1.1 - formulae

• Atom


  The smallest part of an element that can exist. All substances are made up of atoms.
• Compound


  A substance that combines two or more different elements through the formation of chemical bonds.
• Ion


  An ion is formed when an atom/molecule loses or gains electrons. This gives it an overall charge - a positive charge if it has lost at least one electron and a negative charge if it has gained at least one electron.
• Ionic equation


  A chemical equation that involves dissociated ions.
• Molecular formula


  The actual number of atoms of each element in a molecule.
• Oxidation number


  The charge of an ion or a theoretical charge of an atom in a covalently bonded compound assuming the bond becomes ionic.
• State Symbol


  State symbols show the physical state of the substance during the reaction, they are usually in brackets: gas (g), liquid(l), solid(s) and aqueous(aq). Aqueous means the substance is dissolved in water.
• Formula of a compound

  Shows which elements are present, as well as the ratio for the number of atoms of each element
• Charges on some common ions


  Group 1: +

  Group 2: 2+

  Group 6: 2-

  Group 7: -

  Hydrogen: H+

  Hydroxide: OH-

  Silver: Ag+

  Nitrate: NO3-

  Zinc: Zn2+

  Sulfate: SO4 2-

  Ammonium: NH4+

  Carbonate: CO3 2-
• Constructing a formula from ions

  1. Write the symbols for the ions required

  2. If the charges on the ions balance, simply write the formula without the charges

  3. If the charges on the ions do not balance, then choose the ratio of positive to negative ions needed to balance the charges

  4. Sometimes brackets are needed for clarity when compound ions are involved
• Oxidation number


  Indicates the number of electrons that need to be lost or gained by the element to make it neutral
• Oxidation numbers increasing or decreasing during a redox reaction show which species is oxidised and which is reduced
• The sum of the oxidation numbers in a compound is 0
• The sum of the oxidation numbers in an ion is equal to the overall charge on the ion
• The most electronegative element in a compound is assigned the negative oxidation number
• Assigning oxidation numbers


  • CaCl2: Ca +2, Cl -1

  HCO3-: H +1, O -2, C +4
• Chemical equation


  Shows what happens during a chemical reaction

  Must be balanced - same number of atoms of each element on each side
• State symbols


  (s) solid

  (l) liquid

  (g) gas

  (aq) solution in water
• Ionic equation


  Shows only the ions that take part in a chemical reaction

  Ions that do not change during the reaction - 'spectator ions' - are left out
• Ionic equation example


  • CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq)

  Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)