Structure and Bonding

Structure and Bonding

1. Atomic Structure

   • The atomic number and Mass number describe different features of an atom.
   • Relative atomic masses
   • An isotope is: ' A different atomic form of the same element, with the same number of protons and electrons, but a different number of neutrons.
     • same atomic number- different mass number

2. Ionic Bonding

   • Transferring Electrons
   • Atoms lose or gain electrons to form charged particles (ions)- strongly attracted to each other
     • Group 1 and 2 metals only have to lose one/two electrons to become stable ions (+ charge)
     • Elements in Groups 6 and 7 only have to gain one/two electrons to become stable ions (- charge)
       • Latch onto atoms that the gave/gained electron from
   • Regular Giant Ionic Lattices
     • Very Strong electrostatic forces of attraction between oppositely charged ions in all directions
   • Properties
     • High melting and boiling points- strong attraction between ions
     • When molten, ions are free to move- will carry electric current
     • Dissolve easily in water. Ions separate- free to move- carry electric current
   • Ions and Formulas
     • Ions have the structure of a noble gas
     • Groups 1,2,6 and 7 most readily form ions.
     • Group 1 and 2 elements = metals - form + ions Group 1 elements( alkali metals) form ions with non metals where metal has 1+ charge
     • Group 6 and 7 elements = non-metals- gain electrons to form - ions Group 7 elements (halogens) form ionic compound with alkali metals where halide ion has 1- charge
     • Charge on positive ions is the same as group number e.g. Group 1 + LI+ Group 7 = F-
     • Charges must balance each other e.g. MgCl2

3. Covalent Bonding

   • Sharing electrons
   • Covalent Bond = shared pair of electrons
   • Dot and Cross diagrams
   • Simple Molecular Substances
     • Very strong covalent bonds to form small molecules of several atoms.
     • Intermolecular forces = very weak - Melting and boiling points are very lows - molecules easily parted
     • Most molecular substances are gases/ liquids at room temperature but can be solids
     • Don't conduct electricity - no ions, no electrical charge
   • Giant Covalent (Macromolecules)
     • Similar to giant ionic lattices except no charged ions
     • All atoms bonded to each other by strong covalent bonds- very high melting/boiling points
     • Don't conduct electricity- except graphite
     • Main examples = Diamond and Graphite
       • Diamond- Each carbon atom forms four covalent bonds in a v. rigid giant covalent structure. Diamond = hardest natural substance -used for drill tips
       • Graphite- Each carbon atom has 3 covalent bonds. Creates layers- free to slide over each other. Graphite soft and slippery. Layers can be rubbed off onto paper e.g. a pencil. Weak intermolecular forces between layers. Only non-metal that's a good conductor of heat and electricity. Delocalised electrons that conduct heat and electricity

4. Metallic Structures

   • Giant Structure
   • Metallic bonds involve 'free electrons'- come from outer shell of the metal atoms and produce the properties of metals
   • Electrons free to move- good conductors of heat and electricity
   • These electrons hold atom together in regular structure- strong forces of electrostatic attraction between + metal atoms and - electrons
   • Electrons allow layers of atoms to slide over each other - metals can be bent/shaped
   • Alloys
     • Harder than pure metals-different sized atoms distorting layers- more difficult to slide over each other = harder

5. New Materials

   • Smart Materials
     • Behave differently depending on conditions e.g. temperature
     • Nitinol = shape memory alloy- when cool bends and twist like rubber. When heatedit goes back to a 'remembered' shape- used for braces
   • Nanoparticles
     • Between 1-100 nanometres across 1nm=0.000000001m
     • Contain roughly a few hundred atoms
     • Include fullerenes- molecules of carbon, shaped like hollow balls or closed tubes- arranged in hexagonal rings. Different fullerenes have different numbers of carbon atoms
     • A nanoparticle has v. different properties from the bulk chemical e.g. fullerenes have different properties from big lumps of carbon
     • Fullerens can be joined together to make nanotubes. All the covalent bonds make nanotubes v. strong- Can be used to reinforce graphite in tennis rackets
     • Nanoscience
       • Huge surface area to volume ratio- could help make new catalysts
       • Nanotubes can make stronger, lighter building materials
       • Sun cream can include nanoparticles- do't leave white marks
       • Nanomedicine- fullerenes absorbed more easily by the body - could deliver drugs into the cells where they're needed
       • Nanotubes conduct electricity

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