Simple Molecular Compounds

• Covalent substances tend to have small molecular structures, such as Cl2, H2O or CO2
• These small molecules are known as?simple molecules
• Hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2), hydrogen chloride (HCl), water (H2O), ammonia (NH3) and methane (CH4) are also examples of simple molecules
• Iodine is a simple molecule and can be represented but it exists as a crystalline structure involving a regular structure held together by weak London dispersion forces

Dot cross diagram of an iodine molecule

Covalent bonding & giant covalent lattice structures

• Giant covalent lattices?have?very high?melting?and?boiling points
  • These compounds have a large number of?covalent bonds?linking the whole structure
  • A lot of energy is required to break the lattice

   

• The compounds can be?hard?or?soft
  • Graphite is?soft?as the forces between the carbon layers are weak
  • Diamond and silicon(IV) oxide are?hard?as it is difficult to break their 3D network of strong covalent bonds

   

• Most compounds are insoluble with water
• Most compounds do not?conduct electricity?however some do
  • Graphite has?delocalised?electrons between the carbon layers which can move along the layers when a voltage is applied
  • Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom are involved in a?covalent bond?so there are no freely moving electrons available

   

Graphite

• Each carbon atom in graphite is bonded to?three?others forming?layers?of?hexagons, leaving one free electron per carbon atom
• These free electrons migrate along the layers and are free to move and carry charge, hence graphite can?conduct electricity
• The covalent bonds within the layers are very strong, but the layers are attracted to each other by weak?intermolecular forces, so the layers can?slide?over each other making graphite?soft?and?slippery

Diamond

• In diamond, each carbon atom bonds with four other carbons, forming a?tetrahedron
• All the covalent bonds are identical, very strong and there are no?intermolecular forces

Diagram showing the structure and bonding arrangement in diamond

Graphene

• Graphene consists of a single layer of graphite which is a sheet of carbon atoms covalently bonded forming a continuous hexagonal layer
• It is essentially a 2D molecule since it is only one atom thick
• It has very unusual properties make it useful in fabricating?composite?materials and in?electronics

Graphene is a truly remarkable material that has some unexpected properties