The orbital-phase theory was applied to propose pentagon stability in a well-defined manner. Cyclic delocalization of the lone pair electrons on the five-membered ring atoms through the vicinal sigma bonds was shown to be favored by the orbital-phase properties. The pentagon stability was found to be outstanding in saturated phosphorus five-membered rings in the puckered conformation, and was substantiated by the negative strain energy of cyclopentaphosphane, P5H5 (3). The relative increments of the remarkable increase in the strain energies of protonation on the different atoms in the most stable conformers supported the significance of the cyclic delocalization of the lone pairs. Pentagon stability led to the design of three novel polycyclic phosphanes, P12H4 (18), P13H3 (19), and P14H2 (20), with low strain energies due to many puckered pentagon units in them. The low stability of the dodecahedron P-20 (22) was suggested by the high strain energy due to its planar pentagon units. The pentagon stability is less significant in the saturated nitrogen ring molecules due to the greater energy gap between the n and sigma* orbitals.