5.13: Network Covalent Atomic Solids- Carbon and Silicates

Covalent Network Solids

Covalent solids are formed by networks or chains of atoms or molecules held together by covalent bonds. A perfect single crystal of a covalent solid is therefore a single giant molecule. For example, the structure of diamond, shown in part (a) in Figure \(\PageIndex{2}\), consists of carbon atoms, each bonded to four other carbon atoms in a tetrahedral array to create a giant network. The carbon atoms form six-membered rings. In, graphite, the other common allotrope of carbon, has the structure shown in part () in Figure \(\PageIndex{1}\). It contains planar networks of six-membered rings carbon atoms in which each carbon is bonded to three others. This leaves a single electron in a 2p orbital that can be used to form C=C double bonds, resulting in a ring with alternating double and single bonds.

a)           b)              

Figure \(\PageIndex{1}\) a) Carbon bonding in graphite; b) Carbon bonding in diamond

Figure \(\PageIndex{2}\) The Structures of Diamond and Graphite. (a) Diamond consists of sp3 hybridized carbon atoms, each bonded to four other carbon atoms. The tetrahedral array forms a giant network in which carbon atoms form six-membered rings. (b) These side (left) and top (right) views of the graphite structure show the layers of fused six-membered rings and the arrangement of atoms in alternate layers of graphite. The rings in alternate layers are staggered, such that every other carbon atom in one layer lies directly under (and above) the center of a six-membered ring in an adjacent layer.

Elemental silicon has the same structure, as does silicon carbide (SiC), which has alternating C and Si atoms. The structure of crystalline quartz (silicon dioxide, SiO₂), shown in \(\PageIndex{3}\) can be viewed as being derived from the structure of silicon by inserting an oxygen atom between each pair of silicon atoms. 

\(\PageIndex{3}\) . Network covalent solid structure of SiO₂.Image by Smokefoot, CC BY-SA 4.0, via Wikimedia Commons

All compounds with the diamond and related structures are extremely hard, high-melting-point solids that are not easily deformed. Instead, they tend to shatter when subjected to large stresses, and they usually do not conduct electricity very well. In fact, diamond (melting point = 3500°C at 63.5 atm) is one of the hardest substances known, and silicon carbide (melting point = 2986°C) is used commercially as an abrasive in sandpaper and grinding wheels. It is difficult to deform or melt these and related compounds because strong covalent (C–C or Si–Si) or polar covalent (Si–C or Si–O) bonds must be broken, which requires a large input of energy.

 

 

Fullerenes

Until the mid 1980's, pure carbon was thought to exist in two forms: graphite and diamond. The discovery of C₆₀ molecules in interstellar dust in 1985 added a third form to this list. The existence of C₆₀, which resembles a soccer ball, had been hypothesized by theoretians for many years. In the late 1980's synthetic methods were developed for the synthesis of C₆₀, and the ready availability of this form of carbon led to extensive research into its properties.

Figure \(\PageIndex{4}\): Example of fullerenes: a buckyball (\(C_{60}\) on left and an extended bucktube. Images used with permission from Wikipedia.

The C₆₀ molecule (Figure \(\PageIndex{4}\); left), is called buckminsterfullerene, though the shorter name fullerene is often used. The name is a tribute to the American architect R. Buckminster Fuller, who is famous for designing and constructing geodesic domes which bear a close similarity to the structure of C₆₀. As is evident from the display, C₆₀ is a sphere composed of six-member and five-member carbon rings. These balls are sometimes fondly referred to as "Bucky balls". It should be noted that fullerenes are an entire class of pure carbon compounds rather than a single compound. A distorted sphere containing more than 60 carbon atoms have also been found, and it is also possible to create long tubes (Figure \(\PageIndex{4}\); right). All of these substances are pure carbon.