Covalent Bonds(simple and giant structure)

• Giant covalent structures


  In some substances, such as sand, diamond and graphite, millions of atoms are joined together by covalent bonds
• Giant covalent structures


  • All the bonds are covalent, so they have very high melting and boiling points, and are usually hard
  • The covalent bonds in these substances do not form molecules but vast networks of atoms
• Sand


  Mostly made of the mineral quartz, which is silicon dioxide. It has a giant covalent structure made up of silicon and oxygen atoms
• Structure of sand


  1. Each silicon atom (2.8.4) is bonded to four oxygen atoms

  2. Each oxygen atom (2.6) is bonded to two silicon atoms
• Allotropes of carbon


  Different forms of the same element
• Diamond and graphite are both made up of carbon atoms
• Diamond and graphite have different properties because the atoms are bonded in different arrangements which create different giant structures
• Graphite


  Only three of the four electrons in the outer shell of each carbon atom (2.4) are involved in covalent bonds
• Graphite


  • Soft and slippery - layers can easily slide over each other because the weak forces of attraction are easily broken
  • Conducts electricity - the only non-metal to do so. The free electron from each carbon means that each layer has delocalized electrons, which can carry charge
• Fullerenes


  A third class of carbon compounds that have been discovered in recent years
• Fullerenes


  • Buckminsterfullerene - contains 60 carbon atoms, each of which is bonded to three others by two single bonds and one double bond
• The atoms in this allotrope of carbon form a sphere, like the shape of a football. The molecules can be called 'bucky balls'
• Fullerenes are large but are not classified as giant structures
• Bonds


  Involve the electrons in the outer shells of atoms
• Filled electron shells


  • Very stable
• Maximum number of electrons in each shell


  • 1st shell holds a maximum of 2 electrons
  • 2nd shell holds a maximum of 8 electrons
  • 3rd shell holds a maximum of 8 electrons
• Electrons fill the shells nearest the nucleus first
• Noble gases


  Atoms have completely full outer shells and are stable
• Atoms of other elements


  Have incomplete outer electron shells and are unstable
• Noble gases


  Very unreactive and do not usually form bonds
• By forming bonds, atoms of unstable elements are able to have filled outer shells and become stable
• Covalent bond


  Shared electrons join non-metal atoms together
• Non-metal atoms cannot form a bond by transferring electrons from one to another
• Each atom in a covalent bond has a full, stable outer shell
• Covalent bond


  • Shared pair of electrons
• Ways to represent a covalent bond


  • Solid line
  • Simplified dot and cross diagram
• Only outer shells of electrons are involved in bonding, so the inner shells do not always have to be included in diagrams
• Diatomic molecules


  Contain covalent bonds
• How a covalent bond is formed in hydrogen

  1. Each hydrogen atom shares its single unpaired electron

  2. The shared pair of electrons forms a covalent bond and creates a diatomic molecule
• Some molecules contain double or triple covalent bonds
• Covalent compounds


  Covalent bonding between atoms of different non-metals
• How a covalent bond is formed in hydrogen chloride (HCl)


  1. Hydrogen and chlorine both need one more electron to fill outer shells

  2. By sharing one electron each, they both have a stable outer shell and a covalent bond is formed
• How covalent bonds are formed in water (H2O)


  Oxygen shares 1 electron with 1 hydrogen atom, and a second electron with another hydrogen atom
• Oxygen needs 2 more electrons, but hydrogen only needs 1 more
• Water (H2O)


  The molecule formed by covalent bonding of oxygen and hydrogen
• Calculating the ratio of atoms in a stable covalent compound

  1. Work out how many electrons are needed by each non-metal element to complete its outer electron shell

  2. Work out the ratio of atoms that will provide enough shared electrons to fill all the outer shells
• Ratio of atoms in ammonia (NH3)

  • 1 nitrogen, 3 hydrogen
• Ratio of atoms in methane (CH4)

  • 1 carbon, 4 hydrogen
• Ratio of atoms in carbon dioxide (CO2)

  • 1 carbon, 2 oxygen
• Double bond


  Two pairs of electrons shared