C2 - Bonding, structure and the properties of matter

• Ionic bonding is the electrostatic attraction between positive and negative ions. It is a relatively strong attraction
• Ionic compounds are held together in a giant lattice structure that extends in all directions. The structure is held together by the electrostatic attraction between positive and negative ions
• Properties of ionic substances:

  • High melting and boiling point (due to strong electrostatic forces between oppositely charged ions)
  • Do not conduct electricity when solid (ions are in fixed positions)
  • Conduct electricity when molten or dissolved in water (ions are free to move)
• Examples of positive ions: Na+, Mg2+, Al3+, Ca2+, Rb+

  Examples of negative ions (anions): Cl−, Br−, SO42−, NO3−, OH− (chloride, bromide, sulfate, nitrate, hydroxide)

  When working out a formula of an ionic compound, it is important to ensure that positive and negative charges balance each other, making the compound electrically neutral
• Ionic compounds are formed through the reaction of a metal with a non-metal. In the case of MgO, electron transfer occurs where magnesium (Mg) gives away its 2 outer shell electrons to oxygen (O) which accepts the electrons to become Mg2+ and O2− (oxide)
• Covalent bond is a shared pair of electrons between two atoms
• Structure and properties of simple molecular covalent substances:

  • Do not conduct electricity (no ions)
  • Consist of small molecules
  • Have weak intermolecular forces, resulting in low melting and boiling points
• Intermolecular forces increase as the mass/size of the molecule increases, leading to higher melting and boiling points (more energy needed to overcome these forces)
• Polymers are very large molecules with atoms linked by covalent bonds. Thermosoftening polymers melt/soften when heated due to no bonds between polymer chains. Strong intermolecular forces keep the structure solid at room temperature, melting when heated
• Giant covalent substances are solids where atoms are covalently bonded in a giant lattice. They have high melting/boiling points due to strong covalent bonds and mostly do not conduct electricity. Examples include diamond, graphite, and silicon dioxide
• Allotropes of carbon:

  • Diamond: very hard with strong covalent bonds, high melting point, does not conduct electricity
  • Graphite: layers of hexagonal rings, high melting point, soft and can conduct electricity
  • Fullerenes: hollow shaped molecules, may have 5/7-carbon rings, conductive
  • Nanotubes: cylindrical fullerene with high tensile strength and conductivity
  • Graphene: a single layer of graphite
• Metallic bonding is the forces of attraction between delocalised electrons and nuclei of metal ions
• Properties of metals:

  • High melting/boiling points
  • Good conductors of heat and electricity
  • Malleable and soft due to layers of atoms sliding over each other while maintaining attraction forces
• Alloys are mixtures of metal with other elements, usually metals. They are harder than pure metals because different sizes of atoms distort the layers, preventing them from sliding over each other
• The amount of energy needed to change state from solid to liquid or liquid to gas depends on the strength of the forces between the particles of the substance, which is influenced by the type of bonding and structure of the substance
• A pure substance will melt or boil at a fixed temperature, while a mixture will melt over a range of temperatures
• The three states of matter are solid, liquid, and gas
• Nanoscience studies particles that are 1 - 100nm in size
• Uses of nanoparticles:

  • Medicine (drug delivery systems)
  • Electronics
  • Deodorants
  • Sun creams for better skin coverage and protection against cell damage
• Fine particles (soot) have a diameter of 100-2500 nm, while coarse particles (dust) have a diameter of 2500-10^5 nm
• Nanoparticles have different properties from those of the same materials in bulk due to their high surface area to volume ratio