The Aufbau Principle and Hund's Rule

Recall that atomic orbitals have different energies and can accommodate two electrons each. The aufbau principle dictates the distribution of electrons among the subshells of the atom, and Hund’s rule of maximum multiplicity explains the filling of orbitals in the subshells.

The aufbau principle states that in the ground state, electrons fill atomic orbitals from lowest to highest energy to achieve the lowest-energy configuration.

Though energy generally increases with shell number, the greater penetration of s orbitals often leads to the four-s and five-s orbitals having lower energies than the three-d and four-d orbitals, respectively. The order can be remembered using diagrams like this one, where the path of the arrow reveals the sequence in which electrons are assigned to orbitals.

Consider writing the electron configuration for carbon — an element with atomic number six. Certainly, the 1s orbital, which has the lowest energy, should be filled before the 2s orbital. Each orbital can hold a maximum of two electrons.

The fifth electron enters a 2p subshell. But which of the three 2p orbitals does it populate?

Well! The orbitals in any subshell are presumed to be degenerate, whic