chemistry O’level

• Atom


  The smallest particle of a substance that can exist
• Element


  Contains only one type of atom and cannot be split by chemical means
• Compound


  Two or more elements chemically combined, cannot be separated back into constituent elements
• Mixture


  Contains two or more elements not chemically combined, can be separated
• Atoms contain protons, neutrons and electrons
• Proton


  Positively charged particle in the nucleus
• Neutron


  Neutral particle in the nucleus
• Electron


  Negatively charged particle orbiting the nucleus
• Atomic number


  Number of protons in the nucleus
• Mass number


  Total number of protons and neutrons in the nucleus
• Group number


  Number of electrons in the outer shell
• Period number


  Number of electron shells
• Elements in the same group have similar chemical properties due to the same number of outer shell electrons
• Noble gases


  Group 0 elements, highly unreactive due to full outer shells
• Isotopes


  Atoms of the same element with the same number of protons but different number of neutrons
• Ion


  Charged particle formed by gaining or losing electrons
• Ionic bonding


  1. Metal atom loses electron(s)

  2. Non-metal atom gains electron(s)

  3. Oppositely charged ions are attracted
• Covalent bonding


  1. Sharing of electrons between non-metal atoms

  2. Atoms achieve full outer shells
• Covalent bonding examples


  • Water (H2O)
  • Methane (CH4)
• Water (H2O) has a central oxygen atom with two hydrogen atoms coming to the side
• Hydrogen


  Has one electron in its outer shell
• Oxygen


  Has six electrons in its outer shell, which becomes full with eight electrons
• Methane (CH4) has a central carbon atom with four hydrogen atoms
• Carbon dioxide (CO2) has double covalent bonds
• Ethene (C2H4) has a central carbon-carbon double bond with four hydrogen atoms
• Chemical structures


  • Giant covalent
  • Giant ionic
  • Giant metallic
  • Simple molecular
• Ionic bond


  Electrostatic forces of attraction between oppositely charged ions
• Giant ionic structures


  • Have high melting and boiling points due to strong electrostatic forces
  • Don't conduct electricity when solid as ions aren't free to move
  • Conduct electricity when molten/liquid as ions are free to move
  • Are brittle as layers of ions with same charge repel when force is applied
• Diamond


  • Has a giant tetrahedral structure with many strong covalent bonds, requiring high energy to break
  • Does not conduct electricity as it has no free electrons
• Graphite


  • Has a structure of bonded carbon layers that can slide over each other, making it a good lubricant
  • Conducts electricity as each carbon has a fourth free electron
• Covalent bond


  A shared pair of electrons
• Simple molecular substances


  • Have low melting points due to weak intermolecular forces
  • Have increasing boiling points with increasing molecular mass as intermolecular forces are stronger
• Giant metallic structures


  • Have high melting points due to strong metallic bonds
  • Are good conductors of heat and electricity due to delocalized electrons
  • Are malleable and ductile as layers of ions can slide over each other
• Balancing chemical equations


  1. List elements on both sides

  2. Make tally chart of elements

  3. Adjust coefficients to balance
• Relative atomic mass (Mr)


  Ratio of average mass of an element compared to carbon-12
• Calculating moles


  1. Use formula: mass = Mr x moles

  2. Moles = mass / Mr
• Empirical formula


  Simplest whole number ratio of atoms of each element in a compound
• Molecular formula


  Actual number of atoms of each element present in a compound
• Determining empirical formula


  1. List elements and masses

  2. Calculate moles of each element

  3. Divide moles by smallest to get simplest ratio
• Calculating water of crystallization

  1. Set up table with components (e.g. Na2CO3, H2O)

  2. Use mass, Mr and moles to solve for unknown