WRITING FORMULAE
Compounds-two or more elements chemically combined in fixed proportions and not easily separated. E.g.
sugar-C12H22O11
water-H2O
ammonia-NH3
Writing Formulae
Compounds are electrically neutral
The superscript represents the charge while the subscript shows the number of that particular atom is used in the formula.
1. The total number of positive charge must cancel out the total number of negative charge
2. NB: all metals form positive ions. The name of the metal ion tells the charge on it. E.g. lead (II) oxide contain PB2+ ion
3. Work out the number of charges based on group the element is from.
Element in Periodic Table Group | Charge on ion |
1 | +1 |
2 | +2 |
3 | +3 |
6 | -2 |
7 | -1 |
Charges on Some Ions
Positive ions
Charge | Name of ion | Formula |
1+ | Ammonium | NH+4 |
| Copper (I) | Cu+ |
| Hydrogen | H+ |
| Lithium | Li+ |
| Potassium | K+ |
2+ | Barium | Ba2+ |
| Calcium | Ca2+ |
| Copper (II) | Cu2+ |
| Iron (II) | Fe2+ |
| Lead (II) | Pb2+ |
| Magnesium | Mg2+ |
| Nickel (II) | Ni2+ |
| Strontium | Sr2+ |
| Zinc | Zn2+ |
3+ | Aluminium | Al3+ |
| Iron (III) | Fe3+ |
Negative Ions
Charge | Name of Ion | Formula |
1- | Bromide | Br- |
| Chloride | Cl- |
| Hydroxide | OH- |
| Fluoride | F- |
| Iodide | I- |
| Nitrate | NO-3 |
2- | Carbonate | CO2-3 |
| Oxide | O2- |
| Sulphate | SO2-4 |
| Sulphide | S2- |
3- | Nitride | N3- |
| Phosphate | PO3-4 |
Polyatomic ions- are ions that contain more than one type of atoms. e.g. carbonate, sulphate, phosphate, hydroxide. These formulae cannot be altered. They must be placed in a bracket and given a subscript number to indicate if more than one molecule is used.
Write formulae for lead (II) oxide, magnesium sulphate, potassium carbonate, sodium sulphate, aluminium sulphate, calcium hydroxide, calcium, silver nitrate, zinc chloride, iron (III) hydroxide, iron...