def electron_binding_energy(charge_number):
    """Return the electron binding energy for a given number of protons (unit
    is MeV). Expression is taken from [Lunney D., Pearson J. M., Thibault C., 
    2003, Rev. Mod. Phys.,75, 1021]."""
    return 1.44381e-5 * charge_number ** 2.39\
            + 1.55468e-12 * charge_number ** 5.35