Normalized abundances of four trace neutral atomic species N(X)/N(H_tot) vs. molecular fraction f(H₂). The stars denote the sight line toward HD 62542: red = main (14 km s⁻¹) component; green = all other components; blue = all components. The other symbols are for sight lines in the Galactic ISM, mostly from Welty & Hobbs (2001), Jenkins & Tripp (2011), and Welty et al. (2016) for Na I and C I and from Welty et al. (2003) for Ca I and Fe I. The dotted lines have slopes of 1 (Na I, C I) or −1 (Ca I, Fe I). The differences in the behavior of those trace species for f(H₂) > 0.1 reflect differences in their ionization and depletion in denser gas.