The electron is the dominant species in the radiation chemistry of those gases where the G-value has been observed to be independent of linear energy transfer. Taking the radiolysis of ammonia vapor as such an example, we enumerate the various electron-molecule scattering channels and compute the rate of electron energy loss. Assuming a specific reaction mechanism, we compute the rate of disappearance of ammonia molecules. From the rate of electron energy loss and the rate of ammonia molecule disappearance, we obtain a formula for G(-NH₃). We then note that the electric dipole approximation in electron-molecule scattering permits us to consider the same mechanism from the point of view of photochemistry. We compute the photochemical quantum yield and derive a formula linking it to G(-NH₃). We discuss the chemical implications of our formula, noting its application to plasma chemistry and its restriction to gases which do not attach electrons.