Descriptions of the physical properties of three nonmetals that are characteristic of molecular solids follow. Carbon occurs in the uncombined elemental state in many forms, such as diamond, graphite, charcoal, coke, carbon black, graphene, and fullerene. Diamond, shown in [link] , is a very hard crystalline material that is colorless and transparent when pure. Each atom forms four single bonds to four other atoms at the corners of a tetrahedron sp 3 hybridization ; this makes the diamond a giant molecule.
Graphite, also shown in [link] , is a soft, slippery, grayish-black solid that conducts electricity. These properties relate to its structure, which consists of layers of carbon atoms, with each atom surrounded by three other carbon atoms in a trigonal planar arrangement. Many resonance forms are necessary to describe the electronic structure of a graphite layer; [link] illustrates two of these forms.
London dispersion forces hold the layers together. To learn more, see the discussion of these weak forces in the chapter on liquids and solids. Other forms of elemental carbon include carbon black, charcoal, and coke.
Carbon black is an amorphous form of carbon prepared by the incomplete combustion of natural gas, CH 4. It is possible to produce charcoal and coke by heating wood and coal, respectively, at high temperatures in the absence of air.
Recently, new forms of elemental carbon molecules have been identified in the soot generated by a smoky flame and in the vapor produced when graphite is heated to very high temperatures in a vacuum or in helium. One of these new forms, first isolated by Professor Richard Smalley and coworkers at Rice University, consists of icosahedral soccer-ball-shaped molecules that contain 60 carbon atoms, C This is buckminsterfullerene often called bucky balls after the architect Buckminster Fuller , who designed domed structures, which have a similar appearance [link].
Nanotubes and Graphene Graphene and carbon nanotubes are two recently discovered allotropes of carbon. Both of the forms bear some relationship to graphite. Graphene is a single layer of graphite one atom thick , as illustrated in [link] , whereas carbon nanotubes roll the layer into a small tube, as illustrated in [link]. Graphene is a very strong, lightweight, and efficient conductor of heat and electricity discovered in As in graphite, the carbon atoms form a layer of six-membered rings with sp 2 -hybridized carbon atoms at the corners.
Resonance stabilizes the system and leads to its conductivity. Unlike graphite, there is no stacking of the layers to give a three-dimensional structure.
The simplest procedure for preparing graphene is to use a piece of adhesive tape to remove a single layer of graphene from the surface of a piece of graphite.
This method works because there are only weak London dispersion forces between the layers in graphite. Alternative methods are to deposit a single layer of carbon atoms on the surface of some other material ruthenium, iridium, or copper or to synthesize it at the surface of silicon carbide via the sublimation of silicon.
There currently are no commercial applications of graphene. However, its unusual properties, such as high electron mobility and thermal conductivity, should make it suitable for the manufacture of many advanced electronic devices and for thermal management applications.
Carbon nanotubes are carbon allotropes, which have a cylindrical structure. Like graphite and graphene, nanotubes consist of rings of sp 2 -hybridized carbon atoms.
Unlike graphite and graphene, which occur in layers, the layers wrap into a tube and bond together to produce a stable structure. The walls of the tube may be one atom or multiple atoms thick. Carbon nanotubes are extremely strong materials that are harder than diamond.
Depending upon the shape of the nanotube, it may be a conductor or semiconductor. For some applications, the conducting form is preferable, whereas other applications utilize the semiconducting form. The basis for the synthesis of carbon nanotubes is the generation of carbon atoms in a vacuum. It is possible to produce carbon atoms by an electrical discharge through graphite, vaporization of graphite with a laser, and the decomposition of a carbon compound.
The strength of carbon nanotubes will eventually lead to some of their most exciting applications, as a thread produced from several nanotubes will support enormous weight. However, the current applications only employ bulk nanotubes. The addition of nanotubes to polymers improves the mechanical, thermal, and electrical properties of the bulk material.
There are currently nanotubes in some bicycle parts, skis, baseball bats, fishing rods, and surfboards. The name phosphorus comes from the Greek words meaning light bringing.
When phosphorus was first isolated, scientists noted that it glowed in the dark and burned when exposed to air. Phosphorus is the only member of its group that does not occur in the uncombined state in nature; it exists in many allotropic forms. We will consider two of those forms: white phosphorus and red phosphorus. White phosphorus is a white, waxy solid that melts at It is insoluble in water in which it is stored—see [link] , is very soluble in carbon disulfide, and bursts into flame in air.
As a solid, as a liquid, as a gas, and in solution, white phosphorus exists as P 4 molecules with four phosphorus atoms at the corners of a regular tetrahedron, as illustrated in [link].
Each phosphorus atom covalently bonds to the other three atoms in the molecule by single covalent bonds.
White phosphorus is the most reactive allotrope and is very toxic. Its structure is highly polymeric and appears to contain three-dimensional networks of P 4 tetrahedra joined by P-P single bonds. For example, the element carbon has two common allotropes: diamond , where the carbon atoms are bonded together in a tetrahedral lattice arrangement, and graphite , where the carbon atoms are bonded together in sheets of a hexagonal lattice.
Note that allotropy refers only to different forms of an element within the same phase or state of matter i. For some elements, allotropes have different molecular formulae which can persist in different phases - for example, the two allotropes of oxygen dioxygen , O 2 and ozone , O 3 , can both exist in the solid, liquid and gaseous states. Conversely, some elements do not maintain distinct allotropes in different phases: for example phosphorus has numerous solid allotropes, which all revert to the same P 4 form when melted to the liquid state.
The concept of allotropy was originally proposed in by the Swedish scientist Baron Jons Jakob Berzelius who offered no explanation. In the early 20th century it was recognized that other cases such as carbon were due to differences in crystal structure. By , Ostwald noted that the allotropy of elements is just a special case of the phenomenon of polymorphism known for compounds, and proposed that the terms allotrope and allotropy be abandoned and replaced by polymorph and polymorphism.
Although many other chemists have repeated this advice, IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only. The walls of the tube may be one atom or multiple atoms thick. Carbon nanotubes are extremely strong materials that are harder than diamond. Depending upon the shape of the nanotube, it may be a conductor or semiconductor. For some applications, the conducting form is preferable, whereas other applications utilize the semiconducting form.
The basis for the synthesis of carbon nanotubes is the generation of carbon atoms in a vacuum. It is possible to produce carbon atoms by an electrical discharge through graphite, vaporization of graphite with a laser, and the decomposition of a carbon compound. The strength of carbon nanotubes will eventually lead to some of their most exciting applications, as a thread produced from several nanotubes will support enormous weight.
However, the current applications only employ bulk nanotubes. The addition of nanotubes to polymers improves the mechanical, thermal, and electrical properties of the bulk material.
There are currently nanotubes in some bicycle parts, skis, baseball bats, fishing rods, and surfboards. The name phosphorus comes from the Greek words meaning light bringing. When phosphorus was first isolated, scientists noted that it glowed in the dark and burned when exposed to air.
Phosphorus is the only member of its group that does not occur in the uncombined state in nature; it exists in many allotropic forms. We will consider two of those forms: white phosphorus and red phosphorus. White phosphorus is a white, waxy solid that melts at It is insoluble in water in which it is stored—see Figure 6 , is very soluble in carbon disulfide, and bursts into flame in air.
As a solid, as a liquid, as a gas, and in solution, white phosphorus exists as P 4 molecules with four phosphorus atoms at the corners of a regular tetrahedron, as illustrated in Figure 6. Each phosphorus atom covalently bonds to the other three atoms in the molecule by single covalent bonds. White phosphorus is the most reactive allotrope and is very toxic.
Its structure is highly polymeric and appears to contain three-dimensional networks of P 4 tetrahedra joined by P-P single bonds. Red phosphorus is insoluble in solvents that dissolve white phosphorus. When red phosphorus is heated, P 4 molecules sublime from the solid. The allotropy of sulfur is far greater and more complex than that of any other element.
Sulfur is the brimstone referred to in the Bible and other places, and references to sulfur occur throughout recorded history—right up to the relatively recent discovery that it is a component of the atmospheres of Venus and of Io, a moon of Jupiter. The most common and most stable allotrope of sulfur is yellow, rhombic sulfur, so named because of the shape of its crystals. Rhombic sulfur is the form to which all other allotropes revert at room temperature.
Cooling this liquid gives long needles of monoclinic sulfur. At room temperature, it gradually reverts to the rhombic form. Both rhombic sulfur and monoclinic sulfur contain S 8 molecules in which atoms form eight-membered, puckered rings that resemble crowns, as illustrated in Figure 7. Each sulfur atom is bonded to each of its two neighbors in the ring by covalent S-S single bonds. When rhombic sulfur melts, the straw-colored liquid is quite mobile; its viscosity is low because S 8 molecules are essentially spherical and offer relatively little resistance as they move past each other.
As the temperature rises, S-S bonds in the rings break, and polymeric chains of sulfur atoms result. These chains combine end to end, forming still longer chains that tangle with one another. The dangling atoms at the ends of the chains of sulfur atoms are responsible for the dark red color because their electronic structure differs from those of sulfur atoms that have bonds to two adjacent sulfur atoms.
This causes them to absorb light differently and results in a different visible color. Cooling the liquid rapidly produces a rubberlike amorphous mass, called plastic sulfur. As seen in this discussion, an important feature of the structural behavior of the nonmetals is that the elements usually occur with eight electrons in their valence shells.
If necessary, the elements form enough covalent bonds to supplement the electrons already present to possess an octet. For example, members of group 15 have five valence electrons and require only three additional electrons to fill their valence shells. These elements form three covalent bonds in their free state: triple bonds in the N 2 molecule or single bonds to three different atoms in arsenic and phosphorus.
The elements of group 16 require only two additional electrons. Oxygen forms a double bond in the O 2 molecule, and sulfur, selenium, and tellurium form two single bonds in various rings and chains. The halogens form diatomic molecules in which each atom is involved in only one bond. This provides the electron required necessary to complete the octet on the halogen atom.
The noble gases do not form covalent bonds to other noble gas atoms because they already have a filled outer shell. Nonmetals have structures that are very different from those of the metals, primarily because they have greater electronegativity and electrons that are more tightly bound to individual atoms.
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