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, which means that they have the same energy. Thus, the fifth electron can enter any of the three degenerate 2p orbitals.

What about the sixth electron? Does it enter the 2p orbital with an electron or one of the vacant 2p orbitals?

According to Hund’s rule of maximum multiplicity, electrons singly occupy all orbitals of a given energy level before they start pairing, and the unpaired electrons cannot have opposite spins: their spins must be parallel.

Hence, for carbon, the two 2p electrons must occupy two different orbitals and have parallel spins. This way, electrons can spread over a larger area. This decreases their shielding of each other, thereby minimizing the energy of the atom.

For nitrogen, each of the three 2p orbitals is singly occupied.

For oxygen, once the degenerate 2p orbitals are filled singly, the last electron must pair with another 2p electron. The atom has two unpaired electrons.

The electron configuration of neon reveals that the outermost shell is filled to its maximum capacity of eight electrons. Neon has two electrons, called core electrons in the inner shell, and eight electrons, called valence electrons, in the outermost shell.