Editing Adhesion (section)

===Chemical===
Two materials may form a [[Chemical compound|compound]] at the joint. The strongest joints are where atoms of the two materials share or swap electrons (known respectively as [[covalent bonding]] or [[ionic bonding]]). A weaker bond is formed if a [[hydrogen]] atom in one [[molecule]] is attracted to an atom of [[nitrogen]], [[oxygen]], or [[fluorine]] in another molecule, a phenomenon called [[hydrogen bonding]].

Chemical adhesion occurs when the [[surface atom]]s of two separate surfaces form ionic, covalent, or hydrogen bonds. The engineering principle behind chemical adhesion in this sense is fairly straightforward: if surface molecules can bond, then the surfaces will be bonded together by a network of these bonds. It bears mentioning that these attractive ionic and covalent forces are effective over only very small distances – less than a [[nanometer]]. This means in general not only that surfaces with the potential for chemical bonding need to be brought very close together, but also that these bonds are fairly brittle, since the surfaces then need to be kept close together.<ref name=Kendall>{{cite journal|author=K. Kendall|title=Adhesion: Molecules and Mechanics|pmid=17795378|year=1994|volume=263|issue=5154|pages=1720–5|doi=10.1126/science.263.5154.1720|journal=Science|bibcode=1994Sci...263.1720K |s2cid=1525799 }}</ref>