2.1 Bonding

• Bulk properties are the properties such as strength, melting point, conductivity, flexibility and hardness which are related to the different types of bonds the compound contains, the bond strengths in relation to intermolecular forces and the ways in which the bonds are arranged. The atoms themselves do not have these properties.
• Conductor: A material that contains charged particles which are free to move to carry electrical or thermal energy. Metals are good conductors due to the delocalised electrons. Covalent bond: A shared pair of electrons between two non-metals
• Diamond: A giant covalent structure which is made up of carbon atoms each of which form four covalent bonds with four other carbon atoms. The structure makes diamond very hard, making it suitable for use as drill bits
• Electrostatic forces: The strong forces of attraction between oppositely charged ions
• Fullerenes: Molecules of carbon atoms with hollow shapes. The structures are based on hexagonal rings of carbon atoms but they may also contain rings with five or seven carbon atoms. Examples include graphene and C60
• Giant covalent structure: A molecular structure containing many atoms covalently bonded together. The strong covalent bonds mean that giant covalent structures have high melting points.
• Graphite: A giant covalent structure which is made up of carbon atoms each of which form three covalent bonds with three other carbon atoms. These atoms form layers of hexagonal rings which have no covalent bonds between them. There is one delocalised electron per carbon atom which is free to move to carry charge.
• Graphene: A single layer of graphite with properties that make it useful in electronics and composites
• Ion: An atom or molecule with an electric charge due to the loss or gain of electrons. A positive ion is formed when an atom loses electrons, and a negative ion is formed when an atom gains electrons.
• Ionic bond: The bond formed between the oppositely charged ions when a metal atom loses electron(s) to form a positively charged ion and a non-metal gains these electron(s) to form a negatively charged ion.
• Ionic compound: Chemical compound formed of oppositely charged ions, held together by strong electrostatic forces.
• Lattice: A repeating regular arrangement of atoms/ions/molecules. This arrangement occurs in crystal structures
• Malleable: Capable of being deformed and moulded into various shapes. Metals are malleable since the uniform layers of atoms can slide over each other
• Metallic bond: The bonds present in metals between the positive metal ions and negatively charged delocalised electrons
• Nanoparticles: Particles with diameters between 1 nm to 100 nm in size. Nanoparticles can exhibit properties different to those for the same material in bulk.
• Simple molecules: Molecules containing a fixed number of atoms covalently bonded together. Simple molecules have low boiling points since they have weak intermolecular forces which are easy to overcome
• Smart materials: Materials which respond to certain external stimuli such as temperature and moisture. Examples include thermochromic pigments (change colour depending on the temperature), polymer gels and shape memory alloys.
• A compound is a substance in which 2 or more elements are chemically combined
• The three types of chemical bonds are:

  • Ionic - bonding in ionic compounds
  • Metallic - bonding in metals
  • Covalent - bonding in giant covalent structures and simple covalent molecules
• Properties of ionic compounds:

  • Strong electrostatic attraction between oppositely charged ions means ionic compounds have high melting and boiling
  • In a solid state, ionic compounds do not conduct electricity as the ions are fixed in place
  • When molten/ dissolved, ions can move around so then ionic compounds conduct electricity
  • They are brittle
• Properties of metallic compounds:

  • The layers of ions in metals are able to slide over each other so metals are malleable and ductile
  • The delocalised electrons can move through the metal and carry charge, so metals conduct electricity and heat
  • Metallic bonds are very strong and require large amounts of energy to be broken, giving most metals high melting and boiling points
• Properties of simple molecular covalent substances:

  • Simple molecules have weak intermolecular forces between the molecules, so they have low melting and boiling points
  • Do not conduct electricity because simple molecules do not have an overall charge
  • They are usually gases or liquids
• Substances with a giant covalent structure are solids with very high melting points
• Bonding and structure in metallic compounds:

  • The electrons in the outer shell of metals atoms are delocalised and so they are free to move through the whole structure - 'sea' of delocalised electrons
  • Metallic bonds form due to the electrostatic attraction between the positively charged metal ions and the negatively charged delocalised electrons
• How does ionic bonding take place?

  • Electrons in the outer shell of the metal are transferred
  • Metal atoms lose electrons to become positively charged ions
  • Non-metal atoms gain electrons to become negatively charged ions
  • The electrostatic attraction from these oppositely charged ions forms a giant ionic lattice
  • The electron transfer can be represented by a dot and cross diagram
• A compound is a substance in which 2 or more elements are chemically combined
• Properties of metallic compounds:

  • The layers of ions in metals are able to slide over each other so metals are malleable and ductile
  • The delocalised electrons can move through the metal and carry charge, so metals conduct electricity and heat
  • Metallic bonds are very strong and require large amounts of energy to be broken, giving most metals high melting and boiling points
• Properties of simple molecular covalent substances

  • Simple molecules have weak intermolecular forces between the molecules, so they have low melting and boiling points
  • Do not conduct electricity because simple molecules do not have an overall charge
  • They are usually liquids or gases
• Properties of giant covalent substances:

  • Solids
  • Very high melting point
• Bonding and structure in metallic compounds:

  • The electrons in the outer shell of metal atoms are delocalised and so they are free to move through the whole structure - 'sea' of delocalised electrons
  • Metallic bonds form due to the electrostatic attraction between the positively charged metal ions and the negatively charged delocalised electrons
• Covalent bonds are formed when atoms share pairs of electrons, they form covalent bonds. The bonds between these atoms are strong. Dot and cross diagrams can be drawn the represent the sharing of electrons
• Simple molecular substances have low melting and boiling points because of their weak intermolecular forces between molecules - these are broken in melting or boiling not the strong covalent bonds.
• Intermolecular forces increase with the size of the molecules, larger molecules have higher melting and boiling points
• Diamond is very hard, has a very high melting point and does not conduct electricity. Each carbon is joined to 4 other carbons covalently, covalent bonds need a lot of energy to be broken = very high melting point.
• Graphite properties with respect to bonding and structure:

  • Each carbon is covalently bonded to 3 other carbons, forming layers of hexagonal rings which have no covalent bonds between the layers.
  • The layers can slide over each other due to the absence of covalent bonds between layers, but there are weak intermolecular forces between layers so graphite is soft and slippery.
  • One electron from each carbon atom is delocalised so it can conduct electricity
• Properties of graphene with respect to bonding and structure:

  • Structure resembles single layer of graphite
  • Graphene has very high melting point due to the strong covalent bonds between the carbon atoms that require large amounts of energy to be broken
  • Conducts electricity due to the delocalised electrons that are free to move through its structure
• Fullerenes:

  • Molecules of carbon atoms with hollow shapes
  • Based on hexagonal rings of carbon atoms but they may also contain rings with five or seven carbon atoms
• Carbon nanotubes:

  • Cylindrical fullerenes with very high length to diameter ratios
  • Very high tensile strength and conduct electricity due to the delocalised electrons present
• Individual atoms do not have the same properties as bulk materials. For example, carbon atoms on their own don't have any of the properties exhibited by any of the different structures (diamond, graphite, graphene)
• Properties of nanoparticles:

  • 1-100 nanometres across
  • Contain a few hundred atoms
  • Have different properties from the 'bulk' properties which they form because of their high surface area to volume ratio