Chp5 structure and properties of materials chemistry

• Element


  Pure substance that cannot be broken down into two or more substances by chemical methods
• Element


  • Can appear as atoms if not chemically bonded with each other
  • Can appear as molecules if covalently bonded
• Compound


  Pure substance containing two or more elements that are chemically bonded to form covalent or ionic compounds
• Compound


  • Has fixed melting and boiling points
  • Has different properties from its constituent elements
• All atoms are elements but not all molecules are compounds
• Separation of compounds into constituents


  1. Exposing compound to strong heat (thermal decomposition)

  2. Passing electric current through compound (electrolysis)
• Difference between compound and mixture

  • Compound has fixed proportion of elements
  • Mixture does not have fixed ratios and can be made up of elements and compounds
• Atom


  Purest form of an element
• Molecule


  Two or more elements chemically combined together
• Components of air


  • Oxygen (21%)
  • Nitrogen (78%)
  • Carbon dioxide
  • Water
  • Other noble gases (1%)
• Air does not contain hydrogen
• Ionic compounds have high melting and boiling points because they have strong electrostatic forces of attraction between ions of opposite charges, hence a large amount of energy is needed to overcome them
• Ionic compounds are physically hard but brittle. the strong attraction between oppositely charged ions makes ionic compounds resistant to deforming
• Ionic compounds are often soluble in water but insoluble in organic solvents. (dissolved in water=aqueous state)
• Ionic compounds conduct electricity when molten or in aqueous solution but not in solid state.
• When ionic compound is in solid state, the ions in the lattice are only able to vibrate about their fixed positions. Hence ions are not mobile. However when compound is molten (liquid) or in aqueous state, ions are mobile and can conduct electricity.
• Structural properties of covalent molecules

  Molecules held together by weak intermolecular forces of attraction
• Simple covalent substances


  • Small amount of energy needed to overcome intermolecular forces
  • Usually gaseous or in liquid state at room temperature and pressure
• Melting and boiling points of simple covalent substances


  Low
• Structural properties of a molecule

  Cause simple covalent substances to have low melting and boiling points
• As the molecule of the covalent substance gets larger

  The melting and boiling points are usually higher
• Solubility

  Simple covalent substances are usually insoluble in water and soluble in organic solvents
• Electrical conductivity

  Simple covalent substances do not conduct electricity as there are no mobile electrons/ions. They are insulators.
• Alloys


  Mixture of metal with one or more elements. Exact number and distribution unknown. Melt over a range of temperatures.
• Pure metals


  • Malleable and ductile
• Metals and alloys

  • Good conductors of heat
  • Have high melting and boiling points
• Electrical conductivity

  Metals and alloys are good electrical conductors
• Steel- iron and carbon

  used to increase hardness.
• Reason for why alloys are stronger than pure metals.

  
  

  Atoms of the constituent elements in the mixture are of different sizes, which breaks up the regular arrangement of atoms in pure metal hence atoms of different sizes cannot slide over each other easily resulting in the alloy being stronger than the pure metal.