filled before higher energy orbitals.

Here's the general order of filling:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p