A crossed beam chemical reaction, Al(2P1/2, 3/2) plus O2(X 3(Sigma) g-) yields AlO(X 2(Sigma) +) plus O(3Pj), has been investigated by fluorescence imaging techniques. It has been found experimentally that only the ground spin-orbit state, i.e., Al atoms in the 2P1/2 level are reactive towards O2 molecules. An electrostatic interaction model is examined to reveal the dynamics in the region of large separation between the two reactants. It is shown that Al atoms in the 2P3/2 spin-orbit state experience repulsive potential energy surfaces and are non-reactive. In the entrance valley of the allowed channel, Al atoms in the 2P1/2 spin-orbit state are governed by a barrierless, attractive potential energy surface. From the microscopic mechanism of the Al plus O2 reaction, its activation energy has been calculated to be -0.19 kcal/mol, in accordance with experimental measurements.