An attractive force between atoms strong enough to permit the combined aggregate to function as a unit. A more exact definition is not possible because attractive forces ranging upward from 0 to those involving more than 250 kcal/mole of bonds are known. A practical lower limit may be taken as 2–3 kcal/mole of bonds, the work necessary to break approximately 1.5 × 1024 bonds by separating their component atoms to infinite distance. All bonds appear to originate with the electrostatic charges on electrons and atomic nuclei. Bonds result when the net coulombic interactions are sufficiently attractive. Different principal types of bonds recognized include metallic, covalent, ionic, and bridge. Metallic bonding is the attraction of all the atomic nuclei in a crystal for the outer shell electrons which are shared in a delocalized manner among all available orbitals. Metal atoms characteristically provide more orbital vacancies than electrons for sharing with other atoms. Covalent bonding results most commonly when electrons are shared by two atomic nuclei. Here the bonding electrons are relatively localized in the region of the two nuclei, although frequently a degree of delocalization occurs when the shared electrons have a choice of orbitals. The conventional single covalent bond involves the sharing of two electrons. There may also be doublebonds with four shared electrons, triple bonds with six shared electrons, and bonds of intermediate multiplicity. Covalent bonds may range from nonpolar, involving electrons evenly shared by the two atoms, to extremely polar, where the bonding electrons are very unevenly shared. The limit of uneven sharing occurs when the bonding electron spends full time with one of the atoms, making the atom into a negative ion and leaving the other atom in the form of a positive ion. Ionic bonding is the electrostatic attraction between oppositely charged ions. Bridge bonds involve compounds of hydrogen in which the hydrogen bears either a + or −charge. When hydrogen is attached by a polar covalent bond to one molecule, it may attract another molecule, bridging the two molecules together. If the hydrogen is +, it may attract an electron pair of the other molecule. This is called a protonic bridge. If the hydrogen is −, it may attract through a vacant orbital the nucleus of an atom of a second molecule. This is called a hydridic bridge. Such bridges are at the lower range of bond strength but may have a significant effect on the physical properties of condensed states of those substances in which they are possible.See Hydrogen Bond.
