Published online by Cambridge University Press: 12 April 2016
Observational evidence related to the chemical composition across the disk of the Galaxy is reviewed. The H2 density distribution derived for the Galaxy is poorly known, consequently it is still not possible to compare theoretical models of the chemical evolution of the Galaxy with the gaseous density distribution. The H2 density distribution is particularly sensitive to the fraction of carbon atoms embedded in CO molecules and to the possible presence of a C/H abundance gradient.