<P> The hydrogen - bond networks make both natural and synthetic polymers sensitive to humidity levels in the atmosphere because water molecules can diffuse into the surface and disrupt the network . Some polymers are more sensitive than others . Thus nylons are more sensitive than aramids, and nylon 6 more sensitive than nylon - 11 . </P> <P> A symmetric hydrogen bond is a special type of hydrogen bond in which the proton is spaced exactly halfway between two identical atoms . The strength of the bond to each of those atoms is equal . It is an example of a three - center four - electron bond . This type of bond is much stronger than a "normal" hydrogen bond . The effective bond order is 0.5, so its strength is comparable to a covalent bond . It is seen in ice at high pressure, and also in the solid phase of many anhydrous acids such as hydrofluoric acid and formic acid at high pressure . It is also seen in the bifluoride ion (F − H − F). </P> <P> Symmetric hydrogen bonds have been observed recently spectroscopically in formic acid at high pressure (> GPa). Each hydrogen atom forms a partial covalent bond with two atoms rather than one . Symmetric hydrogen bonds have been postulated in ice at high pressure (Ice X). Low - barrier hydrogen bonds form when the distance between two heteroatoms is very small . </P> <P> The hydrogen bond can be compared with the closely related dihydrogen bond, which is also an intermolecular bonding interaction involving hydrogen atoms . These structures have been known for some time, and well characterized by crystallography; however, an understanding of their relationship to the conventional hydrogen bond, ionic bond, and covalent bond remains unclear . Generally, the hydrogen bond is characterized by a proton acceptor that is a lone pair of electrons in nonmetallic atoms (most notably in the nitrogen, and chalcogen groups). In some cases, these proton acceptors may be pi - bonds or metal complexes . In the dihydrogen bond, however, a metal hydride serves as a proton acceptor, thus forming a hydrogen - hydrogen interaction . Neutron diffraction has shown that the molecular geometry of these complexes is similar to hydrogen bonds, in that the bond length is very adaptable to the metal complex / hydrogen donor system . </P>

When does a hydrogen atom form an ionic bond