QTAIM properties for uracil and 18 derivatives containing the substituents -NH2, -OH, -OCH3, -SH, -F, -Cl, -CH3 -NO2, and -Li in position 5 or 6 were computed on MP2/6-31++G**//MP2/6-31G** charge densities. The results indicate that -OH, -OCH3, and -NH2 groups are really retrieving charge from the ring. Also, the activating ability of the substituent groups, usually considered as the variation of electron population at the carbon where the electrophilic attack takes place, C*, was studied. The study shows that the activating ability is reflected by the variation of pi charge or quadrupole moment at C*, and also by the variation of the Laplacian of the charge density in the secondary charge concentration points around C* (SCC-C*). They indicate a similar, but not exactly equal, graduation of activating ability. The relative behavior of the substituents is basically the same as in benzene, though benzene has more tendency to concentrate charge in the SCC-C* regions than uracil, where this tendency is larger for 6- than for 5-derivatives. sigma(+/-)(R) Taft parameters are found to display good correlations with the above indicated activating indexes. Finally, the resonance model predicts most of the main variations displayed by QTAIM atomic pi electron populations of derivatives with regard to uracil, but there are still some significant variations of the pi electron charge that it cannot predict.