Carbon has six electrons, and a question arises of how to assign the two 2p
electrons. Do they go into the same orbital with paired spins or
do they occupy different orbitals with unpaired spins ? Experiments
show that the energetically preferred configuration is the latter, in
which the spins are aligned. This is one illustration of Hund’s rule,
which states that electrons usually fill different orbitals with unpaired
spins, rather than the same orbital with paired spins. Hund’s rule can be
partly understood by noting that electrons in the same orbital tend to be
closer together, where their mutual repulsion contributes to a higher energy
than if they were separated in different orbitals. Some exceptions to
this rule do occur in those elements with subshells that are nearly filled
or half-filled. The progressive filling of the 2p subshell illustrating Hund’s
rule is shown schematically in Figure 9.15. With neon, the 2p subshell is
also closed. The neon atom has ten electrons in the configuration
1s22s22p6. Because the energy gap separating the 2p level from the next
available level—the 3s—is quite large, the neon configuration is exceptionally
stable and the atom is chemically inert.