Covalent Substances: Two Kinds

Covalent Substances: Two Kinds

1. Substances with covalent bonds (electron Sharing) can either be simple molecules or giant structures

2. Simple Molecular Substance

   • The atom forms a very strong covalent bond to form small molecules of several atoms.
   • By contrast, the forces of attraction between these molecules are very weak
   • The result of these feeble intermolecular forces
     • The melting and boiling point are very low
     • Because molecules are easily parted from each other.
   • It’s the intermolecular forces that get broken when simple molecular substances melt or boil – not the much stronger covalent bonds.
   • Most molecular substances are gases or liquids at room temperature but they can be solids
   • Molecular substances don’t conduct electricity – there are not ions so there’s no electrical charge.

3. Giant covalent Structure are macromolecules

   • These are similar to giant ionic Structure (lattice)
     • Except that there are no charged ions
   • All the atoms are bonded to each other by strong covalent bonds
     • This means that they have a very high melting and boiling points
   • They don’t conduct electricity - not even when molten (except for graphite)
     • The main example are diamond and graphite as they are both made only from carbon atoms and silicon atoms (Silica)
   • Diamond
     • Each carbon atom forms four covalent bonds in a very rigid giant structure
       • Made of the hardest natural substance
         • Used for drill tips and it’s pretty and sparkly too
   • Silicon Dioxide (Silica)
     • This is what sand made of - each grain of sand is one giant structure of silicon and oxygen.
   • Graphite
     • Each carbon atom only forms three covalent bonds
     • These creates layers which are free to slide other each other (like a pack of cards) - Graphite is soft and slippery
     • This is because there are weak intermolecular forces between the layers

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