The temporal evolution of the electron velocity-distribution function (EVDF), the concentration, mean energy and the drift velocity of the electrons is studied on a kinetic basis in a weakly ionized Ar/F-2 mixture plasma under conditions when the electron concentration temporally decreases as a result of the electron attachment to fluoride molecules. Using an appropriate relaxation model, the time-dependent electron Boltzmann equation,vas solved in multiterm and two-term approximations of the velocity distribution function. The multiterm results confirmed predictions on the occurrence of negative electron mobilities in such a decaying Ar/F-2 plasma, which were made in a former study using the conventional two-term approximation. The investigations particularly showed that this approximation gives almost accurate results for the EVDF and related electron swarm parameters except for in the very beginning of the relaxation process. It has been further shown that for a certain range of the reduced electric field strength, the drift velocity becomes negative in the process of temporal evolution and remains negative even when approaching the hydrodynamic stage of the electron swarm. In addition, the role played by the back heating from the gas by elastic collisions on the EVDF formation is studied and various comparisons with corresponding Monte Carlo results are performed.