Of the major molecular constituents of the atmosphere (see Table 8.1 and Fig. 8.1), only water vapor and ozone, owing to their bent structure, have a non-zero electric dipole moment. Molecular nitrogen, an homonuclear species, and CO2, a linear symmetric species, have no permanent electric or magnetic dipole moment in their lowest energy states. These latter molecules, as is the case for 99.9% of gaseous molecules, are singlet states, with electrons arranged two-by-two with opposite spins.