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Bonding in solids

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 (sometimes called simply "covalent solids")


 * Ionic bonding, which forms  ionic solids


 * Metallic bonding, which forms  metallic solids


 * Weak intermolecular bonding, which forms  molecular solids (sometimes anomalously called "covalent solids")

Typical members of these classes have distinctive electron distributions , 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.

Network covalent solids
A network covalent solid consists of atoms held together by a network of covalent bonds (pairs of electrons shared between atoms of similar electronegativity), and hence can be regarded as a single, large molecule. The classic example is diamond; other examples include silicon, quartz, and graphite.

Covalent network solids are typically have high strength, high elastic modulus, and high melting temperatures. Their strength, stiffness, and high melting points are consequences of the strength and stiffness of the covalent bonds that hold them together. They are also characteristically brittle because the directional nature of covalent bonds strongly resists the shearing motions associated with plastic flow, and are, in effect, broken when shear occurs. This property results in brittleness for reasons studied in the field of fracture mechanics. Network covalent solids vary from insulating to semiconducting in their behavior, depending on the band gap of the material.

Ionic solids
A standard ionic solid consists of atoms held together by  ionic bonds, that is, by the electrostatic attraction of opposite charges (the result of transferring electrons from lower- to higher-electronegativity atoms). 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. Vapor pressures in all instances are extraordinarily low; this is a consequence of the large energy required to move a bare charge (or charge pair) from an ionic medium into free space. Ionic solids have large band gaps, and hence are insulators.

Metallic solids
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 see intermediate forms

Solids with purely metallic bonding are ductile and, in their pure forms, have low strength; melting points can be very low (e.g., Mercury melts at 234 K (–39°C). These properties are consequences of the non-directional and non-polar nature of metallic bonding, in which planes of atoms can slide past one another without disrupting their interactions with the surrounding sea of delocalized electrons. Greater strength can be provided by interfering with the dislocations that mediate plastic deformation. Further, some transition metals exhibit directional bonding in addition to metallic bonding; this increases shear strength and reduces ductility. Metallic solids have, by definition, no band gap at the Fermi level and hence are conducting.

Molecular solids
A classic molecular solid consists of small, non-polar covalent molecules, and is held together by London dispersion forces (van der Waals forces); a classic example is paraffin wax. These forces are weak, resulting in pairwise interatomic binding energies on the order of 1/100 those of covalent, ionic, and metallic bonds. Binding energies tend to increase with increasing molecular size and polarity (see intermediate forms).

Solids that are composed of small, weakly bound molecules are mechanically weak and have low melting points; an extreme case is solid molecular hydrogen, which melts at 14 K (–259°C). The non-directional nature of dispersion forces typically allows easy plastic deformation, as planes of molecules can slide over one another without seriously disrupting their attractive interactions. Molecular solids are typically insulators with large band gaps.

Solids of intermediate kinds
The four classes of solids would permit, in principle, six pairwise intermediate forms, but only four of these are physically consistent and encountered in reality:

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 (for example, C–C and C–H bonds in aliphatic hydrocarbons). 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; the bonds between Mg and O in magnesium oxide, by contrast, are chiefly ionic in 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 organization of their covalent structures or because they are held together by intermediate forms of bonding, or because of the organization of their covalent bonds.

Intermediate organization of covalent bonds:

Regarding the organization of covalent structures, 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 increasing chain length, with high-density polyethylene at the far end of the series. High-density polyethylene is a strong material, and 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 cross-linked hydrocarbon polymers, 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 forms of bonding:

A solid with some degree of 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 akin to that found in ionic solids.

Molecular to ionic
A large molecule with an ionized group is technically an ion, but its behavior may be largely the result of non-ionic interactions. For example, sodium stearate (the main constituent of traditional soaps) consists entirely of ions, yet it is a soft material quite unlike a typical ionic solid. There is a continuum between ionic solids and molecular solids with little ionic character in their bonding.

Metallic to ionic or molecular
Metallic solids are characterized by bonding interactions that result from a high density of shared, delocalized electrons. This form of bonding is incompatible with the existence of separate, weakly bound molecular components, and is likewise incompatible with the existence of charged ions bound by electrostatic forces.