Bonding in solids
can be classified according to the nature of the bonding between their atomic or molecular components. The traditional classification distinguishes four kinds of bonding:
- Covalent bonding, which forms network covalent solids
- Ionic bonding, which forms ionic solids
- Metallic bonding, which forms metallic solids
- Weak inter molecular bonding, which forms molecular solids
thermodynamic, electronic, and mechanical properties. In particular, the binding energies of these interactions vary widely. Bonding in solids can be of mixed or intermediate kinds, however, hence not all solids have the typical properties of a particular class, and some can be described as intermediate forms.
Basic classes of solids
Network covalent solids
A network covalent solid consists of atoms held together by a network of covalent bonds, and hence can be regarded as a single, large molecule. The classic example is diamond; other examples include silicon, quartz and graphite.Properties
- High strength
- High elastic modulus
- High melting point
- Brittle
Ionic solids
A standard ionic solid consists of atoms held together by ionic bonds, that is by the electrostatic attraction of opposite charges. Among the [|ionic solids] are compounds formed by alkali and alkaline earth metals in combination with halogens; a classic example is table salt, sodium chloride.Ionic solids are typically of intermediate strength and extremely brittle. Melting points are typically moderately high, but some combinations of molecular cations and anions yield an ionic liquid with a freezing point below room temperature. Vapour pressures in all instances are extraordinarily low; this is a consequence of the large energy required to move a bare charge from an ionic medium into free space.
Metallic solids
are held together by a high density of shared, delocalized electrons, resulting in metallic bonding. Classic examples are metals such as copper and aluminum, but some materials are metals in an electronic sense but have negligible metallic bonding in a mechanical or thermodynamic sense. Metallic solids have, by definition, no band gap at the Fermi level and hence are conducting.Solids with purely metallic bonding are characteristically ductile and, in their pure forms, have low strength. Melting points can vary widely, from very low to extremely high. However, almost all metals, with the exception of the alkali metals and Group 12 metals, have high boiling points. These properties are consequences of the non-directional and non-polar nature of metallic bonding, which allows atoms to move past one another without disrupting their bonding interactions. Metals can be strengthened by introducing crystal defects that interfere with the motion of dislocations that mediate plastic deformation. Further, some transition metals exhibit directional bonding in addition to metallic bonding; this increases shear strength and reduces ductility, imparting some of the characteristics of a covalent solid.
Solids of intermediate kinds
The four classes of solids permit six pairwise intermediate forms:Ionic to network covalent
Covalent and ionic bonding form a continuum, with ionic character increasing with increasing difference in the electronegativity of the participating atoms. Covalent bonding corresponds to sharing of a pair of electrons between two atoms of essentially equal electronegativity. As bonds become more polar, they become increasingly ionic in character. Metal oxides vary along the iono-covalent spectrum. The Si–O bonds in quartz, for example, are polar yet largely covalent, and are considered to be of mixed character.Metallic to network covalent
What is in most respects a purely covalent structure can support metallic delocalization of electrons; metallic carbon nanotubes are one example. Transition metals and intermetallic compounds based on transition metals can exhibit mixed metallic and covalent bonding, resulting in high shear strength, low ductility, and elevated melting points; a classic example is tungsten.Molecular to network covalent
Materials can be intermediate between molecular and [|network covalent solids] either because of the intermediate organization of their covalent bonds, or because the bonds themselves are of an intermediate kind.Intermediate organization of covalent bonds:
Regarding the organization of covalent bonds, recall that classic molecular solids, as stated above, consist of small, non-polar covalent molecules. The example given, paraffin wax, is a member of a family of hydrocarbon molecules of differing chain lengths, with high-density polyethylene at the long-chain end of the series. High-density polyethylene can be a strong material: when the hydrocarbon chains are well aligned, the resulting fibers rival the strength of steel. The covalent bonds in this material form extended structures, but do not form a continuous network. With cross-linking, however, polymer networks can become continuous, and a series of materials spans the range from Cross-linked polyethylene, to rigid thermosetting resins, to hydrogen-rich amorphous solids, to vitreous carbon, diamond-like carbons, and ultimately to diamond itself. As this example shows, there can be no sharp boundary between molecular and network covalent solids.
Intermediate kinds of bonding:
A solid with extensive hydrogen bonding will be considered a molecular solid, yet strong hydrogen bonds can have a significant degree of covalent character. As noted above, covalent and ionic bonds form a continuum between shared and transferred electrons; covalent and weak bonds form a continuum between shared and unshared electrons. In addition, molecules can be polar, or have polar groups, and the resulting regions of positive and negative charge can interact to produce electrostatic bonding resembling that in ionic solids.