A simple bond index is developed which permits predicting weak bonds in a radical cation. The model is based on the perturbed first-order wave function resulting from a bond elongation in a radical cation. By referring to the first-order wave function, the model contains implicitly the relaxation term of the force constant but also important terms of the anharmonicity constant. The band strength is related to an interaction between occupied and unoccupied molecular orbitals. The bond stretch force operator is applied to show that an interaction is bond weakening provided the occupied and the unoccupied orbital both have a nodal surface in the bond region. When only the occupied orbital is bonding but the unoccupied orbital is antibonding a weak bond is not indicated. The theoretical scheme is applied to estimate carbon bond strengths in the n-butane radical cation. The bond index indicates that a rupture of the terminal carbon bond is involved in the main fragmentation pathway as observed in the mass spectrum. This qualitative result is supported by ab initio Hartree-Fock calculations. They show in addition that the preferred fragmentation pathway comprises also a hydrogen shift leading to low-energy fragments.