An extra-orbital tight-binding scheme and a repulsive Born-Mayer model are used to determine the total adsorption energy of a 2p adatom (B, C or N) at the surface of graphite. The corresponding electronic structure of the adsorbed atom is deduced for the most stable (hollow) position. Our model allows us to exhibit an increase of the local density of states on the adatom and a decrease of charge transfer (from adatom to carbon) when going from boron to nitrogen