1.39 Activity. Finding formulae
Students should:
- 1.39 write formulae for compounds formed between the ions listed
Watch the video closely pausing where instructed. Use the information given to work out the correct formulae for the following compounds:
1.3.9 A neat trick
A simple way to work out the formula of an ionic compound.
You can use this method to balance the charges and therefore work out the formula of any combination of positive and negative ions.
Note :
If the charge is 1- or 1+ we do not put the number 1 in the formula. For example : for sodium chloride we write NaCl rather than Na1Cl1
If the charges are 2+ and 2- ( as in magnesium and oxygen) , the crossover method would give us Mg2O2 but we simplify this and write it as MgO.
So far we have considered only monatomic ions. These are ions formed when single atoms gain or lose electrons.
1.3.9 Activity. Polyatomic ions
Polyatomic ions are ions which are made up of more than one atom . Each of these ions can exist as a single particle. Use the key and the formulae to match the images below to the formulae
1.3.9 Activity 4. Polyatomic formulae
When writing formula involving polyatomic ions you need to be careful to use round brackets appropriately. The video here explains why.
Watch the video carefully and use it to note down the formulae of the following compounds :
- sodium nitrate
- magnesium hydroxide
- potassium sulfate
- copper (II) chloride
- copper (I) oxide
- ammonium sulfate
- aluminium carbonate
** Check your answers by rolling your cursor over the relevant name.