Unit 2: bonding

• Compounds


  Substances in which 2 or more elements are chemically combined
• Types of strong chemical bonds

  • Ionic
  • Covalent
  • Metallic
• Ionic bonding


  • Particles are oppositely charged ions
  • Occurs in compounds formed from metals combined with non-metals
• Covalent bonding


  • Particles are atoms which share pairs of electrons
  • Occurs in most non-metallic elements and in compounds of non-metals
• Metallic bonding


  • Particles are atoms which share delocalised electrons
  • Occurs in metallic elements and alloys
• Formation of ionic compounds

  1. Metal atoms lose electrons to become positively charged ions

  2. Non-metal atoms gain electrons to become negatively charged ions

  
• An ion is an atom that has lost or gained electron(s)
• Ions produced by metals in Groups 1 and 2 and by non-metals in Groups 6 and 7 gain full outer shell of electrons, so they have the same electronic structure as a noble gas (Group 0 element)

  
• Electron transfer during the formation of an ionic compound

  • Dot and cross diagram (e.g. for NaCl)

  
• Ionic compounds


  • Giant structure of ions
  • Held together by strong electrostatic forces of attraction between oppositely charged ions
  • Forces act in every direction since the structure is in 3D
• Ionic compound


  • Sodium chloride (salt)
• Covalent bonding


  Atoms share one or more pairs of electrons
• Small molecules with covalent bonds

  • HCl, H2, O2, Cl2, NH3, CH4
• Polymers


  Large covalently bonded molecules
• Giant covalent structures (macromolecules)

  Many atoms covalently bonded in a lattice structure
• Giant covalent structures


  • Diamond, silicon dioxide
• Metallic bonding


  Positive ions (atoms that have lost electron(s)) and delocalised electrons arranged in a regular pattern

  
• Metallic bonding


  • Delocalised electron system consists of the electrons 'lost' from the atoms to form positive ions
  • Delocalised electrons are free to move through the structure
  • Delocalised electrons are shared through the structure so metallic bonds are strong

  
• The three states of matter

  • Solid
  • Liquid
  • Gas

  
• Melting and freezing


  Take place at the melting point
• Boiling and condensing


  Take place at the boiling point
• Particle theory


  Can help to explain melting, boiling, freezing and condensing
• The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance
• The nature of the particles involved depends on the type of bonding and the structure of the substance
• The stronger the forces between the particles the higher the melting point and boiling point of the substance
• Limitations of the simple particle model include that there are no forces, all particles are represented as spheres, and the spheres are solid
• State symbols


  Solid (s), liquid (l), gas (g), aqueous (aq)
• Ionic compounds


  • Have regular structures (giant ionic lattices)
  • Have strong electrostatic forces of attraction in all directions between oppositely charged ions
• Ionic compounds


  Have high melting and boiling points because a lot of energy is required to break the many strong bonds
• Ionic compounds


  Conduct electricity when melted or dissolved in water because the ions are free to move and carry current, but can't conduct electricity when solid because the ions are fixed in place
• Substances consisting of small molecules

  • Are usually gases or liquids that have low boiling and melting points
  • Have weak intermolecular forces between the molecules
• The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points
• Substances consisting of small molecules don't conduct electricity because small molecules do not have an overall electric charge
• Polymers


  • Have very large molecules
  • Atoms in the polymer molecules are linked to other atoms by strong covalent bonds
  • Intermolecular forces between polymer molecules are relatively strong and so these substances are solids at room temperature
• Substances consisting of giant covalent structures

  • Are solids with very high melting points
  • All of the atoms in these structures are linked to other atoms by strong covalent bonds that must be overcome to melt or boil these substances
• Giant covalent structures


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


  • Have giant structures of atoms with strong metallic bonding
  • Most metals have high melting and boiling points
  • The layers of atoms in metals are able to slide over each other, so metals can be bent and shaped
• Alloys


  • Are made from 2 or more different types of metals
  • The different sized atoms distort the layers in the structure, making it harder for them to slide over each other, so alloys are harder than pure metals
• Metals as conductors


  • Good conductors of electricity because the delocalised electrons in the metal carry electrical charge through the metal
  • Good conductors of thermal energy because energy is transferred by the delocalised electrons