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4. Large Covalent Molecules and their Propertiesmacromolecules - giant covalent networks and polymers

What is the bonding, structure and properties of the carbon allotropes diamond, graphite & buckminsterfullerenes (fullerenes), silica

(silicon dioxide), thermosets, thermoplastics?Because covalent bonds act in a particular direction i.e. along the 'line' between the two nuclei of the atoms bonded together in an

individual bond, strong structures can be formed, especially if the covalent bonds are arranged in a strong three dimensional giant

covalent lattice.The structure of the three allotropes of carbon (diamond, graphite and fullerenes), silicon and silicon

dioxide (silica)DIAGRAMS

It is possible formany atoms to link up to form a giant covalent structure or lattice. The atoms are usually

non-metals. This produces a very strong 3-dimensional covalent bond network or lattice. This gives them significantly different properties from the small simple covalent molecules

mentioned above. This is illustrated by carbon in the form of diamond (an allotrope of carbon). Carbon has four outer

electrons that form four single bonds, so each carbon bonds to four others by electron pairing/sharing. Pure

silicon, another element in Group 4, has a similar structure.o NOTE: Allotropes are different forms of the same element in the same physical state. They occur

due to different bonding arrangements and so diamond, graphite(below) and fullerenes(below)

are the three solid allotropes of the element carbon . Oxygen (dioxygen), O2, and ozone (trioxygen), O3, are the two small gaseous allotrope

molecules of the element oxygen. Sulphur has three solid allotropes, two different crystalline forms based on small

DIAMOND

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S8 molecules called rhombic and monoclinic sulphur and a 3rd form of long chain ( -S-S-S-

etc.) molecules called plastic sulphur. TYPICAL PROPERTIES of GIANT COVALENT STRUCTURES

This type of giant covalent structure is thermally very stable and has a very high melting and boilingpoints because of the strong covalent bond network (3D or 2D in the case ofgraphitebelow).

A relatively large amount of energy is needed to melt or boil giant covalent structures. Energy changes for

the physical changes of state of melting and boiling for a range of differently bonded substances are

compared in a section of theEnergetics Notes. They are usually poor conductors of electricity because the electrons are not usually free to move as they

can in metallic structures. Also because of the strength of the bonding in all directions in the structure, they are often very hard,

strong and will not dissolve in solvents like water. The bonding network is too strong to allow the atoms tobecome surrounded by solvent molecules

Silicon dioxide (silica, SiO2) has a similar 3D structure and properties to carbon (diamond) shown below. The hardness of diamond enables it to be used as the 'leading edge' on cutting tools . Energy changes for the physical changes of state of melting and boiling for a range of differently bonded

substances is given in a section of theEnergetics Notes. Many naturally occurring minerals are based on -O-X-O- linked 3D structures where X is often silicon

(Si) and aluminium (Al), three of the most abundant elements in the earth's crust.o Silicon dioxide is found as quartz in granite (igneous rock) and is the main component in sandstone

- which is a sedimentary rock formed the compressed erosion products of igneous rocks.o Many some minerals that are hard wearing, rare and attractive when polished, hold great value

as gemstones.

SILICAsilicon dioxide

Carbon also occurs in the form of graphite. The carbon atoms form joined hexagonal rings forming layers

1 atom thick. GRAPHITE

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There are three strong covalent bonds per carbon (3 C-C bonds in a planar arrangement from 3 of its 4

outer electrons), BUT, the fourth outer electron is 'delocalised' or shared between the carbon atoms to form

the equivalent of a 4th bond per carbon atom (this situation requires advanced level concepts to fully

explain, and this bonding situation also occurs in fullerenes described below, and in aromatic compounds

you deal with at advanced level). The layers are only held together by weak intermolecular forces shown by the dotted lines NOT by

strong covalent bonds. Like diamond and silica (above) the large molecules of the layer ensure graphite has typically very high

melting point because of the strong 2D bonding network (note: NOT 3D network).. Graphite will not dissolve in solvents because of the strong bonding BUT there are two crucial differences compared to diamond ...

o

Electrons, from the 'shared bond', can move freely through each layer, so graphite is aconductor like a metal(diamond is an electrical insulator and a poor heat conductor). Graphite is

used in electrical contacts e.g. electrodes in electrolysis.o The weak forces enable the layers to slip over each otherso where as diamond is hard

material graphite is a 'soft' crystal, it feels slippery. Graphite is used as a lubricant. These two different characteristics described above are put to a common use with the electrical contacts in

electric motors and dynamos. These contacts (called brushes) are made of graphite sprung onto the

spinning brass contacts of the armature. The graphite brushes provide good electrical contact and are self-

lubricating as the carbon layers slide over each other.

A 3rd form of carbon are fullerenes or 'bucky balls'! It consists of hexagonal rings like graphite and

alternating pentagonal rings to allow curvature of the surface. Buckminster Fullerene C60 is shown and the bonds form a pattern like a soccer ball. Others are oval

shaped like a rugby ball. It is a black solid insoluble in water. They are NOT considered giant covalent structures and are classed as simple molecules. They do

FULLERENES

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dissolve in organic solvents giving coloured solutions (e.g. deep red in petrol hydrocarbons, and although

solid, their melting points are not that high. They are mentioned here to illustrate the different forms of carbon AND they can be made into

continuous tubes to form very strong fibres of 'pipe like' molecules called 'nanotubes'. These 'molecular

size' particles behave quite differently to a bulk carbon material like graphite. Uses of Nanotubes - carbon nanotechnology - examples ofnanochemistry

o They can be used as semiconductors in electrical circuits.o They act as a component of industrial catalysts for certain reactions whose economic efficiency is

of great importance (time = money in business!). The catalyst can be attached to the nanotubes which have a huge surface are per mass of

catalyst 'bed'. They large surface combined with the catalyst ensure two rates of reaction factors work in

harmony to increase the speed of an industrial reaction so making the process more

efficient and more economic.o Nanotube fibres are very strong and so they are used in 'composite materials' e.g. reinforcing

graphite in carbon fibre tennis rackets.o Nanotubes can 'cage' other molecules and can be used as a means of delivering drugs in

controlled way to the body because the thin carbon nanotubes can penetrate cell walls. I've written NEW pages with more examples and details on

o General introduction to nanoscience and commonly used terms explainedo NANOCHEMISTRY - an introduction and potential applications

o NANOCHEMISTRY including fullerenes and CARBON NANOTUBESo SMART MATERIALS SCIENCE (alphabetical index at top of page)

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Bonding in polymers and 1-3 'dimension' concepts in macromolecules The bonding in polymers or plastics is no different in principle to the examples described above, but there is quite a range of properties

and the difference between simple covalent and giant covalent molecules can get a bit 'blurred'.o Bonds between atoms in molecules, e.g. C-C, are called intra-molecular bonds.o The much weaker electrical attractions between individual molecules are called inter-molecular forces.

In thermosoftening plastics like poly(ethene) the bonding is like ethane except there are lots of carbon atoms linked together to form

long chains. They are moderately strong materials but tend to soften on heating and are not usually very soluble in solvents. The

structure is basically a linear 1 dimensional strong bonding networks. The polymer molecules are held together by weak

intermolecular forces and NOT strong chemical bonds. The long polymer molecules mean the intermolecular forces are appreciable but

the material is flexible and softens on heating. Graphite structure is a layered 2 dimensional strong bond network made oflayers of joined hexagonal rings of carbon atoms with

weak inter-molecular forces between the layers. (more details on graphite) Thermosetting plastic structures like melamine have a 3 dimensional cross-linked giant covalent structure network similar to

diamond or silica in principle, but rather more complex and chaotic! Because of the strong 3D covalent bond network they do not dissolve

in any solvents and do not soften and melt on heating and are much stronger than thermoplastics. More on polymers inOil NotesandExtra Organic Chemistry Notes.

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A couple of Advanced Level scribbles', yet to be typed up!

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