Theories of valency are based on physical properties of molecules, most of which are expressed in terms of so-called ``additive molecular constants.'' Dipole moments, molecular volumes, bond distance, energies of formation, etc., belong to this class. The following paper intends to show that such constants are capable of being consistently interpreted in various entirely different ways. Hence a number of entirely equivalent but different and discordant theories of valency are possible, leading, for instance, to classical or to resonance structures of inorganic molecules. The choice between them therefore is arbitrary and not due to necessity. In the following part, I, one of these additive molecular constants, namely, the molecular volume in terms of the parachor, is studied in detail. The existence of coordinate types of covalent linkage having been ruled out by new experimental evidence for many inorganic first-order molecules formed by a definite central atom (Section 1), a new set of atomic parachors is calculated on the basis of the classical formulae of chemistry (Section 2). Instead of resorting to different types of bonds and their respective parachor constants, it ascribes different constants to the same atom in different valence states (Section 3). The contributions of the constituent atoms to the volume of the covalent molecule thus calculated bear a close relation to their atomic numbers and to their ionization potentials (Section 4). The new atomic parachors account for such molecules as N2, O2, N2O, CO, CO2, SO2, which earlier had to be considered as anomalous, and also give the correct structure of the azides. However, as the molecular volume apparently can be separated into atomic contributions according to entirely different concepts of valency, the parachor (i.e., the molecular volume) is an example of an additive molecular constant which is unable to distinguish between different possible structures of a molecule or the different conflicting chemical theories on which they are based.
