2.4 CHEM From models to materials

• Chemical bonding is best described as a continuum rather than as discrete bonding types.
• Position of an element or compound in the bonding triangle is determined from the electronegativity values, as these give measure of the tendency of an atom to gain electrons, and therefore the type of bond it will form.
• x-axis: average electronegativity of bonded electrons. y-axis: electronegativity difference of bonded elements.

  
• Elements have zero electronegativity difference between their atoms, so are positioned on the x-axis. Ionic compounds with high electronegativity differences are found at the top centre, while covalent compounds with low electronegativity differences are found in the lower right corner. Polar covalent compounds occupy the intermediate position.
• The model of the bonding continuum in the bonding triangle allows us to make more accurate predictions of properties.
• Composite materials, such as fibreglass and concrete, are heterogenous mixtures of at least two different materials, which are present as separate phases. As these are mixtures, each component retains its individual properties. This results in a material with enhanced properties.
• Alloys are produced by adding one metal element to another metal element(or carbon) in liquid state, so that the different atoms can mix. As mixture solidifies, ions of the different metals are scattered through the lattice forming a structure of uniform composition.
• Alloys contain metallic bonds as the delocalised electrons bind lattice. The production of alloys is possible due to the non-directional nature of the delocalized electrons and the fact that the lattice can accommodate ions of different size.
• Alloys have some properties that are distinct from their component elements due to the different packaging of the cations in the lattice. The regular arrangement of atoms in a pure metal is interrupted in the alloy by the presence of different cations, making it more difficult for atoms to slip over each other and so change the shape.
• The alloy is often stronger, more chemically stable, and more resistant to corrosion than its component elements.
• Steel - iron with carbon and other elements - high tensile strength but corrodes, used as structural material. Brass - copper and zinc - variety of plumbing fittings
• Alloys have no fixed composition. Alloys cannot be represented by a chemical formula, form without a chemical reaction, so retain their metallic properties such as thermal and electrical conductivity and magnetism.
• Polymers are large molecules made up of repeating subunits called monomers
• Monomers are able to react together to form a linked chain reaction held together by covalent bonds, known as a polymer.
• Polymers are also known as macromolecules, as they are composed of thousands of atoms. Polymers vary in nature compared to monomers, and the length and amount of branching in the chain, and so have a wide range of properties.
• Polymers are such large molecules, we show their structure as a repeating unit.

  
• Synthetic polymers are human-made polymers. --> Plastics
• Features that make plastics so useful such as: unreactivity, water repellency, which mean they are often non-biodegradable(not be broken down by natural processes.
• Biodegradable compounds can undergo breakdown by microorganisms into end products that are found in nature and therefore not harmful to the environment.
• Compostable plastics breakdown along with food and garden waste in specific conditions
• Addition reactions occur when multiple bond in a molecule breaks down and creates new bonding positions. Alkenes and Alkynes are organic compounds which have double and triple C-C bonds so undergo addition reactions.
• Atom economy is a measure of the proportion of reactant that ends up in the desired product, based on the reactions stoichometry.
• Atom economy formula:

  
• Condensation reactions occur when two functional groups react to form a new covalent bond, with the release of a small molecule, such as H20.
• Condensation reaction:

  
• In order to form a condensation polymers, monomers must each have two functional groups 'active ends' where condensation reactions can occur. This allows them to form new covalent bonds with neighbours on both sides.
• Carboxylic acid and Alcohol react to form polyester:

  
• Carboxylic acid and amine react to form polyamide: Amide link is formed. Polymer is known as a polyamide.