topic 2- bonding, structure and properties of matter

• Ions


  Charged particles - can be single atoms or groups of atoms
• Formation of ions


  1. Atoms lose or gain electrons to form ions

  2. Atoms try to get a full outer shell like a noble gas
• Groups that most readily form ions


  • Group 1
  • Group 2
  • Group 6
  • Group 7
• Positive ions (cations)


  Formed when metals lose electrons
• Negative ions (anions)


  Formed when non-metals gain electrons
• The number of electrons lost or gained is the same as the charge on the ion
• Elements in the same group all have the same number of outer electrons, so they form ions with the same charges
• Ion formation


  • Sodium atom loses 1 electron to form sodium ion
  • Magnesium atom loses 2 electrons to form magnesium ion
  • Chlorine atom gains 1 electron to form chloride ion
  • Oxygen atom gains 2 electrons to form oxide ion
• Ionic bonding


  Formed when a metal and a non-metal react, with the metal losing electrons and the non-metal gaining them. The oppositely charged ions are strongly attracted.
• Ionic compounds


  • Have a regular lattice structure
  • Have high melting and boiling points due to strong bonds
  • Don't conduct electricity when solid but do when molten or dissolved
• Finding the formula of an ionic compound


  1. Look at the charges of the ions

  2. Balance the charges so the overall charge is zero
• Covalent bonding


  Non-metal atoms share pairs of electrons to fill their outer shells
• Covalent bonds


  • Are very strong due to electrostatic attraction between nuclei and shared electrons
  • Occur in compounds of non-metals and in non-metal elements
• Molecular formula shows the number of atoms of each element in a molecule
• Simple molecular substances


  Made up of molecules containing a few atoms joined together by covalent bonds
• Common examples of simple molecular substances


  • Hydrogen
  • Oxygen
  • Methane
  • Chlorine
  • Nitrogen
• Hydrogen


  Atoms have just one electron, they only need one more to complete the first shell
• Oxygen


  Atoms need two more electrons to complete their outer shell
• Methane, CH4


  Carbon has four outer electrons which is half a full shell, it can form four covalent bonds with hydrogen atoms to fill up its outer shell
• Chlorine


  Each chlorine atom needs one electron to complete the outer shell
• Nitrogen


  Atoms need three more electrons to complete their outer shell
• Water, H2O


  Oxygen shares a pair of electrons with two H atoms to form two single covalent bonds
• Hydrogen Chloride, HCl

  Both atoms only need one more electron to complete their outer shells
• Properties of simple molecular substances


  • Substances containing covalent bonds usually have simple molecular structures
  • Atoms within the molecules are held together by very strong covalent bonds, but the forces of attraction between these molecules are very weak
  • To melt or boil, only the weak intermolecular forces need to be broken, not the covalent bonds, so melting and boiling points are low
  • Most are gases or liquids at room temperature
  • As molecules get larger, the strength of the intermolecular forces increases, more energy is needed to break them, and the melting and boiling points increase
  • Don't conduct electricity because they can't have charged particles
• The weak intermolecular forces between molecules are broken when a simple molecular substance melts or boils
• Polymers


  Long chains of repeating units linked by strong covalent bonds
• Properties of polymers


  • All the atoms are joined by strong covalent bonds
  • The intermolecular forces between polymer molecules are larger than between simple covalent molecules, so more energy is needed to break them, meaning most polymers are solid at room temperature
  • The intermolecular forces are still weaker than ionic or covalent bonds, so they generally have lower boiling points than ionic or giant molecule compounds
• Molecular formula of a polymer


  (Repeating unit formula)n
• Repeating unit


  The shortest repeating section of a polymer molecule
• Giant covalent structures


  Macromolecules where all the atoms are bonded to each other by strong covalent bonds
• Properties of giant covalent structures


  • Have very high melting and boiling points as lots of energy is needed to break the covalent bonds between the atoms
  • Don't contain charged particles, so they don't conduct electricity (except for a few exceptions like graphite)
• Main examples of giant covalent structures

  • Diamond
  • Graphite
  • Silicon dioxide (silica)
• Diamond


  Each carbon atom forms four covalent bonds in a very tight giant covalent structure, making it very hard
• Graphite


  Each carbon atom forms three covalent bonds to create layers held together weakly, with one delocalised electron, making it soft and a conductor of electricity
• Graphene


  A sheet of carbon atoms joined in hexagons, just one atom thick, very strong and can conduct electricity
• Fullerenes


  Molecules of carbon shaped like closed tubes or hollow balls, made up of hexagons, pentagons and heptagons of carbon atoms
• Properties of fullerenes


  • Can 'encage' other molecules, have a huge surface area so can be used as catalysts, and can form nanotubes which can conduct electricity and heat, and have high tensile strength
• Metallic bonding


  Involves delocalised electrons in the outer shell of metal atoms, which are attracted to the positive metal ions, holding the atoms together in a regular structure
• Properties of metals


  • Most are solid at room temperature due to the strong metallic bonds
  • Good conductors of electricity and heat due to the delocalised electrons
  • Malleable as the layers of atoms can slide over each other
• Alloys


  Mixtures of two or more metals or a metal and another element, harder than pure metals because the different sized atoms distort the layers of metal atoms, making them more difficult to slide over each other