2.1 Bonding, structure, properties

• Bonding and structure of ionic compounds

  
  

  • Bonding occurs between metals and non-metals
  • lonic bonds are electrostatic attractions between oppositely charged ions
  • The ions form a giant ionic lattice
  • 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
  • Electron transfer during the formation of an ionic compound can be represented by a dot and cross diagram
• Properties of an ionic compound:

  • The strong electrostatic attraction between oppositely charged ions means ionic compounds have high melting and boiling points
  • When solid, ionic compounds don't conduct electricity because the ions are fixed in place.
  • However, the ions can move when molten/dissolved so then ionic compounds conduct electricity
  • Ionic compounds are brittle
• Bonding and structure of metallic compounds

  • Metals consist of giant structures of positive metal ions
  • The electrons in the outer shell of metal atoms are delocalised and so are free to move through the whole structure
  • Metallic bonds form due to the electrostatic attraction between the positively charged metal ions and negative delocalised electrons
• Properties of a metallic bond

  • The layers of ions in metals are able to slide over each other, so metals can be bent and shaped making them malleable and ductile
  • The delocalised electrons can move through the metal and carry charge, so metals conduct electricity and heat
  • The metallic bonds are very strong and require large amounts of energy to be broken, so most metals have very high melting and boiling points
• Simple molecules

  • Substances that consist of simple molecules are usually gases or liquids that have low boiling and melting points
  • Substances that consist of simple molecules have weak intermolecular forces between the molecules. These are broken in boiling or melting - not the covalent bonds
  • The intermolecular forces increase with the size of the molecules, so larger molecules have higher melting and boiling points
  • Substances that consist of simple molecules don't conduct electricity, because simple molecules do not have an overall electric charge
• Giant covalent structures

  • Substances that consist of giant covalent structures are solids with very high melting points
  • All of the atoms in these structures are lined to other atoms by strong covalent bonds
  • These bonds must be overcome to melt or boil these substances
• Diamond

  • In diamond, each carbon is joined to 4 other carbons covalently
  • This is the maximum number bonds each carbon atom can make
  • It is very hard, has a very high melting point and does not conduct electricity
• Graphite

  • In graphite, 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 no covalent bonds between the layers, but weak intermolecular forces (meaning that graphite is soft and slippery)
  • One electron from each carbon is delocalised
  • This makes graphite similar to metals, because of its delocalised electrons
  • It can conduct electricity - unlike Diamond

  
• Nano-particle uses

  • Nano-silver

  1. Kills bacteria, so used in wound dressings and deodorants; also used to line socks and fridges to kill bacteria that cause bad smells

  • Nano-titanium dioxide

  1. Titanium dioxide nanoparticles are so small they do not reflect visible light, so cannot be seen. They are used in sunblock creams to block harmful ultraviolet light without appearing white on the skin
  2. Used in self-cleaning windows as they help break down dirt
• Risks of nano-particles

  • Nanoparticles are so small that they could potentially enter the bloodstream. Many people feel the risks of them aren't yet known, so more testing should be done before they are used
  • They are a relatively new material so long term effects are unknown
  • Could enter and potentially damage the environment
• Smart materials are responsive to certain stimuli, such as temperature and moisture
• Shape memory alloys and shape memory polymers

  • These materials can be bent and deformed but return to their original shape when heated
  • Used for shape memory polymers include sports equipment, such as gum shields and medical stitches
  • Used for shape memory alloys include car bodies and plates for bone fractures
• Thermochromic materials

  • Change colour when they reach a certain temperature
  • Used in mugs and spoons which change colour when their contents are hot
• Photochromic pigments

  • These pigments change colour when exposed to light
  • An application of this is sunglasses that darken when in bright sun
• Polymer gels

  • Hydrogels absorb up to 1,000 times their volume in water
  • Certain stimuli (changes in pH and temperature) can cause the water to be released
  • Used in nappies, fake snow and hair gel
• A huge number of natural and synthetic organic compounds we use today occur due to the ability of carbon to form families of similar compounds, chains and rings
• Graphene

  • Single layer of graphite
  • Has properties that make it useful in electronics and composites
• Carbon can also form fullerenes with different numbers of carbon atoms, which are molecules of carbon atoms with hollow shapes.

  • Molecules of carbon atoms with hollow shapes
  • They are 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
  • Their properties make them useful for nanotechnology, electronics and materials
• Fullerenes can be used as lubricants, to deliver drugs in the body and catalysts
• Nanotubes can be used for reinforcing materials, for example tennis rackets