covalent compounds

• types of substance made out of COVALENT bonds:

  1. SIMPLE MOLECULAR SUBSTANCES

  2. GIANT COVALENT SUBSTANCES

  3. POLYMERS
• Simple Molecular Substances (Small Molecules)


  consist of SMALL MOLECULES formed by COVALENT BONDS and are not large networks but small groups of atoms bonded together.

  • e.g. Hydrogen (H₂), Oxygen (O₂), Water (H₂O) and Chlorine (Cl₂).
• Properties of Simple Molecular Substances

  INTERMOLECULAR FORCES: the forces BETWEEN MOLECULES which are weak and do NOT need a lot of energy to overcome.

  
• Properties of Simple Molecular Substances


  COVALENT BONDS: the bonds BETWEEN ATOMS which are STRONG and need A LOT of energy to overcome.

  
• When simple molecular substances MELT or BOIL.

  • it's the intermolecular FORCES that need to be broken, NOT the covalent bonds.
  • the forces are WEAK, so a SMALL amount of energy is needed to break them.
  • this gives simple molecular substances LOW melting & boiling points.
  • They are found as LIQUIDS or GASES at room temperature.
• There're two bonds involved in simple molecular substances:

  • covalent bonds
  • intermolecular forces
• Intermolecular forces are WEAK when compared to COVALENT bonds.

  • but some intermolecular forces are weaker than others.
• The LARGER the molecule, the STRONGER the intermolecular forces.

  • As the molecule has MORE ELECTRONS.
• Iodine atoms are LARGER as they are further down the periodic table.


  • meaning Iodine molecules have MORE ELECTRONS.
  • So there's STRONGER INTERMOLECULAR forces between the molecules.
  • MORE ENERGY is required to break these forces.
  • So Iodine has a HIGHER melting & boiling point than Chlorine.

  
• Polymers:


  Long chains made up of REPEATING UNITS known as monomers.

  • These atoms within the chains are held together by COVALENT BONDS, creating LARGE MOLECULES with unique properties.

  Used to make things like plastic bags & t-shirts

  
• Polymers are very LARGE:

  • so they have STRONG INTERMOLECULAR FORCES.
  • which require A LOT of energy to overcome.
  • So They have HIGH melting & boiling points
  • which means they are usually SOLIDS at room temperature.

  
• Polymers can be represented as molecular formulas using the format:

  • This shows the REPEATING UNIT of the polymer poly(ethene).
  • The structure within the brackets is repeated several times to give the structure of the polymer.

  The molecular formula of poly(ethene) is (C₂H₄)n.

  
• Giant Covalent Structures:


  Has huge numbers of non-metallic atoms bonded by strong COVALENT BONDS arranged in a regular repeating lattices.

  
• Giant Covalent Structures:


  They have HIGH MELTING AND BOILING POINTS.

  • Because a LARGE AMOUNT OF ENERGY is required to break the strong covalent bonds in the network.
• Examples of giant covalent structures:

  • Diamond
  • Graphite
  • Silicon Dioxide (Silica)

  
• Simple molecules:


  Has multiple atoms strong covalent bonds between each other.

  • But the forces of attraction between separate molecules are weak.
  • So simple molecules are easily separated from each other.

  
• Diamond & graphite are both made from the element carbon, and are examples of giant covalent structures:


  Their atoms are arranged in a regular repeating lattice structure with many multiple covalent bonds between each atom. 

  
• Summary of giant covalent structures:

  • Every atom is connected by strong covalent bonds
  • No weak intermolecular forces as there is only one structure
  • High melting and boiling points
  • Cannot conduct electricity (except graphite)
  • Strong

  
• In chemistry, to conduct electricity or heat, substances have to have some electrons or ions that are free to move about.
• Summary of simple molecular substances:

  • Strong covalent bonds between the atoms of each molecule. 
  • Weak intermolecular forces between molecules. 
  • Low melting and boiling points (so normally gaseous as room temperature).
  • Cannot conduct electricity, as there’s no free moving electrons or ions

  
• As you go down the group, the atoms get larger.

  • Meaning there will be more intermolecular forces, & more energy & a higher temperature required to break them.


  Meaning they have different melting & boiling points, So they're in different states at room temperature:

  • Chlorine is a gas at room temperature
  • Bromine is a liquid at room temperature
  • Iodine is a solid at room temperature & gives off purple fumes. 

  
• The forces between iodine molecules are stronger.

  
• In graphite, each carbon atom uses one electron to forms bonds with other carbon atoms.