An explanation of the polyatomic molecules shape changing in certain excited states using the group theory is presented. The method of electron configurations, deduced from molecular orbital theory was used. It is possible to see the mechanism that makes a molecule change its shape in some excited states by the correlation between the united atom orbitals, the molecular orbitals and the separated atoms orbitals. The hybridization of the atomic orbitals is favored in some symmetries of the molecule. As a direct result, the changing of the molecule symmetry is given by the filling of these orbitals with electrons.