. Atomic oxygen
Atomic oxygen has a total of eight electrons. Using the electronic orbitals and the Pauli exclusion
principle to fill the orbitals with electrons, the electronic configuration of atomic oxygen is 1s2
2s2
2p4
.
Since the 2p orbitals are partially filled, there are three different ways to arrange the electrons in the
2px, 2py and 2pz orbitals (Figure 1). According to Hund's first rule, the ground state configuration
corresponds to an orbital occupancy that gives the highest multiplicity1
. The first configuration has a
multiplicity of 3 (hence termed triplet), and the other configurations have a multiplicity of 1 (hence
termed singlet). Therefore, the ground state of atomic oxygen has two unpaired electrons (bi‐radical),
and is designated as 3
P ("triplet P") state. The Hund's second rule states that for two configurations with
the same multiplicity, the configuration with the highest total orbital angular momentum (L) has the
lowest energy. Since the second configuration has the higher L value, its energy is lower. It is the first
excited state of atomic oxygen, designated as the 1
D ("singlet D") state. The configuration 1
S is the
second excited state.