Hund's Rule of Maximum Multiplicity

2.6.4c Hund's Rule of Maximum Multiplicity

This rule governs how electrons fill orbitals within the same subshell — that is, orbitals that have the same energy (called degenerate orbitals). The rule states:

Pairing of electrons in orbitals of the same subshell ($p$, $d$, or $f$) does not take place until each orbital in that subshell has one electron (i.e., is singly occupied).

In other words, electrons spread out as much as possible before they pair up. This is because electrons repel each other, and it is energetically favourable for them to occupy different orbitals (with parallel spins) rather than share the same orbital.

Application to Different Subshells

• $p$ subshell (3 orbitals): The first three electrons each go into a separate $p$ orbital. Pairing begins only with the fourth electron.
• $d$ subshell (5 orbitals): The first five electrons each occupy a separate $d$ orbital. Pairing begins with the sixth electron.
• $f$ subshell (7 orbitals): The first seven electrons each occupy a separate $f$ orbital. Pairing begins with the eighth electron. …