In this work, we study the electronic and chemical properties of a graphene sheet doped with S or P, by means of ab initio calculations. We consider one, two and three impurity atoms by substitution on the graphene and obtain doping formation energies of 5.78, 7.43 and 10.53eV for sulphur impurities and 2.73, 0.54 and 1.82 eV for phosphorous impurities. We find that doping induces a large local curvature that tends to increase the system local reactivity. We characterize the electronic structure by the electronic density of states, the electron localization function and the maximally localized Wannier functions. Some potential applications in electronic nanodevices are highlighted.