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CHEMICAL BONDING Part 4 Covalent Bonding - giant covalent structures and polymers

Doc Brown's Science-Chemistry Chemical Bonding GCSE/IGCSE/O/AS/A2 Level  Revision Notes

DIAGRAMS of GIANT COVALENT STRUCTURES and their PROPERTIES EXPLAINED - This section describes how covalent bonds can lead to large linear ('1D') e.g. thermoplastic polymer macromolecules, two dimensional ('2D') structures like graphite layers and three dimensional ('3D') giant covalent structured molecules like diamond, silica and thermosetting plastics. The physical properties of these structures are described and explained using models of their molecular structure.

Part 1 Introduction - why do atoms bond together? (I suggest you read 1st)

Part 2 Ionic Bonding - compounds and properties

Part 3 Covalent Bonding - small simple molecules and properties

Part 4 Covalent Bonding - macromolecules and giant covalent structures (this page)

Part 5 Metallic Bonding - structure and properties of metals

Part 6 More advanced concepts for advanced level chemistry (in preparation, BUT a lot on intermolecular forces in Equilibria Part 8)


Part 4. COVALENT BONDING - macromolecules & giant covalent structures

 giant network bonding - giant molecules e.g. carbon C-diamond/graphite, silicon Si/silica SiO2

properties of giant covalent structures * polymers/plastics * properties of polymers

carbon (diamond), carbon (graphite), carbon (buckminsterfullerene/fullerenes), silica/silicon dioxide SiO2

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 BIG!(c) doc b4. Large Covalent Molecules and their Properties

macromolecules - 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.

Its a good idea to have some idea of where the elements are in the periodic table

The black zig-zag line 'roughly' divides the metals on the left from the non-metals on the right of the elements of the Periodic Table.

Pd metals Part of the modern Periodic Table

Pd = period, Gp = group

metals => non-metals
Gp1 Gp2 Gp3 Gp4 Gp5 Gp6 Gp7 Gp0
1

1H  Note that H does not readily fit into any group

2He
2 3Li 4Be atomic number Chemical Symbol eg 4Be 5B 6C 7N 8O 9F 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
6 55Cs 56Ba Transition Metals 81Tl 82Pb 83Bi 84Po 85At 86Rn
Gp 1 Alkali Metals  Gp 2 Alkaline Earth Metals  Gp 7 Halogens  Gp 0 Noble Gases

Chemical bonding comments about the selected elements highlighted in white

The non-metallic elements carbon and silicon form giant covalent structures

The structure of the three allotropes of carbon (diamond, graphite and fullerenes), silicon and silicon dioxide (silica)

DIAGRAMS

  • It is possible for many 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 which gives the structure great thermal stability e.g. very high melting point and often great physical strength.
  • 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.
    • 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 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 boiling points because of the strong covalent bond network (3D or 2D in the case of graphite below).
  • A relatively large amount of energy is needed to melt or boil giant covalent structures because strong chemical bonds must be broken (and not just weakening intermolecular forces as in the case of small covalent molecules like water).
    • 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 the Energetics Notes.
  • They are usually poor conductors of electricity because the electrons are not usually free to move as they are 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 to become surrounded by solvent molecules
  • Silicon dioxide (silica, SiO2) has a similar 3D structure and properties to carbon (diamond) shown below.
  • DIAMOND
    • The hardness of carbon in the form of diamond enables it to be used as the 'leading edge' on cutting tools, the hardness is derived from the very strong rigid three-dimensional carbon-carbon bond network.
    • Diamond also has a very high melting point because of this very strong giant covalent lattice in which every carbon atom is strongly bonded to four other carbon atoms (see diagram above on right).
    • The strong bond network in diamond (and graphite and silica) prevents these materials from dissolving in any conventional solvent.
    • 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 the Energetics Notes.
  • SILICON DIOXIDE (SILICA)
    • 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.
    • Silicon dioxide ('silica') 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.
      • Looking at the diagram on the right, each silicon atom (black blobs) forms four strong covalent bonds with the linking oxygen atoms (yellow blobs).
    • Silica (SiO2) is a very hard substance with a very high melting point and won't dissolve in any solvent.
    • Many some minerals that are hard wearing, rare and attractive when polished, hold great value as gemstones, but sand is also mainly silica, but not quite as valuable!

DIAMOND

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  SILICA

silicon dioxide

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  • GRAPHITE
    • Carbon also occurs in the form of graphite. The carbon atoms form joined hexagonal rings forming layers 1 atom thick.
    • 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, so graphite, for a giant covalent structure, is unusually physically weak.
    • 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 ...
      • Electrons, from the 'shared bond', can move freely through each layer, so graphite is a conductor like a metal (diamond is an electrical insulator and a poor heat conductor). Graphite is used in electrical contacts e.g. electrodes in electrolysis.
      • The weak forces enable the layers to slip over each other so where as diamond is hard material graphite is a 'soft' crystal, it feels slippery.
        • This enables graphite to be used as a lubricant.
        • Carbon in the form of graphite is the only non-metal that is a significant electrical conductor.
    • 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.

GRAPHITE

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  • 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 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 of nanochemistry
    • They can be used as semiconductors in electrical circuits.
    • 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.
    • Nanotube fibres are very strong and so they are used in 'composite materials' e.g. reinforcing graphite in carbon fibre tennis rackets.
    • 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

FULLERENES

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(c) doc bBonding 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'.

    • Bonds between atoms in molecules, e.g. C-C, are called intra-molecular bonds.

    • 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 of layers 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 in Oil Notes and Extra Organic Chemistry Notes.

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

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keywords-phrases formulae: giant covalent lattice structures SiO2


Revision notes information to help revise KS4 Science Additional Science Triple Award Separate Sciences GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10) basic aid notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA OCR Edexcel Salters CIE, CCEA/CEA & WJEC advanced level courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses)


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