What defines density of chemical substances at the same pressure and temperature?
What type of density are you talking about? Mass density?
Density of molecular substances is defined by forces of attraction and repulsion between its molecules. I wish to know why hydrogen molecules, for example, repulse each other much more than say, Lithium molecules.
Are you comparing the densities of gases or densities of condensed phases (solid, liquid)?
IDK about that. I always thought the density of a substance was defined by how much mass of the substance was contained in a unit volume. Can you provide another source for your definition of density?
Separate names with a comma.