How would you explain the Aufbau principle?

According to the principle, electrons fill orbitals starting at the lowest available (possible) energy levels before filling higher levels (e.g. 1s before 2s). The number of electrons that can occupy each orbital is limited by the Pauli exclusion principle to two.

For example, hydrogen has one electron in the s-orbital of the first shell, so its configuration is written 1##s^1## Lithium has two electrons in the 1s-subshell and one in the (higher-energy) 2s-subshell, so its configuration is written 1##s^2## 2##s^1## (pronounced "one-s-two, two-s-one"). Phosphorus (atomic number 15) is as follows: 1##s^2## 2##s^2## 2##p^6## 3##s^2## 3##p^3##.