Covalent bonding

• • Non-metal atoms can share electrons with other non-metal atoms to obtain a full outer shell of electrons
  • When two atoms share pairs of electrons, they form covalent bonds
• • Covalent bonds between atoms are very strong
• • When two or more atoms are covalently bonded together, they form ‘molecules’
  • Covalently bonded substances may consist of small molecules or giant molecules
  • Weak intermolecular forces exist between individual molecules
  • For example, in methane, each molecule consists of four hydrogen atoms covalently bonded to a carbon atom, and in between individual methane molecules there are weak intermolecular forces
• • Simple covalent molecules do not conduct electricity as they do not contain free electrons
• • Covalent substances tend to have small molecular structures, such as Cl2, H2O or CO2
  • Small covalent molecules can be represented by dot and cross diagrams
• • You need to be able to describe and draw the structures of the following molecules using dot-and-cross diagrams: hydrogen (H2), chlorine (Cl2), oxygen (O2), nitrogen (N2), hydrogen chloride (HCl), water (H2O), ammonia (NH3) and methane (CH4)
• Simple covalent molecules are small and can be separated into individual molecular units without breaking any chemical bonds (although there will still be strong covalent bonds holding the atoms in each individual molecule together.) Giant ionic and covalent structures form huge continuous networks of atoms that are bonded together and cannot be separated into individual units without breaking bonds.
• • Not all covalent molecules are small; covalent molecules can also be very large
  • For example, polymers and giant covalent structures
  • Common polymers include polythene which is used extensively in plastic bags and polyvinylchloride (PVC) which has many industrial applications, most notably in the production of water pipes.
• • Some covalently bonded substances have giant covalent structures, such as graphite, diamond, and silicon dioxide
  • These substances form giant crystal structures made from many atoms held together by covalent bonds
• Dot and Cross Diagrams

  • Advantages:
  • Useful for illustrating the transfer of electrons
  • Indicates from which atom the bonding electrons come from
  • Disadvantages:
  • Fails to illustrate the 3Darrangements of the atoms and electron shells
  • Doesn’t indicate the relativesizes of the atoms
• Ball and Stick Model

  • Advantages:
  • Useful for illustrating the arrangement of atoms in 3Dspace
  • Especially useful for visualizing the shape of a molecule
  • Disadvantages:
  • Fails at indicating the movement of electrons
  • The atoms are placed far apart from each other, which in reality is not the case as the gaps between atoms are much smaller

  
• 2D Representations of Molecules

  • Advantages:
  • Displayed formulae are 2D representations and are basically simpler versions of the ball and stick model
  • Adequately indicate what atoms are in a molecule and how they are connected
  • Disadvantages:
  • Fail to illustrate the relative sizes of the atoms and bonds
  • Cannot give you an idea of the shape of a molecule and what it looks like in 3D space