1. Ionic Compounds How to write formulae

2. Writing a formula: 1.Use the “table of ions” to find the charges 2.Write the positive ion first, then the negative ion. Example: Magnesium sulfateMg 2+ and SO 4 2- 3.The finished formula must be neutral – number of negative charges must equal number of positive.

3. If the charge on the positive ion is the same size as the charge on the negative ion then all you have to do is to write down the symbols for the ions. Sodium chloride Na + Cl - is written as NaCl Magnesium oxide Mg 2+ O 2- is written as MgO (NOTE: don’t show charges in final the formula)

4. If the charge on the positive ion is different from the charge on the negative ion, then you “swap and drop” Aluminium oxide Al 3+ O 2- Al O Swap and drop (Remember: not to show charges in final the formula) For polyatomic ions you use brackets Aluminium sulphate Al 3+ SO 4 2- is Al 2 (SO 4 ) 3 23