4a. Introduction to giant covalent structures

Large 3D 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)?, why does diamond have such a high melting point? why is silica (silicon dioxide) a giant covalent structure, 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 forming giant covalent structures 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

Materials that consist of giant covalent structures are solids with very high melting points and usually physically hard materials (not graphite).

All of the atoms in these structures are linked to other atoms by strong covalent bonds in specific directions eg a grain of sand (silica) is one giant molecule!

These substances usually have an extended 3D network of strong covalent bonds.

These strong bond networks must be overcome to melt or boil these giant covalent substances and this requires very high temperatures to give the particles sufficient kinetic energy to weaken the bonds and cause melting of the substance.

Diamond and graphite (forms of carbon), silicon and silicon dioxide (silica) are examples of giant covalent structures.

You should be able to recognise giant covalent structures from diagrams showing their bonding and structure.

You also need to be able to explain the properties of giant covalent substances in terms of their molecular structure.

Most giant covalent structures don't have freely moving charged particles like ions or electrons to carry an electric current, so they are poor conductors of electricity.

• It is possible for many atoms to link up to form a giant covalent structure or lattice.
  • The structures of giant covalent structure are usually based on non–metal atoms like carbon, silicon and boron.
  • The atoms in a giant covalent lattice are held together by strong directional covalent bonds and every atoms is connected to at least 2, 3 or 4 atoms.
  • What you might call 'atomic networking'!
• This very strong 3–dimensional covalent bond network or lattice gives the structure great thermal stability e.g. very high melting point and often great physical strength.
  • This is because it takes so much thermal kinetic energy to weaken the bonds sufficiently to allow melting.
  • The covalent bonds are very directional, giving rise to a strong and fixed network that we call a giant covalent structure.
• This gives them significantly different properties from the small simple covalent molecules (see simple molecular substances).
• 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, graphene and nanotubes are the four solid allotropes of the element carbon. Liquid fullerenes are a 5th form of 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.
• 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 (and giant covalent structures are NOT made up of ions).
  • All the valency bonding electrons are tightly held and shared by the two atoms of any bond, so in giant covalent structures they are rarely free to move through the lattice and not even when molten either, since these giant molecular covalent structures do NOT contain ions.
• 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 and also pure silicon itself.