covalent bonds and carbon allotropes

• ionic and covalent bonding difference

  in ionic bonding, one atom donates electrons to the other. in covalent bonding, the two atoms share electrons
• covalent bonds are formed between...

  non-metal atoms
• covalent bond


  a chemical bond that involves sharing a pair of electrons between atoms in a molecule
• simple covalent molecules


  - molecules that contain only a few atoms held together by covalent bonds

  - has strong covalent bonds, but weak inter-molecular forces, therefore are usually gases and sometimes liquids
• simple covalent molecules examples

  hydrogen (H2), chlorine (Cl2), oxygen (O2), water (H2O), ammonia (NH3), hydrogen chloride (HCl), methane (CH4), ethene (C2H4).
• why does covalent bonding occur?

  so atoms share electrons to get to their full outer shell which is more stable
• van der Waals forces

  - weak attraction that develops between the oppositely charged regions of nearby molecules

  - in covalent bonding this is the attraction between the bonding electrons and the two nuclei of the atoms bonded together
• intermolecular forces


  forces of attraction between molecules
• covalent bond diagram

  
• covalent compound properties


  - low melting and boiling points.

  - poor conductors of electricity.

  - soft and flexible.

  - nonpolar covalent compounds dissolve poorly in water.
• giant covalent structures are...

  macromolecules
• giant covalent structure


  a huge 3D network of covalently bonded atoms
• macromolecules


  a very large organic molecule composed of many smaller molecules
• examples of giant covalent structures

  diamond, graphite, silicon dioxide
• allotropes


  different structural forms of the same element in the same physical state
• carbon allotropes


  diamond, graphite, fullerene
• why does diamond have a high melting/boiling point?

  because of the covalent bonds that hold the atoms together - these covalent bonds are extremely difficult to overcome
• diamond structure diagram

  
• diamond properties

  - high melting/boiling point

  - very hard

  - do not conduct electricity

  - insoluble
• diamond structure


  tetrahedral coordination, each carbon atom forms four covalent bonds in a very rigid giant covalent structure
• why is diamond so hard?

  each of the carbon atoms in diamond are covalently bonded to four other in a tetrahedral structure, making it very hard
• why does diamond not conduct electricity?

  because they have no charged particles that are free to move, they cannot conduct electricity
• why is diamond insoluble?

  due to their strong covalent bonds, they are generally inert and so do not react with water, making it impossible to dissolve
• silicone dioxide structure


  tetrahedral coordination, with four oxygen atoms surrounding a central Si atom with strong covalent bonds
• silicone dioxide structure diagram

  
• graphite structure

  each carbon atom only forms 3 covalent bonds, creating sheets of carbon atoms arranged in hexagon that do not have any covalent bonds between them
• graphite structure diagram

  
• graphite properties


  - high melting/boiling point

  - soft

  - conduct electricity
• why does graphite have a high melting/boiling point?

  because the covalent bonds in the layers need a lot of energy to break
• why is graphite soft?

  due to having no covalent bonds between the layers, the layers are free to move over each other. this also makes graphite slippery and a good lubricant
• polymers


  large compound formed from combinations of many monomers, joined together with covalent bonds
• graphene


  a single layer of graphite joined together in hexagons
• graphene is ... dimensional

  two

  (because the sheet is just one atom thick)
• the network of covalent bonds in graphene make it very...

  strong
• why is graphene being light beneficial?

  it can be added to composite materials to improve their strength without adding much weight
• why can graphene be used in electronics?

  like graphite, it contains delocalised electrons that can conduct energy through the whole structure
• why does graphite conduct electricity?

  only three out of each carbon's four outer electrons are used in bonds, so each carbon atom has one delocalised electron which can move freely through the layers and carry charge
• fullerenes form...


  spheres and tubes
• fullerenes are molecules of...

  carbon
• fullerenes are shaped like...


  close tubes or hollow balls