Structures and Properties (Chp 6)

• Ionic compounds form giant ionic structures. They are arranged in giant lattice structures or crystal lattice
• Ionic compounds consists of ions held by strong electrostatic forces of attraction between positive and negative charged ions
• Physical Properties of ionic compounds

  • Ionic compounds have high melting and boiling points and are non-volatile substances. The ions are held together by strong electrostatic attraction between the oppositely charged ions and a lot of energy is required to break down the ionic lattice structure.
• Physical properties of ionic compounds

  • Ionic compounds conduct electricity in the molten state or in the aqueous state. In the molten or aqueous state, the ions are mobile and so electrical conduction is possible.
• Physical properties of ionic compounds

  • in the solid state, the ions are held in fixed positions in the lattice structure so ionic compounds cannot conduct electricity as there are no free-moving charge carriers.
• ionic compounds dissolve in inorganic solvents such as water but not in organic solvents such as ethanol
• Structure and bonding of covalent substances

  • Covalent substances have simple molecular structures or giant molecular structures.
• Simple Molecular structure

  • Many simple molecular substances are volatile (evaporates easily) as a result of their low melting and boiling points.
  • This is due to weak intermolecular forces between simple molecules.
  • Very little heat energy is required to overcome the intermolecular forces.
• Examples of Substances with

  Simple molecular structures: Water, Methane and carbon dioxide

  Giant molecular structures: Diamond and Graphite
• Structure and bonding of covalent substances

  • Most simple molecular substances do not conduct electricity whether in the liquid, solid or gaseous state. This is because they do not have free-moving ions or electrons to conduct electricity
• Most simple molecular compounds are insoluble in water and soluble in organic solvents. However, alcohol and sugar are exceptions which are soluble in water. Some simple molecules like chlorine and hydrogen chloride dissociate in water
• Examples of common simple molecular substances

  Ammonia (NH³), Hydrogen peroxide (H²O²), Methane (CH⁴), Water (H²O)
• Giant Molecular structures (macromolecules) are extremely large molecules. It contains billions of atoms per molecule.
• Examples of Giant molecular structures: Diamond, Graphite, Silicon
• Bonding in Diamond

  • Consists of only carbon atoms
  • Each carbon uses all its 4 valence electrons to form 4 strong covalent bonds with 4 other carbon atoms in a tetrahedral arrangement.
  • All the carbon atoms are bonded together by strong covalent bonds in a three-dimensional lattice
• Properties of diamond

  • Carbon atoms are not able to slide over each other as they are all bonded together by strong covalent bonds between carbon atoms in a giant molecular structure which requires large amount of energy to break
  • High melting and boiling point
  • Cannot conduct electricity as it uses all valence electrons for bonding hence no free-moving electrons to carry electric current through the structure
• Bonding in Graphite

  • Consists of only carbon atoms
  • Another allotrope of carbon (came element but different form)
  • Consists of hexagonal layers of carbon atoms which lie on top of each other and are held by weak intermolecular forces.
  • Each carbon atom in graphite forms covalent bonds with three other carbon atoms
  • Can conduct electricity as each carbon atom uses only 3 valence electrons for bonding. Hence, there are free moving (delocalised) electrons contributed by each atom which can carry the electric current between layers of graphite
• Uses of diamond and graphite:

  Diamond : jewellery , tips of drills and other cutting tools

  Graphite: pencil lead, lubricant for machine parts
• Metallic bonding

  • It is the electrostatic force of attraction between positively charged metal ions and the sea of delocalised electrons
• Alloys

  • A mixture of a metal with one or more elements
  • Alloys are mixtures as the metals and other elements are physically combined, not chemically combined
• Structural Properties of alloys

  • Since the lattice arrangement is disrupted, a larger force is needed to make the layers slide over each other
  • Alloys tend to be less malleable and less ductile than the pure metals they are made from
  • Harder and stronger than pure metals
• Structural Properties of pure Metals

  • Have a regular structure
  • if enough force is applied, the layers of atoms can slide over one another easily
  • Is malleable and also ductile
• Physical Properties of Metals

  • Usually have high melting and boiling points because the atoms are held together in a lattice by strong metallic bonds
  • Good conductors of heat
  • Metals and alloys are good conductors of heat. The delocalised valence electrons allow efficient transfer of thermal energy throughout the giant metallic lattice.