I know that several of you like to see chemical compounds written as formulae. This goes a bit beyond the CE syllabus so if you are only popping by for a casual visit you might want to read something a little gentler – like this or this.
I posted about corundum recently, which is aluminium oxide with the formula AL2O3, but it is not immediately obvious why the atoms arrange in that specific way. This is an easy guide to working out the correct formula for simple ionic compounds.
You will need to know how many electrons there are in each element’s outer shell so that you can work out the charge on each ion. These electrons are known as valence electrons, and they are what determine the element’s capacity to combine to form compounds. It is more complicated than I have shown below, but this is enough for now, with several elements able to have variable valences.
You can use the table to work out the formula of aluminium oxide.
Step One
Write down the elements in the compound – in this case Al O (aluminium oxide)
Step Two
Work out the charge on the aluminium ion and the oxygen ion. Aluminium has three electrons in its outer shell so when it loses those it becomes Al3+. The oxygen has six electrons in its outer shell so when it gains two more (to fill its shell with eight) it gets a double minus charge thus O2- which means Al3+ O2-
Step Three
Remove the charges producing Al3 O2
Step Four
Swap the numbers and make them subscript not superscript i.e. Al2O3
Step Five
If the numbers can be cancelled down, then do that eg Zn2S2 would become ZnS
Let’s look at some examples…
Questions…
- For what does Z stand in the table at the top of the page?
- Iodine is Group VII of the Periodic Table. Name another Group VII element.
- Why are group VII elements so reactive?
- Which is the most reactive set of metal elements? What do they have in common with hydrogen?
- What is the chemical formula of…
- hydrogen sulfide
- magnesium chloride
- silicon oxide
- iron (II) chloride
- iron (III) sulfide
- phosphorous (V) oxide
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