Covalent Compounds

• Covalent bond

  A shared pair of electrons between atoms
• Covalent bonds


  • Occur in non-metallic elements, e.g. Oxygen
  • Occur in compounds of non-metals, e.g. sulfur dioxide
• Covalent bonding in chlorine atoms

  1. An electron from each atom is shared

  2. This gives each chlorine atom eight electrons in the outer shell

  3. Each atom now has a complete outer shell
• Dot and cross diagrams

  Used to show covalent bonds in molecules
• Covalent bonds

  • Very strong
  • Some covalently bonded substances consist of small molecules
  • Others have giant covalent structures, e.g. diamond and silicon dioxide
• Small molecules

  Contain a relatively small number of non-metal atoms joined together by covalent bonds
• Small molecules

  • Have no overall electrical charge, so they cannot conduct electricity
  • Substances that consist of small molecules usually have low melting and boiling points
• Intermolecular forces

  Forces of attraction between the molecules
• The larger the molecules are

  The stronger the intermolecular forces between the molecules become
• The stronger the intermolecular forces

  The higher the melting and boiling points
• Going down Group 7 of the periodic table

  1. The molecules get larger

  2. Their melting and boiling points increase
• States of Group 7 elements at room temperature

  • Fluorine and chlorine are gases
  • Bromine is a liquid
  • Iodine is a solid
• Key Point

  Substances made from small molecules, such as water, have low melting and boiling points. This is because there are only weak forces of attraction between the molecules, which are easily overcome
• Giant Covalent Structures

  • All the atoms are linked by strong covalent bonds
  • These bonds must be broken for the substance to melt or boil
  • They have very high melting and boiling points
• Diamond

  • A form of carbon with a giant, rigid covalent structure (lattice)
  • Each carbon atom forms four strong covalent bonds with other carbon atoms
  • It is a very hard substance with a very high melting point
  • There are no charged particles, so it does not conduct electricity
• Graphite

  • Another form of carbon with a giant covalent structure and a very high melting point
  • Each carbon atom forms three covalent bonds with other carbon atoms
  • This results in a layered, hexagonal structure
  • The layers are held together by weak intermolecular forces
  • This means that the layers can slide past each other, making graphite soft and slippery
  • One electron from each carbon atom in graphite is delocalised
  • These delocalised electrons allow graphite to conduct heat and electricity
• Silicon dioxide (or silica)

  • Has a lattice structure similar to diamond
  • Each oxygen atom is joined to two silicon atoms
  • Each silicon atom is joined to four oxygen atoms
• Metals also have delocalised electrons