  By using first principles calculations we report a chemical doping induced
gap in graphene. The structural and electronic properties of CrO$_3$
interacting with graphene layer are calculated using ab initio methods based on
the density functional theory. The CrO$_3$ acts as an electron acceptor
modifying the original electronic and magnetic properties of the graphene
surface through a chemical adsorption. The changes induced in the electronic
properties are strongly dependent of the CrO$_3$ adsorption site and for some
sites it is possible to open a gap in the electronic band structure. Spin
polarization effects are also predicted for some adsorption configurations.
