Chemistry · Ch 2 — Quantum Mechanical Model of Atom
Hund's rule of maximum multiplicity
Hund's rule of maximum multiplicity
The Aufbau principle tells you which subshell to fill next, but it is silent on a separate question: once you reach a set of several orbitals that are all degenerate (equal in energy) - the three p orbitals, say, or the five d orbitals - in what order do individual electrons fill those particular orbitals among themselves? That gap is closed by Hund's rule of maximum multiplicity: electron pairing within a set of degenerate orbitals does not begin until every orbital in that set already holds one electron each.
Concretely, since there are three p orbitals, five d orbitals and seven f orbitals, this rule says that pairing only starts once the 4th electron enters a p subshell, the 6th electron enters a d subshell, or the 8th electron enters an f subshell - every earlier electron instead goes into its own previously-empty orbital, with its spin kept parallel to the other unpaired electrons already there. …
Worked out. The textbook illustrates Hund's rule with carbon (6 electrons, ground-state configuration ). It draws the orbital diagram as (paired), (paired), then the two 2p electrons placed as - i.e. one electron each in two DIFFERENT p orbitals with parallel spin, rather than both electrons paired up together in a single orbital. The explanation given is that placing the sixth electron in the empty orbital instead of pairing it with the electron already in minimises electron-electron repulsion, since two electrons in separate orbitals are on average farther apart than two elect …